Effects of Constant and Fluctuating Rearing Temperatures on Development and Physiology in
Drosophila mojavensis (Diptera: Drosophilidae).
Eric C. Toolson
Department of Biology
University of New Mexico
Albuquerque, NM 87131
ABSTRACT
Drosophila mojavensis are diurnally active flies that breed in rotting cactus tissue throughout
the low-altitude desert regions of the southwestern United States and northwest Mexico.
Nevertheless, they are not particularly tolerant of high temperatures, and populations 'crash' each
summer. Adult activity and breeding are essentially confined to the months from December (when
populations are just beginning to recover from the previous summer's bottleneck) through April
(when population sizes are generally highest) and May.
The mating system is based on a lek: males establish and defend territories (used for courtship
and mating) on the surface of a host cactus. The lek is usually close to the rotting cactus tissue in
which females will lay their eggs and in which larvae complete development. These leks are
occupied only for a relatively short time in the morning and, depending on the season, again in the
late afternoon, after which the flies disperse to refuges. Most matings on the leks involve relatively
large males, and larger females exhibit higher fecundity, so there appears to be significant selection
for large body size in both sexes.
We followed a population of D. mojavensis near Guaymas, Mexico from December until midMay, when the population was in obvious decline. Extensive data were collected detailing the
thermal regimes experienced by larvae and pupae in the cactus rots and by adult D. mojavensis on the
leks and in their refuges. Behavioral studies provided a picture of the response of D. mojavensis to
temperature. Conditions were most favorable (i.e., flies were most active on the leks and population
sizes largest) when daily fluctuations in rot temperature did not go below 15 degrees at night or above
35 degrees during the day. During this same time, adult flies were able to locate refuges where
maximum temperatures stayed below 31-32 degrees. The onset of the population's decline correlated
with (i) rot temperatures reaching or exceeding 38-40 degrees on a daily basis and (ii) the inability of
adult flies to locate refuges where the temperature remained below about 32 degrees.
The field data were used to select appropriate constant and fluctuating temperature regimes for
laboratory studies on the effects of temperature on and physiology, specifically water relations, and
development. Flies raised under conditions when temperature did not exceed 32 degrees at any time
showed relatively low cuticle permeabilities that were independent of rearing temperature. However,
constant or fluctuating rearing temperatures that exceeded 32 degrees yielded flies with significantly
higher permeabilities. These results are consistent with the hypothesis that hot desert species whose
food source provides abundant water should have high integumentary evaporative water loss rates as
a way of reducing the threat of deleterious overheating.
When constant rearing temperatures ranging from 17 degrees to 34 degrees were employed,
higher temperatures produced smaller flies, as has been reported a number of times in the literature.
However, when fluctuating temperature regimes encompassing the same temperature range were
employed, the size of emerging adults showed virtually no dependence on rearing temperature. Only
when temperatures exceeded this range on a daily basis was any significant effect on mass noted. The
developmental 'system' of D. mojavensis thus appears to be well buffered (in terms of producing large
flies) against the effects of daily fluctuations in temperature, at least over the range of temperatures it
experiences during the spring months. This phenomenon was not due to any 'rescue effect' of
moderate temperatures during part of the fluctuating temperature regimes, since flies reared under
fluctuating conditions were generally as large as, or (more commonly) larger than, flies reared under
comparable constant regimes.
We discuss these results in the context of our data on the behavioral ecology and population
biology of D. mojavensis. We conclude with a discussion of the implications of our data for attempts
to model the evolution of adaptive phenotypic plasticity. Among other things, we argue that
inferences about the response of genotypes to environmental fluctuation that are drawn from results
of experiments in which fixed levels of parameters such as temperature were employed must be made
with caution, and that models based on such inferences probably merit reevaluation.
INTRODUCTION
The paper is simply going to report (i) a surprising lack of dependence of mass on rearing Tamb
when it's variable, and (ii) the effect of rearing Tamb regime on 'reproductive success' and
development time. Then, it'll discuss the ecological/evolutionary implications of the patterns.
è The Introduction should talk about phenotypic plasticity models in general,
The contribution of phenotypic plasticity to success in variable regimes has been the focus of
considerable work (**since Darwin?). For the most part the research has been empirical but more
recently a spate of modeling efforts have been published. theoretical.
Variable Tamb conditions can provide serious challenges to homeostatic mechanisms. Shrode
& Gerking (197*) reported that hatching success of pupfish under variable T a conditions was not
readily predictable from constant-T a experimental results. ; Parsons(?); Hofmann(?); Feder(?);
Clark(?); .
Tamb is just one of many abiotic parameters that both vary and impact the homeostatic
mechanisms. As such (or "For reasons discussed elsewhere (**refs**)"), Tamb is an ideal model
system(*?) to use in tests of the hypotheses deriving from the various phenotypic plasticity models.
Some evidence suggests that variable environments select for phenotypic plasticity (acclimation
ability). Data from *** on salamanders; from *** on plants?; Toolson & Kuper-Simbron (reverse
evidence--highly plastic species, D. pseudoobscura, lost the ability to acclimate to changes in rearing
T a in only a few generations’ maintenance under constant T a conditions. trout Achase (***);
Several modeling efforts have been published (Via & Lande; Scheiner; de Jong; Huey's postdoc; others). Most of these models treat systems in which genotypes experience one of two
(***more?) environments during development and predict the *** of a developmental end point.
