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QUANTITATIVE VARIATION
WITHIN AND AMONG CHEATGRASS
POPULATIONS: THE ROLE OF
MULTIPLE INTRODUCTIONS
Stephen J. Novak
ABSTRACT
Cheatgrass (}Jromus tectorum) is a highly selfing annual grass species that occurs worldwide in temperate
grasslands. The native range of this species extends from
the Mediterranean Basin, including northern Africa, to
Tibet and southern Pakistan (Pierson and Mack 1990).
Cheatgrass has been introduced into many locations and
was first observed in eastern North America as early as
1790 (Muhlenberg 1793) and in the Intermountain West
about 1889 (Mack 1981).
Previous results from electrophoretic analysis of cheatgrass show little genetic variation in North American
populations (Novak and others 1991, also reviewed by
Pyke and Novak, these proceedings). Regional differences
in the distribution of allozyme variation as well as the
presence of several novel allelic variants indicate this species has experienced a complex introduction history in
North America. Some populations in the West appear to
have resulted from the combining of genotypes following
mu\tiple introduction of different genotypes from the native range and subsequent gene flow (Novak and others,
submitted). Furthermore, allozyme data suggest a higher
level of population differentiation in the native range than
in the introduced range. Populations from North America
partition their genetic variation evenly within and among
populations, most likely the result of multiple introductions (Novak and Mack, in preparation).
The objectives of this study were to determine the level
and distribution of variation for quantitative traits in six
native and six introduced populations of cheatgrass. I
was interested in how genetic variation for these traits
is partitioned within and among families and populations
and among regions in this widespread selfing weed. Using a comparison of heritable variation within populations
from Eurasia and North America, I asked whether there
is evidence for a reduction in variation for quantitative
traits due to founder effects. Alternatively, would an increase in variation in quantitative traits be detected
within populations as a result of multiple introductions?
Variation within and among populations of cheatgra,ss
(Bromus tectorum) was determined for 35 quantitative
traits. Substantial population differentiation was detected
for most of the 35 traits. Phenotypic plasticity accounts for
the variation seen for most of these traits. Genetic variation for these traits is usually partitioned among populations and regions. Populations from North America possessed higher levels of variation among families, suggesting
the possible importance of multiple introductions in increasing quantitative variation in alien species.
INTRODUCTION
Genetic variation for individuals and populations can
be described as residing at three distinct levels: DNA sequences, single-locus traits, and multilocus (quantitative)
traits. Quantitative traits such as time to emergence,
growth rate, time to reproduction, and fecundity are critical components of the life history of a species because they
can influence the growth and persistence of the species
(Dingle and Hegmann 1982). Venable (1984) states that
studies of quantitative trait variation "... present an interesting bridge between the fields of ecology and evolution
because they are directly interpretable ecologically as adaptation for survival and reproduction." Additionally,
variation in quantitative traits can be genetically based
or the result of phenotypic plasticity (Bradshaw 1965;
Clausen and others 1940). Either mechanism allows a
species to survive under variable environmental conditions (Lewontin 1957).
The ability to survive in a variable or novel environment is potentially made even more difficult for invading
species because founder events and population bottlenecks result in a reduction in genetic variation following
immigration (Barrett and Husband 1990; Barrett and
Richardson 1986; Brown and Marshall1981). The potential difficulty in avoiding extinction is accentuated for
selfing-introduced species since they partition most of
their genetic variability among rather than within populations (Brown 1979; Hamrick and Godt 1990; Loveless
and Hamrick 1984).
MATERIALS AND METHODS
Plant Material-Seeds from individual maternal
plants (families) were collected from a total of 12 populations from the native and introduced ranges of cheatgrass
(table 1). The six native populations span the Mediterranean Region: Jordan, Turkey, Yugoslavia, France, and
Morocco. Populations from North America were sampled
across the introduced range of cheatgrass and in several
cases their locations correspond to sites at or near putative points ofintroduction (7, 9, 10, and 12).
