vol. 166, supplement
the american naturalist
october 2005

Genetic Correlations with Floral Display Lead to Sexual
Dimorphism in the Cost of Reproduction
Lynda F. Delph,* Janet L. Gehring,† A. Michele Arntz,‡ Maureen Levri,§ and Frank M. Freyk
Department of Biology, Indiana University, Bloomington, Indiana
47405
Online enhancement: table.
abstract: In dioecious plants, females typically invest more biomass in reproduction than males and consequently experience stronger life-history trade-offs. Sexual dimorphism in life history runs
counter to this pattern in Silene latifolia: females acquire less carbon
and invest more biomass in reproduction, but males pay a higher
cost of reproduction. The species is sexually dimorphic for many
traits, especially flower number, with males producing many, small
flowers compared to females. We tested whether the cost of reproduction is higher in males because flower number, which we presume
to be under sexual selection in males, is genetically correlated with
traits that would affect life-history trade-offs. We performed artificial
selection to reduce the sexual dimorphism in flower size and looked
at correlated responses in ecophysiological traits. We found significant correlated responses in total vegetative mass, leaf mass, leaf
thickness, and measures of CO2 exchange. Individuals in the manyand-small-flowered selection lines did not grow as large or invest as
much biomass in leaves, and their leaves exhibited an up-regulated
physiology that shortened leaf life span. Our results are consistent
with the hypothesis that genetic correlations between floral display
and ecophysiological traits lead to a higher cost of reproduction for
males.
Keywords: artificial selection, correlated responses, dioecy, genetic
correlations, sexual selection, Silene latifolia.
* Corresponding author; e-mail: ldelph@indiana.edu.
†
Present address: Department of Biology, Bradley University, Peoria, Illinois
61625; e-mail: jgehring@bumail.bradley.edu.
‡
E-mail: amknott1@hotmail.com.
§
Present address: Division of Math and Natural Sciences, Penn State–Altoona,
3000 Ivyside Park, Altoona, Pennsylvania 16601; e-mail: mal24@psu.edu.
k
Present address: Department of Biology, Colgate University, 13 Oak Drive,
Hamilton, New York 13346; e-mail: ffrey@mail.colgate.edu.
Am. Nat. 2005. Vol. 166, pp. S31–S41. 䉷 2005 by The University of Chicago.
0003-0147/2005/1660S4-40685$15.00. All rights reserved.
Life-history theory states that competition among various
activities within an individual can lead to life-history tradeoffs such as a cost of reproduction (Gadgil and Bossert
1970). Moreover, sex-specific differences in the cost of
reproduction can lead to sexual dimorphism in life history
(Trivers 1972; Bell 1980). Sexual dimorphism in life history
is a well-researched phenomenon in dioecious plants,
which have separate male and female individuals. Lloyd
and Webb (1977) first suggested that females should pay
a higher cost of reproduction than males because females
have a greater level of investment in reproduction. Indeed,
comparisons of multiple dioecious species show that
higher reproductive effort by females often is associated
with less growth, higher photosynthetic rates, delayed or
less frequent reproduction, earlier death, and spatial segregation, with males more frequent in low-resource sites
(Delph 1999; Obeso 2002). There are, however, exceptions
to this general pattern that are contrary to the idea that
investing more in reproduction will necessarily lead to a
higher cost of reproduction.
Silene latifolia is one such species whose pattern of sexual dimorphism in life history runs counter to the expected
pattern. In S. latifolia, males suffer greater life-history
trade-offs than females even though females acquire less
carbon and allocate more biomass to reproduction (Gross
and Soule 1981; Gehring and Linhart 1993; Gehring and
Monson 1994; Delph and Meagher 1995; Laporte and
Delph 1996). For example, males are smaller in both vegetative and total biomass and are less tolerant of high
density and limiting nutrients (Lawrence 1963; van Nigtevecht 1966; Lovett Doust et al. 1987; Gehring and Linhart
1993; Delph and Meagher 1995; Delph et al. 2002). Moreover, there is evidence that males die at a younger age than
females (Lovett Doust et al. 1987; Carroll and Mulcahy
1993; Gehring and Linhart 1993; Taylor 1994). Taken together, these results indicate that males exhibit a higher
cost of reproduction.
The hypothesis we set out to test is whether the cost of
reproduction is higher in males of S. latifolia because of
genetic correlations between a trait under sexual selection
(flower number) and traits that affect life-history trade-
S32 The American Naturalist
offs (allocation and physiology). Most differences between
the sexes are thought to arise at least initially from sexual
selection, which acts through differences in reproductive
success caused by competition for mates (Darwin 1871).
Sexual selection is usually considered distinct from viability selection because it operates specifically through variation in mating success and often opposes the action of
viability selection (Arnold 1985). An example of this tension between sexual and viability selection comes from
studies of marine iguanas, in which large individuals of
both sexes suffer from high mortality when food availability is low (Laurie and Brown 1990a, 1990b; Wikelski
and Trillmich 1997). Nevertheless, large males, but not
females, are strongly selected for via increased mating success and, as a consequence, there is sexual dimorphism in
body mass, with males being twice as heavy as females
(Wikelski et al. 1996; Wikelski and Trillmich 1997).