However, many (**most?) environments are rarely constant, even within an individual’s
lifetime, and it remains to be seen if models based on discrete developmental endpoints are
appropriate for traits that must change continuously and reversibly multiple times during an
individual’s life span. **Is this what Gilchrist’s model dealt with?**
In fact, there are few data quantifying the evolutionary response to (environmental?) variation
on phenotypic traits, especially under controlled laboratory conditions, and these, too, have involved
constant Tamb regimes. Examples: D. pseudoobscura; Bennett's E. coli;
Moreover, the association between levels of habitat variability and the degree of plasticity in
relevant phenotypic traits is not consistently observed (Brown & Feldmuth, 197*; **others??). For
example, Brown and Feldmuth (197*) found no correlation ***. **Did Shrode & Gerking do work
with the same population (species) as Brown & Feldmuth? This suggests that ***.
We know very little about the conditions actually experienced in nature (Jones et al.). 6.
Particularly for most Drosophila species, we know little or nothing about environmental
conditions experienced by any life stage.
a. Get the paper by Jones(?) et al. for one exception (this is an indirect way of inferring
larval/pupal Tamb regime).
b.
This paper reports the results of a combined field and lab study on the behavioral,
developmental, and physiological response of D. mojavensis to temperature.
MATERIALS AND METHODS
Field Studies
The study population was located near San Carlos, Sonora, in northwest Mexico. This
population has been used as source flies for a number of published studies (***). We located a large
Organpipe cactus, ***, some of whose arms contained actively rotting tissue. During one field
season, we obtained 11 days of field data detailing thermal regimes and the response of adults thereto.
The field work spanned the interval from January, when population size was low and just starting to
increase, through May, when the population had peaked and numbers were obviously declining.
We conducted basic behavioral surveys to develop a picture of the daily behavior patterns of the
adults. We recorded times of first appearance and disappearance of flies from the leks, and made
special efforts to locate adult flies at times when they were not active on the leks.
Copper-constantan thermocouples were placed into the rotting cactus tissue where eggs, larvae,
and pupae develop and into thermal refuges used by adult flies during hot times of day. Data from the
thermocouples and meteorological instruments (solar radiation intensity, wind speed, ambient
temperature and humidity) were collected and stored by means of a Campbell CR-10 data logger.
Thermal conditions at the leks used for courtship/mating (males establish and defend territories
here, and this is where the preponderance of mating takes place) were monitored by measuring the
surface temperature of lek sites with
We used an infrared surface pyrometer (**Omega. ***) to record surface temperatures (T surf )
of various parts of the host cactus. As part of this study, we observed flies moving about the lek and
were able to identify parts of the lek that at any given time were actively avoided by the flies. Active
avoidance was evidenced by a fly turning sharply away from an area that was directly in its path.
When we observed this, we recorded T surf of the area through which the fly had been walking and
T surf of the area it turned away from.
Laboratory Studies
A laboratory stock was established using a large number of flies collected by baiting at the
study site in April, when population size was at its peak. The flies were kept on standard cornmealmolasses-brewers yeast medium at room Tamb for 4 generations, then used in the developmental
studies described herein.
Parental flies were randomly drawn from the maintenance stock and 10 pair were placed in
bottles containing ca. 250 ml of medium. Three replicate bottles were used in each Tamb regime, and
**each experiment was repeated twice. Emerging progeny were collected daily, placed in bottles
containing medium, and allowed to mature for 5 days before being used for water loss determinations
(**and HC analyses?).
1.
Flies were reared in temperature-controlled environmental chambers under a 12:12 light-dark
photoperiod regime. Rearing Tamb were controlled to within ±0.5o C. Variable Tamb rearing
conditions were obtained by turning the chamber temperature control to the upper Tamb setting at
the beginning of the light cycle and to the lower Tamb setting at the beginning of each dark cycle.
The chambers took approximately 1 h to reach the new Tamb, so all stages experienced 10+ hours
of each extreme Tamb. We chose a 10o C range for the variable Tamb experiment because the
chambers heated and cooled faster than the rots in nature.
Water loss rates (WLRs) of freshly-killed flies were determined gravimetrically as described in
Toolson (1982), except a chamber Tamb of 30o C and an exposure time of 1 h was used.
A subsample of flies from each WLR sample was used for analysis of fluctuating asymmetry.
Right and left wings were removed and mounted on glass microscope slides. The chord length from
the anterior crossvein to the end of the second longitudinal vein (Markow & Ricker, 1992) was
measured on each wing, using a *** coupled through a *** video camera to a *** digital image
analysis system.
RESULTS
Thermal Conditions in Cactus Rots
**Calculate mean & sd for shade Tamb, Trot
Thermal conditions experienced by eggs, larvae, and pupae are presented in Figures 1-4.
Although we have only one day's data for January, the data presented in Figure 1 reflect the typical
thermal regime experienced by flies at this time of year, with shade Tamb ranging from 5-10o at
night to the low to mid-20's in the afternoon. Under such conditions, Trot regimes were comparable
to shade Tamb, except that the highest Trot were reached later in the day than the highest shade
Tamb.