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, m, May 18-22, 1992.
Stephen J. Novak is a Postdoctoral Research Associate, Department of
Botany, Washington State University, Pullman, WA 99164-4238.
103
Table 1-Populations from the native and introduced ranges of
cheatgrass used in the analysis of quantitative varaition
Range
Native
Introduced
1985) to partition total phenotypic variation within and
between families, populations, and ranges. Intra-class
correlation coefficients (t) were used to determine the
amount of differentiation among families within each
population (Zar 1974).
Previous electrophoretic analyses revealed that outcrossing is exceedingly rare in cheatgrass: zero in the
introduced range and 0.17 percent in the native range
(Novak and Mack, in preparation). Given these results,
I assume complete selfing and that the progeny from all
families used in this study are genetically identical full
sibs. Under the assumption of full-sib families the withinfamily variance is considered to be due to the environment (phenotypic plasticity), and the total genetic variation for each trait is partitioned into the family, population,
and range variance components. The proportion of genetic variation among families (PGVAF), among populations (PGVAP), and among ranges (PGVAR) is reported
as a proportion of the total genetic variance (Venable and
Burquez 1989). These values allow the distribution of the
total genetic variance to be compared at several hierarchical levels.
Population
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Amman, Jordan
AJgada, Jordan
DeGoreme, Turkey
Cemerno, Yugoslavia
Conquerac, France
Touna Station, Morocco
Philadelphia, PA
Lexington, KY
Provo, UT
Smoot Hill, WA
Rattlesnake Springs, WA
Cache Creek, BC
Greenhouse growouts were performed for one generation to reduce environmental effects on the expression of
the traits to be measured (Roach and Wulff1987; Schall
1984). Seeds from field-collected plants of each population were sown into wooden flats (50 by 40 by 20 em) containing standard greenhouse potting media and placed in
an unheated greenhouse. Plants were watered as needed,
and the position of the flats on the greenhouse bench was
rotated regularly until the plants reached maturity. Flats
were rotated to minimize the effects of microclimate differences within the greenhouse on the traits being measured (Schall1984). In the spring the mature plants were
harvested and placed separately in envelopes; 10 plants
were chosen at random as the families for the subsequent
experiment.
In the fall of 1988, seeds from 10 families, each consisting of 10 individuals, from each of the 12 populations were
individually weighed with a Cahn Instruments electrobalance to the nearest 0.001 mg. Seeds from all families and
populations were randomly positioned in a grid in flats at
a density of 550 seedslm2 and maintained under the same
conditions described above. This sowing density is lower
than plant densities usually observed in the field for cheatgrass (Mack and Pyke 1983) and was chosen to minimize
intraspecific competition (Mitchell-Olds and Rutledge
1986).
Plants were scored for 35 quantitative traits (Novak
1990). In this paper I will report on a subset of these
traits, including initial seed weight, time to emergence,
plant height at 30-day intervals and at harvest, time to
flowering (defined as the days needed for the panicle to
exert through the leaf sheath), and mean seed number
per plant. Plants were harvested individually, when chlorophyll pigmentation was no longer observable within the
panicle, and were ovendried (70 °C) for 48 h. Plants were
stored with dessicant for 48-72 h before taking morphometric measurements and dry weight readings.
RESULTS AND DISCUSSION
Phenotypic means differ among populations for all
quantitative traits except percent reprodu~tive biomass
(%REPBIOM) and percent vegetative biomass (%VEGBIOM), indicating substantial population differentiation
for most of these traits (data not shown). Initial seed
weight was uniformly higher in native range populations
than in introduced populations (fig. 1). Of the introduced
populations, Provo, UT, had the largest initial seed weight
(2. 76 mg), while only two populations from the native
range, Cememo, Yugoslavia (2.85 mg) and Conquerac,
France (2.74 mg), possessed comparable values. The
initial seed weight does not appear to influence time
to emergence in the cheatgrass populations used in this
study (fig. 2). In fact, there may be an inverse relationship between initial seed weight and time to emergence
since the population from both the native and introduced
ranges with the largest initial seed weight also possessed
the longest time to emergence (Algada, Jordan, and Provo,
UT, respectively).