In S. latifolia, there is extreme sexual dimorphism in
flower production, with males producing as many as 16
times as many flowers as females (Meagher 1992; Laporte
and Delph 1996; Delph et al. 2002, 2004a). High flower
production is likely to have been sexually selected in males
but not in females (Delph et al. 2002, 2004a). Evidence
for this is fourfold. First, pollinators prefer plants with
large floral displays in this species (Shykoff and Bucheli
1995), and higher flower production leads to higher siring
success (F. M. Frey and L. F. Delph, unpublished data).
Second, flower number is the most sexually dimorphic
trait, and the extent of sexual dimorphism in other traits
depends on their correlation with flower number (Delph
et al. 2002). Third, flower size is highly negatively genetically correlated with flower number and is also highly
sexually dimorphic, with males making flowers that are
smaller in dimension and mass than those on females
(Carroll and Delph 1996; Meagher 1999; Delph et al. 2002,
2004a). Finally, although larger staminate flowers do not
produce more pollen per flower, there are more ovules in
larger pistillate flowers (Delph et al. 2004a). Hence, in
terms of size versus number, a male can make more pollen
by making more flowers but not by making larger flowers,
whereas a female can make more ovules by keeping flower
size large. Consequently, sexual selection in S. latifolia is
predicted to favor sexual dimorphism in flower display,
with males producing large numbers of small flowers and
females producing smaller numbers of relatively large
flowers. If flower size and number are genetically correlated with morphological and physiological traits that are
consequential for life-history trade-offs, then the production of elaborate floral displays by males could lead to
their higher cost of reproduction.
Artificial selection is a straightforward and powerful way
to test for genetic correlations among traits (Schluter 1988;
Conner 2003). Traits that are genetically correlated with
one under selection will show a correlated response in
proportion to the strength of selection applied and the
magnitude of the genetic correlation (Lande 1979; Arnold
1992, 1994; Schluter 1996). Therefore, to test the hypothesis that flower production is genetically correlated with
traits imposing life-history trade-offs, we performed
artificial-selection experiments to alter flower size. We
found a strong response to this selection and a strong
correlated response in flower number, indicating the presence of a flower size/number trade-off (Delph et al. 2004a).
Our selection took the form of reducing the sexual dimorphism in flower size (i.e., we selected for large flowers
in males and small flowers in females), but because of a
high between-sex correlation for flower size (Meagher
1999; Delph et al. 2004a), we were also able to expand the
sexual dimorphism in flower size and number. We capitalized on these experimentally altered patterns of dimorphism in flower size and number to test for correlated
responses in a host of morphological, allocation, and physiological traits.
Our intention was to look at correlated responses in
traits that could affect life-history trade-offs. Hence, we
measured allocation traits, with the prediction that individuals producing a relatively large number of flowers
would not be able to allocate as much biomass to vegetative
structures, such as leaves. In addition, we measured leaf
physiological traits, with the prediction that manyflowered individuals would have an up-regulated physiology that would cause them to have lower leaf life spans
and higher carbon costs. Specifically, we predicted that
individuals with many small flowers would grow less overall, invest less biomass in leaves, have thinner leaves
(higher specific leaf area) with lower longevity, have higher
stomatal conductance, and have higher photosynthetic,
transpiration, and dark respiration rates than individuals
with few large flowers. Overall, our results are consistent
with the hypothesis that sex-specific differences in the cost
of reproduction associated with flower production have
led to sexual dimorphism in life history.
Material and Methods
Study Species
Silene latifolia (Caryophyllaceae) is a dioecious, short-lived
perennial that typically flowers in its first year. The flowers
are white petaled and fragrant, open at night, and are
pollinated primarily by night-flying moths (Shykoff and
Bucheli 1995; Young 2002). These moths prefer plants with
large floral displays, that is, those with many open flowers
(Shykoff and Bucheli 1995). The populations used in this
study come from Giles County, Virginia. Although S. latifolia is not native to North America (Baker 1948), com-
Genetic Correlations and Sexual Dimorphism
parisons of multiple populations from Virginia and Europe
show that Virginian populations are intermediate for a
variety of floral and allocation traits (Delph et al. 2002).
S33
tent selection on the timing of reproduction or related lifehistory traits.
Correlated Responses
Artificial Selection
The selection lines used in the experiments described here
come from artificial-selection experiments to alter calyx
width. By performing artificial selection, we hoped to create lines whose means differed in flower size and also
flower number. We found that calyx width responded to
selection and that flower number showed a strong correlated response to this selection, revealing that it was
heritable and negatively genetically correlated with calyx
width (Delph et al. 2004a). Given the heritability of the
two traits and the flower size and number trade-off, we
were successful in creating lines whose means differed for
both flower size and number. The response to selection
on calyx width and correlated response in flower number,
together with a detailed description of the selection procedure, can be found in Delph et al. (2004a). Here, we
provide a brief description of the selection procedure.
Two completely separate sets of artificial-selection lines
were generated, which we will refer to as selection programs 1 and 2. Seeds for the experiments came from two
separate field populations, taken from 23 and 43 maternal
parents, respectively, for the two selection programs. Seeds
were grown in a greenhouse at Indiana University. In selection program 1, we created one line that favored females
with flowers whose calyx width was small, one control
line, and one line that favored males with flowers whose
calyx width was large. In the second selection program,
we created two replicates each of the small and large lines
and one replicate of the control line. Five generations were
grown for each selection program, which gave four bouts
of selection in each case. Selection program 1 ran from
1993 to 1998 and selection program 2 from 2000 to 2003.