By March (Figure 2), the January rot had aged considerably and was considerably drier that in
January. Correspondingly, the Tamb regime of this rot (Rot #1) had changed as well: the mean
Tamb in Rot #1 was ca. 8o C higher than in January and the diurnal range was greater as well. A new
rot had developed in another arm of the cactus (Rot #2 in Figure 2), and its thermal regime differed
significantly from that of Rot #1, exhibiting both a lower mean Tamb (ca. **) and a smaller diurnal
range.
One month later (Figure 3), shade Tamb had not changed dramatically (daily mean shade @
***o ). Rot #1 was now completely dry and devoid of larval activity. Rot #2 had progressed to the
advanced, drier stage exhibited by Rot #1 in March, and a new rot (Rot #3) had developed in another
arm of the cactus.
Finally, in mid-May thermal conditions were becoming extreme (Figure 4). Even on a cloudy
day, shade Tamb reached the mid-30's, and on a cloudless day, shade Tamb exceeded 40o for several
hours. Trot remained above 19o at all times, and Tamb in four new (since April) rots reached the
mid- to high 30's by early afternoon.
Our field data indicate that mean Trot from October through the following May range from 10o
15 in December/January to the low- to mid-30's in May and June. Diurnal fluctuations in Trot
probably don’t exceed 12-17o, at least in fresh rots (**put in a figureFigure??**). In dry rots, daily
fluctuations in T rot may exceed 25o on a regular basis (Figure **).
Surface Temperatures & behaviors on lekking sites & non-lekking sites
In January,
a.
d. Early in season, flies are active until about 1100-1115, then 'disperse' to refugia
(i) Didn't really show up in large numbers in late afternoon
e. In mid-season, flies consistently choose relatively cool spots for lek sites
(i) Bimodal activity pattern firmly established: morning activity *** to ***; afternoon
activity *** to ***.
(ii)
f. Late in the season (late April-June), flies remain on leks until Tsurf < ca. 25-27o C are
no longer available.
(i) Shift lek sites as Tsurf at one site exceeds 29o - 30o .
(ii). Disperse to refugia until late afternoon. (??or even the next day??)
Behavioral Responses to Temperature
Two lines of anecdotal evidence demonstrated that T surf exerted a profound influence on the
activity of flies on the leks. First, by observing individual flies we found that they would not enter
areas where T surf exceeded 29o. At T surf < 29o, flies moved about with no apparent constraints.
This suggests that 30o represents an upper thermal voluntary tolerance limit.
The second line of evidence developed from observations of flies’ behavior during their
morning activity on the leks in late April and early May. By the time flies arrived at the chosen lek
site, the combination of rapidly increasing T amb (Figures ** and **) and high solar radiative flux
could drive local T surf above 29o within only 10-15 minutes after the first arrival of flies. Even lek
sites shaded from direct insolation could become too hot as a result of solar heating of the soil surface
below the lek. Consequently, suitable lek thermal conditions had become quite ephemeral. Under
such conditions, we observed flies abandoning lek sites that no longer provided areas with T surf <
29o and immediately moving to another part of the host cactus where T surf < 29o were still
available. This often involved moving a distance of a meter or more to the new lek site. In the span
of 30-45 minutes, the flies might switch lek sites as many as three times. Frequently, the difference in
Tsurf between the abandoned lek site and the new one was only 2-3o C, indicating that D.
mojavensis is quite sensitive and responsive to relatively small differences in T surf . ?? D.
mojavensis is clearly affected by Tsurf and responds to changes thereof.
4.
a.
Laboratory Data
Effects of Rearing Tamb Regime on Mass
The effects of rearing Tamb regime on mass of adult flies are presented in Figures 5 & 6. With
constant Tamb, higher rearing Tamb produced smaller adults, and the adults emerging earliest (days 1
and 2) were significantly larger than those emerging on subsequent days. This difference is especially
evident at lower Tamb.
When rearing Tamb varied 10o on a diurnal basis, the picture was different. There was
essentially no dependence of mass on rearing Tamb. Only in the case of males developing at the
highest Tamb (24o night, 34o day) was there any definite decrease in mass. Also, the variable-Tamb
flies were larger than flies reared at 27o or 34o. Finally, variation in mass was generally less than
observed under constant Tamb conditions, and the difference in mass between early- and lateemerging flies was smaller in the variable Tamb regimes.
13. Look at difference in MASS of early & late emergers as a function of Tamb regime. It looks like
the difference increases with (constant) Tamb . Certainly the variance in MASS is greater under
constant Tamb conditions.
a.
Make a graph of the difference in MASS versus rearing Tamb.
Later emergers are smaller, but the difference is greater under constant Tamb
conditions_except at intermediate temperatures (22—27o). Are there two genotypes in the
population, one 'adapted' to high Tamb, one to low Tamb?
a.
Effects of Rearing Tamb Regime on Water Loss Rates
10. LOSS was about the same in females and males, with males therefore tending to have slightly
higher WLRs than females--especially at higher Tamb.
Effects of Rearing Tamb Regime on Developmental Rate and Progeny Number
Fluctuating Asymmetry of Wing Chord Length.
2. Fluctuating asymmetry in wings as a measure of developmental buffering against stress.
3. Epicuticular HC data (**optional**).
a. At least check the R-value.
b.