Initial seed weight and plant height (at least at the
early time intervals) does appear to possess a positive relationship. Mean plant heights at the 30-, 60-, and 90-day
intervals for all populations of cheatgrass from the native
range are larger than those observed for the populations
from the introduced range (fig. 3). However, the mean
plant height for the introduced populations at 120 days
exceeds that observed for native populations. By the time
of harvest, differences in plant heights between the native
and introduced ranges are considerable (fig. 3). The patterns suggested by the relationship between seed weight
and plant height appear to be a function of increased
growth rates that are found in plants possessing large
seeds. Interestingly, initial seed weight apparently has a
larger influence on plant heights than on time to emergence.
Time to flowering also appears to be related to large
early season plant heights and generally with initial seed
weight. Populations that had the fastest growth rates in
Data Analysis-Analysis of variance was performed
to detect trait differences among all populations and to
test for family differences within each population. A
nested analysis of variance was performed for all populations using the NESTED program of SAS (SAS Institute
104
5
a
25
a
a
g
-
4
20
3
15
~
Cl
~
~
=
2
c
10
5
0
0
Amman
Algada
DoGorme
Comorno
Conquorac
Amman
Touna Sta.
Algada
E
l:
5
25
4
20
Cl
~
a
3
Comerno
Conquorac Touna Sta.
Native Population
Native Population
Q
DoGormo
15
2
10
5
0
0
Phlla.
Lexington
Provo
Smoot Hill
Phlla.
Rattlesnake Cacho Cr.
Lexington
Provo
Smoot Hill
Rattlesnake Cache Cr.
Introduced Population
Introduced Population
Figure 2-Days to emergence for native (top)
and introduced (bottom) populations of cheatgrass. Different letters at the top of columns
indicate significant differences (P < 0.05).
Figure 1-lnitial seed weight (mg) for native
(top) and introduced (bottom) populations of
cheatgrass. Different letters at the top of columns indicate significant differences (P < 0.05).
cheatgrass populations. The native populations generally
have larger seeds, larger plant heights in the early time
intervals, shorter time to flowering, and produce fewer
seeds when compared to their North American counterparts. The pattern described for native range populations
consistently the first populations to initiate flowering
(fig. 4). The mean time to flowering for all native populations is 95.1 days; the introduced populations initiated
flowering in 103.4 days. The variation in time to flowering observed for native populations circumscribes that
observed for introduced populations: the Algada, Jordan,
population had the shortest time to flowering (68.54 days)
while the Cemerno, Yugoslavia, population had the longest (121.87 days). Time to flowering for introduced populations ranged from 93.42 days (Rattlesnake Springs, WA)
to 116.14 days (Lexington, KY).
A large amount of variation for mean seed number
was observed in the populations used in this experiment
(fig. 5). Introduced populations produced more seeds (41.9)
than populations from the native range (38.8), suggesting
that there is a benefit associated with delayed flowering.
Indeed, the two populations from the native range with the
longest time to flowering, DeQQreme, Turkey, and Cemerno,
Yugoslavia, produced the largest number of seeds (41.5
and 63.4, respectively). The same trend generally held
for the introduced populations, with the exception of the
population from Lexington, KY. This population had the
longest time to flowering in North America (fig. 4); however, it also produced the smallest number of seeds (fig. 5).
Different patterns emerge when these quantitative
traits are compared between the native and introduced
25
---uE
-~
20
Cl
15
a:=
10
-;
•
Native Plant Height
~
Introduced Plant Height
J:
c
5
0
30
60
90
120
Days From Emergence
Figure 3-Piant heights (em) for native and
introduced populations of cheatgrass at 30
day intervals and at harvest.