We measured the calyx width of specific flowers each
generation (selection program 1: second through seventh
flowers to open; selection program 2: third through fifth
flowers to open). To create the line with flowers with small
calyces, we identified the six families with females that
produced flowers with the smallest calyx width in each
generation and reciprocally crossed a subset of their members to produce 30 new families for the next generation.
Likewise, to create the line with flowers with large calyces,
we identified the six families with males that produced
flowers with the largest calyx width and reciprocally
crossed a subset of their members. All plants were allowed
to flower before the selection of families was made. This
increased the amount of time it took to complete each
generation, but it was important to prevent any inadver-
In the last generation of both selection programs, we measured several traits to determine whether they showed a
correlated response to selection on calyx width. All traits
were measured in both selection programs, except where
noted (see table 1). Categories of traits included morphology, allocation, and physiology. Results on correlated
responses in flower number, petal limb length, flower
mass, and ovule or pollen number are reported elsewhere
(Delph et al. 2004a, 2004b).
In selection program 1, we did not have enough greenhouse space to grow all plants for 1 month after they began
flowering, so we randomly selected three plants per family
(two females and one male) to grow and measure for the
1-month-after-flowering data. A small number of additional plants were saved from each family to use as pollen
donors or for measuring stem respiration (see below), but
the rest were discarded. One of the two females per family
had all of it flowers pollinated (randomly chosen), and the
other female from each family was left unpollinated. Flowers open in the evening, and pollinations were made the
following morning by rubbing dehisced anthers from two
or three flowers of an unrelated male across the stigmatic
papillae. This allowed us to compare pollinated and unpollinated females. Data from the two groups were combined for analysis of specific traits only when there were
no significant differences between them.
Measurements of leaf physiology were always made on
the leaf one node down from the node containing the first
flower to open and used an open gas exchange system,
the LI-6400 (Li-Cor, Lincoln, NE). Only undamaged, nonsenescent leaves were measured. In selection program 1,
a tube was run outside one of the greenhouse windows
for the incoming CO2 (hence, the concentration varied
slightly), and in selection program 2, CO2 concentration
was maintained at 400 mmol mol⫺1. Measurements were
taken at flowering or 1 month after flowering began for
each plant, as noted. After the 1-month-after-flowering
measurements, the leaf was removed and its area was determined using a leaf area meter (LI-3000, Li-Cor). It was
then dried to a constant mass at 60⬚C and weighed. Specific
leaf area was calculated by dividing the area of the leaf by
its mass.
In selection program 1, photosynthetic rate, stomatal
conductance, and transpiration rate were measured between the hours of 10 a.m. and 2 p.m. on the day each
plant opened its first flower with the LI-6400, with irradiance set at 1500 mmol m⫺2 s⫺1 (saturating) and a leaf
temperature of 25⬚C. Flow rate was 500 mmol s⫺1, and the
S34 The American Naturalist
stomatal ratio was set at 0.4 (the ratio of stomata on the
two sides of the leaves in this species). All subsequent
measurements of physiology were taken 1 month after
flowering began for each plant for both selection programs.
To measure dark respiration of leaves in selection program
1, plants were watered to capacity and placed after dark
in a room without windows 2 nights before their 1-month
anniversary of flowering. Dark respiration was measured
with the LI-6400 with irradiance set to 0. A low-intensity
orange incandescent bulb, directed away from the plants,
was the only illumination used. Measurements were made
from approximately 6 to 7 a.m. The plants were then
moved into a greenhouse with overhead lighting and watered as needed in the morning and afternoon. The next
morning, the plants were again watered to capacity around
8 a.m. Between the hours of 10 a.m. and 2 p.m., photosynthetic rate, stomatal conductance, and transpiration
rate were measured as listed above for plants from both
selection programs. Dark respiration of the leaves was
measured in selection program 2 directly after the above
measures, by lowering the irradiance to 0 and waiting for
the values to equilibrate. This method of measuring dark
respiration (whereby the whole plant was not in the dark)
is likely to lead to higher values in comparison to the
method used for selection program 1, and consequently
the absolute values cannot be compared across the selection programs. The latter method would be influenced by
recently accumulated substrate for respiration in the
leaves; however, unless there was an interaction between
this substrate and the selection lines, this would only affect
the absolute values and not the relative values across the
selection lines.
Stem respiration was also measured using the LI-6400
on a subset of plants not used for leaf measurements in
selection program 1. Stems from 10 males and nine females
were measured (79 stems from males, 65 stems from females; sex did not have a significant effect on stem respiration, P p .62). We chose stems in order to get the full
range of stem diameters and used multiple stems per plant.
Large stems are necessarily older than small stems because
stem diameter decreases incrementally with each branching event. Plants were treated as above for measuring dark
respiration of the leaves in selection program 1. Stem respiration was measured by cutting stems from the plants
and immediately placing the stem within the cuvette, parallel to the cuvette’s long side (such that the cut ends were
outside the cuvette). This excluded respiration from
wounding. After this measurement, the diameter of each
stem was measured with digital calipers. The stems are
round, so stem area (which must be entered into the LI6400 for calculations of respiration based on area) was
calculated as 2prh (the surface area of the side of a cyl-
inder), where r p diameter/2 and h equaled the length of
the cuvette.