4.
was *** (or **%). The mean error in measurement (due primarily to the pixel size on the ***
screen) was *** (or 0.476%). The results of the FA measurement are presented in Figure ** (or
Table **).
DISCUSSION
Adult D. mojavensis can first be found in the field in the fall, but numbers are small following
the previous summer’s bottleneck. Although courtship, mating, and oviposition commence at that
time, population numbers remain low through January. With the warming of the habitat starting in
February, the population growth rate increases markedly. As inferred from numbers of adult flies
active on the lek or trapped at baits, population size peaks in April then declines rapidly through May
until in June, it becomes essentially impossible to find adult D. mojavensis. Our field data thus span
the time from when the population was just entering its rapid growth phase until it was in significant
decline.
At first light in the morning, adult flies are abundant on flowers of the host cactus and any
others within a few 10's of meters of the host cactus. From here, the adults proceed to the host cactus,
where males establish territories on the cactus’ surface, near openings that provide access to the
rotting cactus tissue in which females oviposit and larval development takes place. Courtship and
mating occur in these territories, and the site thus conforms to the definition of a lek.
Later in the morning, the flies abandon the lek and most apparently move to crevices in the host
cactus where they remain for much of the remainder of the day. As indicated in Figures 2, T amb in
these crevices can remain several degrees below shade Tamb , and the adult flies are thus buffered to
a certain degree against fluctuations in Tamb (Figs. 2 — 4). As we will discuss, we believe that these
thermal refugia are important in the biology of D. mojavensis.
Throughout much of their annual cycle (October through April) flies return to the lek site in the
afternoon for another round of courtship and mating. After this, the flies disperse, with many of
them moving to the same thermal refugium they occupied during the day. However, at least some of
the flies spend the night perched on the spines of the host cactus, which presumably reduces their
exposure to the predators (ants and spiders) that prowl the cactus’ surface at night.
The timing of activity on the leks is occupied each day appears to be determined by a
combination of factors, including photic and temperature effects. From January through April, adults
appeared on the lek when T surf reached 13-15o (Table 1), resulting in their becoming active
progressively earlier in the day. However, in May they did not occupy the lek until well after sunrise,
but which time T surf had reached 20-21o. Similarly, flies left the lek when T surf reached 23-24o,
except in April, when they remained until T surf reached 28o.
From March onward, T surf in some regions of the leks often became quite warm due to
sunlight directly striking the surface. The fact that adults actively avoided regions where Tsurf
exceeded 29-30o suggests that this represents some sort of upper 'tolerance limit'. Yet, the adults
often abandoned the lek in mid- to late morning even though regions with Tsurf < 29o still occur,
indicating that T surf is not the only determinant of adult activity. In the afternoon, adults do not
return to the lek until Tsurf has dropped below 30-31o. (Figs. 2-4; **Table 1?**).
Afternoon activity on the lek always commenced before sunset and usually ended shortly before
sunset. However, the data indicate that T surf must be < 30o before the flies will occupy the lek.
One consequence of this is that afternoon activity did not occur in May at all; T surf < 30o were not
available until after sunset.
Overall, adult D. mojavensis do not appear to be particularly tolerant of high temperatures, in
spite of the fact that they inhabit one of the hottest environments. This relative intolerance of even
moderately high temperatures clearly has significant impacts on the ability of adult D. mojavensis to
be active, on both a daily and seasonal basis. In general, arthropods inhabiting hot deserts exhibit
relative high thermal tolerances, but this does not seem to have been an evolutionary ‘option’ for
Drosophila species, which instead limit their adult activities to the relatively cool parts of the year.
**Phylogenetic constraint? Ask Huey?**
The thermal refugia occupied by adults when not active on the leks also became warmer as the
season progressed. By late April, T amb in the refugia was reaching 30 -31 o on a daily
basis (Fig. **), and in mid -May, adults were unable to avoid daily exposure to T amb
in the range of 32 - 38 o for several hours each day (Fi gure **). **==> a concluding
sentence
Thermal Regimes of the Larvae and Pupae
During the four-month period of our study, much of the host cactus was destroyed as the rot
spread from its initial location in part of one arm to sequentially encompass most of the cactus’ other
arms. As the season progressed, mean T rot of fresh rots increased by roughly 14o C, from ca. 15o in
late January to ca. 29o in mid-May.
Even though the arms of the host Organ Pipe cactus are large, the rots are ephemeral. A given
rot didn’t remain in the juiciest stage for much more than a month and they appear to dry out
completely within 2-3 months at most. As they age and dry, T rot fluctuations increase (Fig. **), and
even in March, T rot can reach nearly 40o on a daily basis. Our laboratory data suggest that
conditions in old rots may be interfering with development and reproductive success at a time (midMarch) when the population is growing the fastest.
Effect of Thermal Regime on Mass
One of the most pervasive observations in the literature is that arthropods developing at higher
Tamb are smaller than those developing at lower T amb (Yanofsky & Scheiner, 199*). The
phenomenon has been recorded in a number of Drosophila species. Interestingly, long -term
maintenance of laboratory stocks of D. pseudoobscura at different temperatures results in
evolutionary changes that mirror the results of the developmental studies: higher maintenance T amb
selects for larger genotypes (Anderson, 19**; did he propose an explanation?**).