105
HAR
140
70
c
120
80
t!
60
60
as
%
100
l
a
50
.B
40
z:I
30
E
J
40
20
20
10
0
0
Amman
Algada
DeGorme
Amman
Cememo Conquemc Touna Sta.
AJgada
DeGorme
Cememo
Conquomc Touna Sta.
Native Population
Native Population
140
70 •
120
i
100
60
a
a
~ 50
.l.i 40
E
60
:I
z
J
40
20
30
20
10
0
0
Phlla.
Lexington
Provo
Smoot HID Rattlesnake Ceche Cr.
Phlla.
Lexington
Provo
Smoot HID Rattlesnake C8che Cr.
Introduced Population
Introduced Population
Figure 4-0ays to flowering for native (top) and
introduced (bottom) populations of cheatgrass.
Criteria used to define the initiating of flowering
are explained In text. Different letters at the top of
columns indicate significant differences (P < 0.05).
Figure 5-Seed numbers in native (top) and introduced (bottom) populations of cheatgrass.
Different letters at the top of columns Indicate
significant differences (P < 0.05).
appears to be consistent with a life history adapted to extremely arid environments (Fox 1990). Plants in such environments flower quickly before the onset of summer
drought, but they do so at a cost because they also produce fewer seeds. In the face of such drastic selection
pressures, however, the production of even a small number of seeds ensures that these plants (and their genotypes) will be represented in the next generation.
reveals a fairly even distribution of genetic variation
among these hierarchical levels (among families =10
traits, among populations =15 traits, and among ranges
= 10 traits).
These results indicate that the majority of genetic variation for 25 of these traits is partitioned among populations and regions and suggest that genetic variation for
quantitative traits in cheatgrass is distributed over an
enormous geographical area (three continents). The
ranges of environments and habitats from which these
plants were collected are extensive and suggest that differentiation among populations and ranges may be the
result of local selection. The analysis of adaptive significance for these traits is confounded because the populations were grown in the greenhouse (Venable 1984). It
should be noted, however, that this was not the goal of
the current study; I was interested in detecting genetic
variation among and within populations and the nearly
uniform greenhouse environment makes this possible
(Schall1984).
Hierarchical ADalysis-The nested ANOVA was used
to partition the total phenotypic variance for all 35 quantitative traits into a hierarchical series (individuals, families, populations, and ranges) (data not shown). The range
variance component accounts for differences among the
native and introduced ranges for a trait, while the population component accounts for differences among populations. The within-population variation is divided into
the family and individual variance components. For 30
of 35 of these traits the individual variance component
accounts for most of the phenotypic variation for these
traits. The individual variance component is assumed to
be due to the environment and indicates that for these 30
traits most of the variation is due to phenotypic plasticity.
The sum of the family, population, and range variance
components comprises the total genetic variation for each
trait. The total genetic variation was partitioned among
families, among populations, and among regions and
Among-Family Variance-Intraclass correlation (t)
analysis was used to identify among-family differences
for all 35 quantitative traits within each native and introduced population. Genetic variation within populations
of selfing species is usually characterized by the amount
of variation found among families of that population
106
populations was similar, and generally high. In contrast,
higher than expected (for a selfing species) levels of
within-population variability were detected in North
American populations for both electrophoretic and quantitative traits. Both results suggest that multiple introductions can play an important role in influencing the level
of genetic variation within populations of introduced plant
species.
Table 2-Signlficant intraclass correlation coefficients (t) for each
native and introduced population of cheatgrass. A total
of 35 traits were examined in each population
Range
Native
Introduced
Population
Amman
tvalues
25
AIgada
15
DeGoreme
Cememo
Conquerac
Touna Station
20
8
14
13
Philadelphia
Lexington
Provo
Smoot Hill
Rattlesnake Springs
Cache Creek
23
25
23
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