Biomass allocation was determined for each plant 1
month after flowering. The plants were pulled apart into
the following components: reproductive parts, leaves, and
stems. The stems were subsequently divided into four categories based on diameter. Here we report on the biomass
invested in leaves and the smallest stem diameter category,
!0.8 mm. All leaves and stems were dried to a constant
dry mass at 60⬚C and weighed to obtain vegetative dry
mass for each plant.
Statistical Analyses
To determine whether each of the traits listed in table 1
showed a correlated response to selection on calyx width,
we performed one-way ANOVAs with sex and selection
line combined into one categorical variable (1 p smallline males, 2 p control males, 3 p large-line males, 4 p
small-line females, 5 p control females, 6 p large-line
females). Separate analyses were run for the two selection
programs. If the ANOVA for a particular trait was significant, a series of planned nonorthogonal pairwise comparisons was used to determine which sex/line combinations differed significantly from each other. A significant
difference among selection lines within a sex was taken as
evidence for a significant correlated response. In addition,
the elimination of a significant difference between the sexes
(by comparing large-line males with small-line females)
or the creation of a significant difference between the sexes
(by comparing small-line males with large-line females)
was also tested. The replicate lines in selection program 2
were combined for analysis. Levene’s test for homogeneity
of variances was run, and t-tests assuming either equal or
unequal variances were employed, depending on the outcome of the test. We employed the sequential Dunn-Sidák
method to adjust our experimentwise error rate (Sokal
and Rohlf 1995, p. 241).
Results
Means of traits by sex and selection line, along with the
one-way ANOVA and significant pairwise comparison results, are all presented in table 1, unless otherwise indicated. Table A1 in the online edition of the American Naturalist contains the P values and degrees of freedom for
all nine pairwise planned nonorthogonal comparisons for
each trait.
Biomass Allocation
Total leaf biomass showed a significant correlated response
to selection in both selection programs. In selection pro-
Table 1: Means (SE) of traits for the different selection lines in the last generation of both selection programs for males and females and results of the one-way ANOVAs
Males
Trait
Selection program 1:
Total leaf biomass (g)
Small stem biomassb (g)
Vegetative biomassb (g)
Specific leaf area (cm2 g⫺1)
Photosynthesis (mmol m⫺2 s⫺1)
Photosynthesisb (mmol m⫺2 s⫺1)
Stomatal conductance (mol m⫺2 s⫺1)
Stomatal conductance (mol m⫺2 s⫺1)
Transpiration (mmol m⫺2 s⫺1)
Transpiration (mmol m⫺2 s⫺1)
Dark respiration (mmol m⫺2 s⫺1)
Selection program 2:
Total leaf biomass (g)
Small stem biomass (g)
Vegetative biomass (g)
Specific leaf area (cm2 g⫺1)
Photosynthesis (mmol m⫺2 s⫺1)
Stomatal conductance (mol m⫺2 s⫺1)
Transpiration (mmol m⫺2 s⫺1)
Dark respiration (mmol m⫺2 s⫺1)
a
Time
F (df)
(5, 199)
(5, 135)
(5, 136)
(5, 174)
(5, 308)
(5, 168)
(5, 308)
(5, 222)
(5, 308)
(5, 222)
(5, 154)
P
MF
MF
MF
MF
F
MF
F
MF
F
MF
MF
5.81
23.29
4.92
8.10
.33
8.49
1.33
5.21
.55
5.70
5.30
MF
MF
7.02 (5, 270) !.001
41.03 (5, 272) !.001
MF
MF
MF
MF
MF
MF
9.82
8.98
3.73
2.67
4.63
11.34
(5, 268)
(5, 273)
(5, 249)
(5, 249)
(5, 249)
(5, 248)
!.001
!.001
!.001
!.001
.894
!.001
.252
!.001
.742
!.001
!.001
!.001
!.001
.003
.023
!.001
!.001
1. Small
2. Control
Females
3. Large
4. Small
5. Control
6. Large
2.3 (.11)
2.1 (.10)
2.9 (.19)
2.4 (.10)
2.6 (.11)
2.8 (.13)
.65 (.054)
.66 (.058)
.46 (.047)
.25 (.55)
.16 (.052)
.11 (.042)
6.4 (.33)
6.3 (.28)
7.5 (.42)
7.7 (.31)
8.0 (.39)
8.1 (.45)
308 (15.1)
314 (9.8)
254 (12.2)
328 (9.9)
295 (6.4)
272 (6.6)
19.2 (.47)
18.6 (.49)
19.3 (.50)
18.7 (.40)
18.7 (.39)
18.9 (.40)
8.5 (.40)
8.1 (.41)
7.8 (.55)
6.3 (.45)
6.2 (.46)
5.5 (.33)
.52 (.020)
.51 (.025)
.48 (.023)
.47 (.018)
.50 (.021)
.46 (.018)
.19 (.016)
.19 (.014)
.17 (.015)
.15 (.010)
.14 (.009)
.13 (.007)
5.1 (.13)
4.8 (.17)
4.9 (.19)
4.8 (.16)
4.9 (.14)
4.9 (.14)
2.8 (.17)
2.7 (.14)
2.5 (.18)
2.2 (.12)
2.1 (.10)
2.1 (.10)
⫺.81 (.086) ⫺.72 (.065) ⫺.52 (.085) ⫺.47 (.058) ⫺.50 (.055) ⫺.40 (.064)
1.9 (.07)
1.3 (.05)
2.4 (.09)
1.0 (.06)
2.3 (.08)