Although the mechanism whereby higher T amb leads to smaller offspring unclear, Yampolsky
& Scheiner (1996) used a life-history approach to develop an hypothesis to account for this
phenomenon. In brief, they argued that under conditions of ***, rapid development to adulthood
would be favored over large body size because it minimized the time spent in a high predation-risk
life stage. **More??**
In the context of the life history of D. mojavensis, the ‘typical’ arthropod response to rearing
T amb presents some problems. Mating success of male D. mojavensis in nature correlates positively
with body size (Markow & Toolson, 1990; Markow & Ricker, 1992), and larger females are
presumably more fecund (Thornhill and Alcock, 1983). There is undoubtedly strong selection on
body size in D. mojavensis, with smaller genotypes suffering significant fitness reductions. If the
mass of D. mojavensis responded to Tamb in the manner suggested by the results of constant rearingTamb experiments, emerging adults would be progressively smaller as the growing season passed.
The premium placed on large body size by the mating system of D. mojavensis would presumably
impose strong selective pressures on its developmental system such that Tamb-invariant genotypes
were favored.
The results of our rearing experiments suggest that this is the case. Over the range of mean
T rot experienced by larvae and pupae during the growing season, the mass of flies developing under
variable T amb regimes shows essentially no dependence on T amb except, perhaps, in the case of
males at the very highest T amb (39o day/29o night). Also, with the exception of earliest flies to
emerge after developing at a constant 17o, flies developing under variable T amb are larger than those
developing at constant T amb .
Finally, for a given rearing T amb regime, later emerging flies are smaller, even when variable
T amb regimes were employed. This seems counter to the usual assumption that developmental time
correlates positively with body size. (**Wasn’t this the crux of the Yanofsky & Scheiner
argument?**). A couple explanations suggest themselves. First, perhaps the earliest emerging flies
developed from the first eggs to be produced. Hatching first, these individuals had a day or two head
start on the competition, allowing them to both develop rapidly and reach large body size.
However, an alternative hypothesis that merits further investigation is that the earliest flies to
emerge were genotypes that developed best under the particular rearing T amb regime they
encountered. This hypothesis is supported by data from D. buzzatii suggest that there is significant
genetic variation for mass and for plasticity in mass within a population (Toolson and Barker, ms. in
prep.). One correlate of the genetic variation in mass plasticity is that there are genotypes that
develop best at low T amb and others that develop best at higher T amb (Toolson and Barker, ms. in
prep.).
**In the context of the overlapping-generations, scramble competition developmental ** for
larval D. mojavensis, the ‘ability’ to develop rapidly while reaching a large body size would seem to
make sense.
b. Since, in overlapping generations, earlier emergers are favored as (presumably) are larger
flies, this pattern makes sense. At a given Tamb , genotypes that develop faster (presumably
because they handle the Tamb regime better) also produce larger flies. Some genotypes
muddle through & complete development (more slowly) and are smaller to boot.
14.. Lots of data suggest that Tamb can be a powerful selective force on body size, implying that it is
disrupting 'something'. But, Yanofsky & Scheiner suggest that there's an adaptive reason for the
effect of Tamb on body size. ??Do variable Tamb 'protect' files from this disruption??
Implications of Our Results for Models of Phenotypic Plasticity
Mass of flies showed the typical negative dependence on rearing T amb only when T amb was
a constant.
We are not aware of other studies that have investigated the consequences of variable T amb
regimes (**what about Feder?**).
Myriad studies have investigated the response of various phenotypic traits to different
intensities of relevant environmental parameters, and a number of models of the
ecological/evolutionary significance of phenotypic plasticity have been developed (***). In some
cases, this experimental approach may be appropriate. For example, the system employed by Via and
Lande (***), in which the **what trait?** of a phytophagous insect on two different host plants
was***,
More generally, our data raise the question of how generally applicable models of phenotypic
plasticity evolution may be, if the models are based on phenotypic responses to different constant
values of environmental parameters. Considerably more empirical research in this area is needed
before generally applicable models can be developed.
********************************************************************************
a. Where does this leave Scheiner & the other plasticity people? I guess they'd argue that shorter
generation time overrides the reduced fecundity associated with smaller size. Does it?
b. Except, smaller males might be at a disadvantage on leks.
c. The outcome would therefore depend on the genetic correlation
25. The lack of plasticity in body size in response to variable Tamb regimes is almost certainly
adaptive in the context of the flies' natural history.
It appears that D. mojavensis’ developmental systems function 'better' under variable Tamb
conditions. They’re also probably responding to a number of selective pressures: short
developmental time, especially at high T amb ; high larval survival, especially at low T amb ; T amb invariance of body size;
Effect of Rearing T amb Regime on Developmental Time and Reproductive Success
Developmental times were minimal when larvae/pupae developed at Tamb (either constant or
the mean of variable Tamb regimes) in the range of 26-31o, but the number of progeny obtained from
the bottles was maximal at somewhat lower rearing Tamb (Figure **).
**Don't know what drives the variation in progeny number: egg-production, larval survival,
pupal survival, or a combination.
Our field data indicate that mean Trot in this range that minimized developmental time occur in
late March through late April, when population growth is most rapid and population sizes are
approaching their peak. However, Reproductive Success (measured by the number of adult progeny
produced from the bottles) was maximal at temperatures corresponding roughly to those
characterizing the rots in late January, when population growth rates are quite low.
********************************************************************************
Developmental Time
16. Flies could possibly get to be larger at 7/17 or 12/22, but it would take a long time. In a world of
overlapping generations, the race is to the swift (shorter generation times), so perhaps they forgo
growth for immediate reproduction. Check Stearns & ?? (1996), also ** & Scheiner (1996).
WLRs
***Need some summary stats***
4. Mention that higher WLRs at highest rearing temperatures may be adaptive (Toolson, 1987).
C. Literature Data
1.
It doesn't appear that D. mojavensis' thermal biology acclimated during the spring; instead, activity
times shifted. **The April data are weird, however.**
Conclusions
A. Constant Tamb results don't do a very good job of predicting results of ecologically relevant
variable Tamb regimes.
1. Must be something very interesting about the developmental system of D. mojavensis.
2. Don't know about other species--has Bennett done anything with his E. coli?
What's Important to D. mojavensis populations?
40. Another thing--I believe the premium would be on flies to do well very early in the season,
produce lots of progeny during the first bout of reproduction===>a 'head-start' on the
competition? more progeny to disperse & look for new rots? **other explanations?
a. A better way of saying this is that progeny have higher reproductive values early in the season,
especially if developmental time is short. Late in the season, reproductive value of progeny
would probably be very low because the population is about to crash and survival probabilities
over the summer are very low.
From this study, the big factor seems to be temperature_.populations crash at about the time it
becomes impossible for adults to find refugia in which Tamb remains below about 35o . We focus on
this because Trot at the same time are only reaching the mid- to high 30's, which doesn't seem to
severely impact larval survival and development under lab conditions.
Do populations crash because conditions in rots become intolerable for immatures, or because
adults can't find refuges? I think it's both, actually: things may be getting tough for larvae/pupae in
April (at least in older rots), and thermal conditions even in April are forcing adults to spend several
hours each day at T amb that they actively avoid while on the leks. The situation is even worse in
May.
**Another problem is that the mean value of temperature experienced by larvae, pupae, and
adults are approaching values that result in changes in surface hydrocarbon composition that reduce
attractiveness of males to females. Markow and Toolson (199*) found that surface hydrocarbons of
males reared at, or acclimated to, 34o were significantly less apt to achieve mating following
courtship with females.
Given adults’ reluctance to expose themselves to T surf > 30 o on the lek and the
failure of laboratory stocks to develop at rearing T amb > 34 o ,
What about some Big Picture verbiage?
b. This is clearly a classic overlapping generations situation, with a crunch thrown in at the end.
I assume this would select for rapidly-developing genotypes that also get large under a given
set of conditions.
1. What did Gilchrist (1995) conclude (I think Lewontin said much the same thing in 1965)?
a. Specialists favored in environments with "significant" within-generation variation;
b. Generalists favored in environments with significant among-generation, but little withingeneration, variation.
c. Although our data
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I believe there are two times when it is especially important for r to be as large as possible:
early in the season when “population pressures” are low and the premium is on early reproduction and
rapid development, and late in the season when ‘population pressures’ are probably intense but the
population size is about to ‘crash’. I assume that once the flies (**whichever life stage is involved)
‘hunker down’ to try and survive the summer that a certain amount of density-independent mortality
is encountered. Under these conditions, production of as many progeny as possible is in a late-season
female’s best evolutionary interest (= will maximize her relative fitness). This corresponds to Case 1
of Kozlowski (1993) for use of r as a fitness measure. (exponential growth stops at the end of a
growing season and the probability of surviving to the next growth period is equal for all genotypes.
Don’t know about the latter, but is probably not a bad assumption in this case.).
Figure ** suggests that the disparity between the optima in the Minimum Developmental Time
and the Progeny Number curves may not be entirely **. For example, if
Species inhabiting hot deserts can realize some benefits (escape from predation, **) if they can
be active during the hot times of the year. However, only a few species have *** to do this. Cicadas,
antelope ground squirrels, Alan Marsh's ants, ***. In general, Drosophila species are adapted to
function at low- to moderate Tamb ranges. D. mojavensis , while able to survive and develop at
higher Tamb than some species, is not as remarkable at this as one might expect, given the nature of
its thermal habitat.
Data from cicadas indicates that thermobiology (Tpref of ovipositing females in the case of
cicadas) can be used to develop models whose output corresponds well to species distributional limits
and key components of a species' life history.
The present results ***.
Unlike a number of other insect taxa, the genus Drosophila does not seem to evolve the
ability to tolerate high T a , even though such an ability clearly can afford some real benefits in hot
deserts. Cicadas, for example, achieve a certain ‘protection’ from predators by being diurnally active
during the hottest months of the year (Toolson, 19**). They are able to do this in part because they
have evolved the ability to tolerate high body temperatures (**).
So where do we go from here? We believe that systems such as this, where it is possible to
develop a detailed picture of the thermal environment of all life stages and 'translate those data into
lab experiments' are a fertile ground for testing of existing hypotheses and development of new ones
relating to the evolution of physiological adaptations.
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**Need to bring in the 'stress & adaptation' theme.
Check Parsons, Hoffmann, etc. Do they make any predictions/statements that I can address
with the D. mojavensis data?