.8 (.04)
2.3 (.07)
1.2 (.04)
2.6 (.10)
.7 (.05)
Significant comparisonsb
1-3,
1-3,
1-6,
1-3,
1-6, 2-3, 2-5, 4-6
1-6, 2-3, 2-5, 3-4
2-5
2-3, 3-4, 4-5, 4-6
1-6, 2-5
1-6, 2-5
1-6, 2-5
1-3, 1-6
2.5 (.10)
.6 (.03)
1-2, 1-3, 1-6, 4-5
1-2, 1-3, 1-6, 2-3, 2-5, 3-4,
4-5, 4-6, 5-6
5.1 (.16)
5.6 (.25)
5.4 (.18)
6.5 (.17)
6.6 (.23)
6.2 (.23)
1-6, 2-5, 3-4
249 (5.9)
220 (6.9)
220 (4.2)
228 (5.2)
196 (6.4)
216 (4.7)
1-2, 1–3, 1-6, 4-5
10.2 (.33)
9.6 (.36)
9.8 (.36)
9.8 (.39)
9.3 (.50)
8.3 (.34)
1-6, 4-6
.16 (.007)
.15 (.010)
.16 (.010)
.14 (.007)
.13 (.008)
.13 (.008) 1-6
2.8 (.13)
2.6 (.21)
2.7 (.13)
2.3 (.09)
2.1 (.11)
2.2 (.10)
1-6
⫺1.26 (.042) ⫺1.03 (.041) ⫺1.00 (.046) ⫺1.07 (.048) ⫺.90 (.040) ⫺.86 (.040) 1-2, 1-3, 1-6, 4-6
Note: For selection program 2, the means for the two replicate small and two replicate large lines are shown combined. Nine planned nonorthogonal pairwise comparisons were made for each trait (see
“Statistical Analyses”). Only significant pairwise comparisons are listed here (designated by the numbers in the column headings); see table A1 for all comparisons. Control values in boldface indicate
significant sexual dimorphism for the trait (comparison 2 vs. 5). Small stems are those with a diameter of !0.8 mm.
a
F p measured on the day the first flower opened on a plant; MF p measured 1 month later.
b
Traits in selection program 1 for which pollinated and unpollinated females differed from one another. The means include only unpollinated females in these cases.
S36 The American Naturalist
Figure 1: Mean (⫹SE) number of flowers produced by pollinated (gray
bars) and unpollinated (black bars) females in the three selection lines
(Small p small-flower line, Control p control line, Large p largeflower line). Different superscripts indicate significant differences among
treatment groups (Tukey’s). See Delph et al. (2004a) for flower number
data for males.
gram 1, males and females in the large line invested 32%
and 17% more in leaf biomass, respectively, than those in
the small line, and these differences were statistically significant. In addition, the large line differed significantly
from the control line for males. Moreover, the significant
sexual dimorphism that existed between the control-line
males and females (control females invested 24% more in
leaves than control males) was reversed when comparing
males in the large line with females in the small line: largeline males invested 21% more than small-line females.
Finally, the sexual dimorphism was expanded when comparing individuals with the most extreme differences in
flower size and number, with females in the large line
investing 27% more in leaves than males in the small line.
Changes in total leaf biomass were similar in selection
program 2. Both males and females showed a significant
correlated response. Males in the small line invested significantly less than those in the control and large lines
(26% and 21%, respectively), and females in the small line
invested significantly less than those in the control line
(13%). Control-line males and females did not differ significantly, but a significant difference was seen when comparing the two extremes: females in the large lines invested
32% more than males in the small lines.
The amount of biomass invested in small stems (!0.8
mm) was affected by pollination, with pollinated females
making significantly fewer small stems than unpollinated
females (mean Ⳳ SE p 0.04 Ⳳ 0.019 g vs. 0.17 Ⳳ 0.029 g,
respectively; one-way ANOVA on log-transformed means
[⫺1.24 vs. ⫺0.73]: F p 9.77, df p 1, 53, P p .003). Because plants produce a new axillary branch, each of which
is smaller than the preceding branch, whenever they make
a new flower, this effect is likely a consequence of pollinated females producing significantly fewer flowers than
unpollinated females (one-way ANOVA on log f lower
number, with pollination treatment and selection line
combined into one categorical variable: F p 176.20,
df p 5, 116, P ! .001; Tukey’s a posteriori contrasts shown
in fig. 1). Note that in spite of the significantly lower overall
production of flowers in pollinated females, pollinated females in the small line produced more flowers than pollinated females in the large line (P ! .001). Hence, fruit
production did not eliminate flower number differences
between the selection lines.