D. mojavensis doesn’t follow the typical arthropod patterns w.r.t. effects of rearing T amb on
body size, cuticle permeability, **.
ACKNOWLEDGEMENTS
Teri Markow introduced me to D. mojavensis and provided much ** during the initial phases of
this study. John Roach, Patty Ashby, Sandy Brantley, Yorgos Maranakos, *** provided assistance
during various phases of the field work. **Who helped with the lab work? Patty, Todd, John, **
performed much of the lab work. Bill Etges told me about D. mojavensis biology.
**Thank Teri & her students for assistance during initial phases of the field work.
LITERATURE CITED
Gibbs**, 1997
Kozlowski, J. 1993. Measuring Fitness in Life-history studies. Trends Ecol. Evol. **:84-85.
Lewontin, R. C. 1965. Selection for Colonizing Ability. pp. 77-91 in H. G. Baker and G. L.
Stebbins, The genetics of colonizing species, Academic Press, New York.
Markow & Ricker, 1992
Markow & Toolson, 1990
Thornhill, R. and J. Alcock. 1983. The Evolution of Insect Mating Systems. Harvard Press,
Cambridge, MA.
Stearns, S. C. 1992. The Evolution of Life Histories. Oxford University Press, Oxford.
Toolson, 1984, 1987
Table 1. Timing of lek activity in D. mojavensis. Start Times are expressed (to the nearest 0.25 h) relative to
local sunrise or sunset. Positive Start Times indicate that activity commenced after sunrise; negative start
times indicate that activity commenced before sunrise (or sunset). Values for T surf at the start and end of
each activity bout represent the mean value (rounded to the nearest degree) obtained by scanning the lek
surface with the infrared surface pyranometer set in the accumulate/averaging mode. n.d. = no data. n/a =
not applicable, no activity on lek.
═══════════════════════════════════════════════════════════════════════════
Morning Activity Period
Afternoon Activity Period
─────────────────────────────
──────────────────────────────
Start Time
T surf
Start Time
T surf
(Relative
────────
(Relative
────────
Date
to Sunrise)
Duration
Start End
to Sunset)
Duration
Start End
───────────────────────────────────────────────────────────────────────────
January 29
+1.5 h
2.25 h
13
23
-2.0 h
1.25 h
22
19
March 23
24
25
+0.5
- 0.5
0.0
2.25
2.50
1.75
13
13
14
24
23
23
-1.5
-2.0
n/d
1.75
2.25
n/d
28
28
n/d
24
24
n/d
April 28
29
- 0.75
- 1.0
3.75
4.5
14
16
28
28
-2.5
-2.5
2.25
2.25
30
30
28
27
May 16
17
+1.5
+1.25
1.0
1.25
21
20
24
25
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
═══════════════════════════════════════════════════════════════════════════
FIGURE LEGENDS
Figure 1. Thermal conditions experienced by different life stages of D. mojavensis during late
January, 1990. — : Shade Tamb at 1m; __ : Rot # 1Tamb; ° ° ° : Lek Tsurf. Times
that adult flies were active on the lek sites are indicated by the horizontal bars just above
the x-axis.
Figure 2. Thermal conditions experienced by different life stages of D. mojavensis during late
March, 1990. Legend as in Figure 1, except: - - - : Rot # 1 Tamb; __ : Rot # 2 Tamb;* :
Adult Refuge # 1 Tamb.
Figure 3. Thermal conditions experienced by different life stages of D. mojavensis during late April,
1990. Legend as in Figure 2, except: - - - : Rot # 2 Tamb; __ : Rot # 3 Tamb; — . . — :
Adult Refuge #1 Tamb; *: Adult Refuge #2 Tamb.
Figure 4. Thermal conditions experienced by different life stages of D. mojavensis during mid-May,
1990. Legend as in Figure 3, except: __ : Rot # 3-# 6 Tamb; — . — : Adult Refuge #1
Tamb; *: Adult Refuge Tamb. ***Check the legend for this figure***
Figure 5. Box plots of mass for female D. mojavensis for constant (upper plot) and variable (lower
plot) rearing-Tamb conditions. Within each Tamb category, the left-hand plot represents
data from the earliest flies to eclose (days 1 & 2), while the right-hand plot represents the
data for later emerging flies (days 3+). Boxes encompass the ** quartiles, the horizontal
line represents the median, and the brackets encompass the 95 %ile limits.
Figure 6. Box plots of mass for male D. mojavensis for constant (upper plot) and variable (lower plot)
rearing-Tamb conditions. Within each Tamb category, the left-hand plot represents data
from the earliest flies to eclose (days 1 & 2), while the right-hand plot represents the data
for later emerging flies (days 3+). Boxes encompass the ** quartiles, the horizontal line
represents the median, and the brackets encompass the 95 %ile limits.
Figure 7. Total number of adult D. mojavensis emerging from the ** bottles and the minimum
developmental time (= number of days between matings and the first emergence of adult
flies). The jagged appearance of the regression line for minimum developmental time
reflects the fact that developmental time can only change in increments of one day, rather
than continuously.
NOTES, QUESTIONS, THOUGHTS, etc.
1.