The amount of biomass invested in small stems showed
a correlated response to selection in both selection programs. In selection program 1, males in the small and
control lines invested significantly more in small stems
than males in the large line (141%), and males from all
three lines invested significantly more in small stems than
females. Males in the control line invested 76% more than
did control-line females, and those that differed the most
in their flower production (males in the small line vs.
females in the large line) differed the most dramatically.
In selection program 2, all nine pairwise contrasts were
significant, indicating a correlated response in both males
and females, sexual dimorphism between the control-line
males and females, and both a reversal and an expansion
of sexual dimorphism when comparing large-line males
Figure 2: Relationship between carbon loss via respiration (mmol m⫺2
s⫺1) and stem diameter (mm).
Genetic Correlations and Sexual Dimorphism
with small-line females and small-line males with largeline females, respectively.
We report on the amount invested in small stems because carbon is lost through stem respiration, and the
amount of carbon lost depends on the diameter of the
stem (fig. 2). Stems with the smallest diameter (!0.8 mm)
have the highest surface-to-volume ratio and lose carbon
at a faster rate than older, thicker stems. Hence, plants
that make a relatively high number of flowers and a relatively high number of small stems will be losing more
carbon via stem respiration than plants with fewer small
stems.
The amount of biomass invested in vegetative structures
was affected by pollination. Unpollinated females invested
25% more in leaves and stems than pollinated females
(mean Ⳳ SE p 7.9 Ⳳ 0.23 vs. 6.3 Ⳳ 0.20; one-way
ANOVA: F p 24.98, df p 1, 120, P ! .001). Control-line
males and females exhibited significant sexual dimorphism
in vegetative biomass, with females investing more than
males. This dimorphism was eliminated by selection to
increase flower size in males and reduce flower size in
females in selection program 1 but not in selection program 2. However, the dimorphism was expanded in both
selection programs, with females in the large lines investing
21% and 18% more than males in the small lines in selection programs 1 and 2, respectively.
Leaf Physiology
Specific leaf area showed a correlated response to selection
in both selection programs: selecting for large flowers in
males led to a decrease in specific leaf area (thicker leaves),
and selecting for small flowers in females led to an increase
in specific leaf area (thinner leaves). Plants in the large
lines generally had thicker leaves than those in the small
lines. Lastly, although the sexes in the control lines were
not significantly dimorphic in either program, males in
the small lines had significantly thinner leaves than females
in the large lines.
Photosynthetic rate was measured on the day the first
flower opened in selection program 1. The one-way ANOVA
was not significant (F p 0.33, df p 5, 308, P p .894), indicating that photosynthesis at the time of first flowering
did not differ significantly by sex or selection line.
Photosynthetic rate 1 month after the onset of flowering
differed significantly between pollinated and unpollinated
females in the large line (one-way ANOVA: F p 5.79,
df p 1, 45, P p .020). The pollinated females in this line
photosynthesized at a higher rate than those that were not
pollinated (mean Ⳳ SE p 7.0 Ⳳ 0.54 vs. 5.5 Ⳳ 0.33 mmol
m⫺2 s⫺1).
The selection programs differed somewhat in terms of
how sex and selection line affected photosynthetic rate 1
S37
month after the onset of flowering, but overall, plants that
made more flowers had higher rates of photosynthesis than
those with relatively fewer flowers. In selection program
1, males and females in the control line differed significantly, with males photosynthesizing at a higher rate (23%
higher), but this sexual dimorphism was reduced to nonsignificance by selection to reduce the sexual dimorphism
in flower size. The dimorphism was also expanded, with
males in the small line photosynthesizing at a 35% higher
rate than females in the large line. In selection program
2, the mean photosynthetic rates of small-line males and
females are likely to be underestimates, because the leaves
of 18 plants in the small lines had already browned and
were no longer capable of photosynthesis 1 month after
flowering (this would also be true for stomatal conductance and transpiration rate; see below). This also occurred
in two control-line plants, but it did not occur in any of
the large-line plants. Nevertheless, in selection program 2,
females showed a correlated response to selection, with
females in the large lines photosynthesizing at a 15%
higher rate than females in the small lines. Finally, significant differences were seen between males in the small
lines and females in the large lines.
Just as with photosynthesis at the time of first flowering,
there were no significant differences between the sexes or
selection lines in stomatal conductance when flowering
first began (one-way ANOVA: F p 1.33, df p 5, 308,
P p .252). However, differences had developed 1 month
after the onset of flowering. In selection program 1, males
in the control line had a 26% higher conductance than
females in the control line, and this difference was significant. This dimorphism both became nonsignificant
(12% difference between males in the large line and females in the small line) and expanded (32% difference
between males in the small line and females in the large
line) by selection. Males and females in the control line
did not differ significantly in selection program 2, but the
difference between the sexes became significant after selection, with males in the small lines exhibiting a 19%
higher conductance than females in the large lines.
Similar to photosynthetic rate and conductance, transpiration rate did not differ significantly among the sexes
or selection lines at the time of first flowering (one-way
ANOVA: F p 0.55, df p 5, 308, P p .742). After 1 month
of flowering, males in the control line in selection program
1 were transpiring at a significantly higher rate than females in the control line (22% higher). Just as with conductance, this dimorphism both became nonsignificant
(12% difference) and expanded (25% difference). In selection program 2 the difference between males in the
small lines and females in the large lines was the only
significant difference.
Dark respiration of leaves showed a correlated response
S38 The American Naturalist
to selection in both selection programs. In selection program 1, males in the small line lost 36% more CO2 than
males in the large line, and this difference was significant.
Furthermore, a significant difference existed between
males in the small lines and females in the large lines, with
males respiring at a rate more than twice that of females.
In selection program 2, dark respiration was measured at
a different time of day than in selection program 1 (see
“Correlated Responses”), so the absolute values may not
be comparable between selection programs. Both males
and females showed a correlated response to selection in
selection program 2. Individuals in the small lines respired
more than 20% more than those in the large lines in both
sexes. In addition, small-line males respired significantly
more than control-line males. Finally, the sexes differed
significantly when comparing small-line males to largeline females.
Discussion
We created artificial-selection lines to determine whether
allocation and physiological traits were genetically correlated with flower size, a highly sexually dimorphic trait in
Silene latifolia. Specifically, we were interested in correlated
responses for traits that could account for males having
greater life-history trade-offs than females even though
they acquire more carbon and allocate less biomass to
reproduction. We hypothesized that sexual selection for
high flower production in males would also alter genetically correlated traits, with negative effects on their growth
and maintenance.
The correlated responses to selection we observed are
concordant with this hypothesis. Plants in the small-flower
lines produced more flowers than those in the large-flower
lines, indicating a flower size and number trade-off (data
reported in Delph et al. 2004a). Moreover, although pollination reduced flower production in females, it did not
eliminate the differences in flower production among the
selection lines. Because plants must produce a new branch
every time they make a new flower, higher flower production would carry any metabolic costs associated with
the differentiation of flowers, as well as maintenance and
nectar costs. A recent study has shown that whole-plant
nectar costs are greater for males than for females under
low-to-moderate levels of pollination as a result of their
greater flower production (Gehring et al. 2004).
In addition to a correlated response in flower number,
we found correlated responses in allocation and physiological traits. Individuals of both sexes with larger floral
displays produced less total vegetative biomass and less
leaf biomass, had thinner leaves, lost more carbon via stem
and leaf respiration, and had lower leaf life span than
individuals that produced relatively few flowers. The suite
of traits seen in the lines with the larger floral displays, as
compared to the suite seen in those with more limited
displays, indicates how males would be experiencing
greater losses of carbon as a result of continuous, high
flower production and therefore pay a higher cost of
reproduction.
Biomass Allocation
In terms of allocation, we found correlated responses in
total leaf biomass and small stem biomass in both selection
programs and for vegetative biomass in selection program
2. Selection to reduce flower size, which thereby increased
flower number, resulted in a decrease in the amount of
biomass a plant invested in leaves for both sexes. It also
resulted in an increase in the amount of biomass invested
in small stems, which were shown to experience relatively
high carbon loss through respiration. Similarly, significant
correlations between the percentage of total mass allocated
to leaves and stems with flower number were found in an
interpopulation study (Delph et al. 2002). These limits on
carbon gain and enhancement of carbon loss led to a
reduction in overall vegetative growth for individuals with
many, small flowers relative to individuals with few, large
flowers. Moreover, comparisons between males selected to
be more similar to females (large flowers) and females
selected to be more similar to males (small flowers) revealed that the sexual dimorphism that existed between
the sexes for two of these traits was eliminated by selection
in both selection programs: males no longer had less leaf
or vegetative biomass relative to females.
Leaf Physiology
Leaf physiological traits also responded to selection on
flower size, with the pattern being that plants with many,
small flowers showed an up-regulated physiology. For example, specific leaf area showed a correlated response to
selection in both selection programs. This trait is phenotypically integrated with other leaf functional traits in
the comparative physiology literature (Reich et al. 1992,
1998, 1999), but its heritability has not been widely studied
(Rebetzke et al. 2004). Specific leaf area appears to be
heritable and negatively genetically correlated with flower
size and hence positively correlated with flower number
in S. latifolia. In other words, plants of either sex that
make relatively many, small flowers have thinner leaves
than plants that make few, large flowers.
Thin leaves are typically associated with increased CO2
exchange and limited leaf life span in interspecific comparisons (Reich et al. 1999), and our results are consistent
with this pattern. Before flowering, neither the sexes nor
the selection lines differed in their photosynthetic rate,
Genetic Correlations and Sexual Dimorphism
stomatal conductance, or transpiration rate. However, 1
month after flowering began, differences emerged for these
traits, with the general pattern being that the values were
higher in the many-and-small-flowered line.
In selection program 1, the control line was sexually
dimorphic for all three of these traits, with males having
higher values than females. Selection to reduce the sexual
dimorphism in flower size resulted in elimination of the
sexual dimorphism in photosynthetic rate, stomatal conductance, and transpiration rate. Furthermore, expansion
of the sexual dimorphism in flower size caused by a strong
between-sex genetic correlation for flower size (see Delph
et al. 2004a) expanded the differences between the sexes,
especially for photosynthetic rate.