2. Check on everything Stearns has done. Also Andy Clark's stuff.
5. Ask Bill Etges:
a. if later emerging flies tend to be smaller;
b. if larger males dominate on leks (Markow says mating males are larger);
c. what's been done with this population
d.
a. There appears to be considerable genetic variation for size in wild Drosophila populations
(Toolson & Barker; **others?).
26. From the abstract: "Conditions were most favorable (i.e., flies were most active on the leks and
population sizes largest) when daily fluctuations in rot temperature did not go below 15 degrees
at night or above 35 degrees during the day. During this same time, adult flies were able to
locate refuges where maximum temperatures stayed below 31-32 degrees." Does this imply
coevolution between thermal 'tolerance' of larvae and adults?
a. Discuss in the context of lab results & vice versa.
b. Compare thermal conditions in the adults' refuges and in the rots. If they're different, it would
suggest coevolution, right?
c. Refuge Tamb fluctuated less than Trot (even new ones).....in particular, they stayed cooler
during the day.
29. Activity on the leks seemed to start when Tsurf reached ca. 13-15o in the morning & lasted until
Tsurf reached: 23-25o in January, 24-25o in March, 26-28o in April, and 24-25o in May. Flies
returned to lek when afternoon Tsurf dropped to: 22o in January, 28o in March, 29-30o in April,
and not at all in May.
a. Not much evidence for acclimatization there.
b. Even though lek Tsurf stayed below 25o in January, flies deserted it for the refuges by ***
am.
c. Flies on leks actively avoided spots with Tsurf > 29-30o .
34. What work has been done with this population (Toolson et al., 199*; Markow and Toolson,
199*; Gibbs, 1997?**others?)
36. What can be said about the FA results in the context of Tamb regimes of flies in nature?
42. Although we have only one season's worth of data—rather than a multi-year study (Toolson,
1998)—nevertheless, the data do provide some insights into the thermobiology of Dmoj .
43. Discuss the thermal regimes experienced by D. mojavensis in nature in the context of their effect
on R-value & possible effects on mating.
44. There appears to be both a temperature and a photic influence on adult activity.
45. Don't forget, thermal conditions in late April are equivalent to those that produce epicuticular HC
profiles that make males less attractive to females (Markow & Toolson, 1990; but see Gibbs,
1997**).
46. Clearly, something in the developmental system of D. mojavensis is 'plastic' in the sense that
something 'changes' to produce the temperature-independence of mass.
50. Presumably there was larval competition in our bottles; there's almost certainly larval
competition in the rots as well.
56. In choosing the variable rearing-T amb regimes, we endeavored to pick ranges whose mean
corresponded to those experienced at ‘crucial’ times by flies in nature -- when population size
was just starting to increase (12/22), increasing rapidly (17/27), peaking (22/32), and starting to
decline(24/34)-- or slightly exceeded conditions experienced in nature (7/12 & 29/39).
57.
59. What about a plot of mass vs. Season in nature?
a. This would have to be relative mass because of the effect of the cactus medium on fly mass
(**)
60. Developmental times weren’t exactly the same for constant and variable T amb regimes, but the
difference was slight. For this paper, we focus on the variable T amb results.
61. Our lowest (7/17) and highest (29/39) T amb regimes appear to be more extreme than the
larvae/pupae generally encounter in nature, but were chosen to represent what we think would
occasionally occur. Both clearly had significant negative impact on reproductive success.
62. Rot #3 (from April 28-May 1) was still juicy in mid-May, but even so T rot reached nearly 40o
on sunny days.
63. T b is determined almost entirely by T amb ( = T surf on the lek), although the higher WLRs at
higher developmental temperatures suggest that there may be some short-term evaporative
cooling going on (cf. Edney’s tsetse fly work; Toolson, 198*, 1987).
64. Note that we found no evidence females were selecting oviposition sites on the basis of T amb .
(in contrast the results for D. melanogaster reported by Jones et al., 19**)
65. If flies were active 2.5 hours before sunset in May, T surf would be 30-32o C, which seems to be
too high. If they were to wait for T surf to drop to 28.5o, they couldn’t become active until after
sunset.
Also, something strange is going on w.r.t. onset of activity in the morning: from January
through April, flies seem to become active when T surf reaches the mid-teens, which results in
their becoming active earlier as the season progresses. However, in mid-May, activity started
later than at any other time and T surf had reached the low 20s.
Also, morning activity seems to stop when T surf reaches 23-24o, except in April, when
flies stayed around until T surf reached 28o.
Evening activity seemed to start at (sunset - 2 h) ± 30 min, when T surf had dropped
below 30o. T surf this low doesn’t occur in May until after sunset ==> no afternoon/evening
activity.
Clearly, the timing of lek activity is not a simple function of time, light intensity, or T surf .
Also, we would have an incorrect picture if we’d only gathered data during one ‘visit’ to
the field site.
66. This paper shows that natural history studies can provide useful data for the design of
experiments and the interpretation of the resulting data. Others, notably Bartholomew, have said
this before.
67. Remember: field flies are smaller!
68.
General Outline for the Manuscript:
What should the manuscript do?
Present the data on T rot .
Present the behavioral data.
Present the life cycle summary.
Title: Thermal physiological ecology of D. mojavensis: effects of temperature variation on
evolution of life-history parameters.
Introduction
Materials and Methods
Results
Discussion
Tables
Figures