In selection program 2, measures of gas exchange were
made difficult by an effect of selection line on leaf life
span, which is an interesting result in and of itself. Nearly
20% of the individuals in the small-flowered line had leaves
that had senesced before the 1-month timetable for
measuring their gas exchange, whereas all of the largeflowered-line plants still had functional, green leaves at this
time. This reduced leaf life span would cost the plant carbon;
most of the carbon and half the nutrients in senesced leaves
are lost to the plant (Chapin 1991), and these leaves would
no longer be fixing carbon. It is likely that the senesced
leaves had the highest CO2 exchange before their senescence,
given that leaf life span is highly negatively correlated with
photosynthetic rate and stomatal conductance in a variety
of species (Ackerly and Reich 1999). Consequently, our
measures of gas exchange are likely underestimates for the
many-and-small-flowered plants. In spite of this, we observed correlated responses in photosynthetic rate in selection program 2, with photosynthetic rate being higher in
the small-and-many-flowered line, indicating that this trait
is heritable and genetically correlated with flower size in
that population. In addition, stomatal conductance and
transpiration were significantly different when comparing
individuals that differed the most in their flower size and
number, that is, males who made the most flowers and
females who made the fewest flowers.
Finally, we observed a correlated response in dark respiration of leaves in both selection programs, with individuals in the many-and-small-flowered lines exhibiting
relatively high rates of carbon loss compared to those in
the few-and-large-flowered lines. These results indicate
that dark respiration of leaves is heritable and negatively
genetically correlated with flower size. Although previous
research indicated sexual dimorphism in dark respiration
rates, with males having higher respiration (Laporte and
Delph 1996), the differences between the sexes in the control lines were not significant in either of our selection
programs, although the trend was for males to have higher
rates. However, this loss of carbon through respiration was
S39
significant when comparing the two most extreme groups,
with males in the small line losing more than twice as
much and 1.5 times as much carbon as females in the
large line in selection programs 1 and 2, respectively.
Trait Integration and Opposing Selection
The suite of leaf functional traits seen across the selection
lines mirrors patterns seen at the macroevolutionary scale,
in which species with thin leaves exhibit relatively fast
growth rate, higher carbon assimilation rates, higher respiration rates, and shortened leaf life span (Reich et al.
1998; Ackerly and Reich 1999; Ackerly et al. 2000; Arntz
and Delph 2001). Coordinated changes in physiology have
also been shown in response to environmental variation,
including environmental stress (Chapin et al. 1993; Ackerly
et al. 2000). Our results indicate that trait integration extends beyond ecophysiological traits.
Importantly, the correlated responses we saw to selection on flower size in S. latifolia indicate that neither traits
related to floral display (flower size and number) nor ecophysiological traits may respond independently to selection. This may be, in part, because physiology is upregulated in this species when sink strength (e.g., flower
number) is increased (Laporte and Delph 1996), and
flower number and physiological features co-varied in our
study. However, quantitative trait loci analysis of a cross
between the selection lines indicates that the correlated
responses of physiological traits was, at least in part, a
consequence of genetic polymorphism at loci controlling
these traits varying among the selection lines (L. F. Delph,
I. Scotti, and A. M. Arntz, unpublished data).
If there is strong sexual selection for elaborate floral
displays in males that is not countered by selection on
ecophysiology, then the genetic correlations between
flower size and ecophysiology seen here will result in selection for an up-regulated physiology. Conversely, there
could be strong selection to down-regulate physiology in
certain environments. For example, selection to limit water
loss in dry habitats might select for thicker leaves (Reich
et al. 1999) and lower rates of CO2 exchange (Farquhar
and Sharkey 1982). Such selection would indirectly cause
the evolution of fewer, larger flowers.
As a consequence of genetic correlations, if variation
existed among populations in the degree of sexual selection
on floral display or selection on ecophysiology, some populations might have relatively many and small flowers and
thin leaves with a shortened life span, whereas other populations might have relatively few and large flowers and
long-lived, thick leaves. This type of variation and covariation among traits is exactly what was found in a ninepopulation study (Delph et al. 2002). The two most extreme populations for these traits were from Croatia and
S40 The American Naturalist
Portugal, with the former resembling the many-and-smallflowered selection line and the latter resembling the fewand-large-flowered selection line. Further work, involving
multivariate-selection analysis combined with reciprocaltransplant experiments, is planned to determine which
traits contribute directly to fitness in different populations.
In summary, we tested the hypothesis that divergent
intersexual selection on one trait, flower number, would
cascade through the phenotype as a result of genetic correlations among traits and lead to a higher cost of reproduction for males. Evidence from our artificial-selection
experiments and interpopulation studies with S. latifolia
supports this premise. The next step will be to evaluate
the siring success of males in the divergent selection lines
and relate this to measures of their viability and longevity
in the field, as a direct test of whether traits that enhance
the one are opposed by the other.
Acknowledgments
We thank the many people who helped with the artificial
selection, including M. Banet, J. Bradlaw, R. Davis, G.
Harp, B. Helman, L. Kao, F. Knapczyk, B. Koskella, M.
Rummel, E. Rumschlag, G. Sellhorn, E. Vozar, and S. Wenzel. This work was supported by grants from Indiana University (IU) and the National Science Foundation (DEB9629774 to L.F.D. and J.L.G. and DEB-0075318 to L.F.D.)
and a grant to IU from the Howard Hughes Medical
Institute.
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Editor: Jonathan B. Losos