ADAPTIVE VARIATION IN AN INVASIVE ANNUAL GRASS:
GERMINATION PATTERNS OF AEGILOPS TRIUNCIALIS IN
CALIFORNIA, USA
A.R. Dyer1, J.D. Gerlach2 and K.J. Rice2
1
University of South Carolina Aiken, 471 University Parkway, Aiken, South Carolina, USA.
E-mail: [email protected]
2
University of California, Davis, One Shields Avenue, Davis, California, USA.
E-mail: [email protected] and [email protected]
1. INTRODUCTION
Many factors influence seed germination and range from internal physiological controls, maternal controls via the seed
coat, environmental signals, and interspecific inhibition (Baskin & Baskin 1998). Different factors are likely to affect
each species variously and the strength and importance of each factor is determined in part by environmental
conditioning and evolutionary history. Both environmental signals and genetic traits vary spatially and the strength and
importance of germination controls should vary from one population to the next. However, while seed dormancy has
been studied in many species, very little is known about the population variation in factors controlling dormancy and
how much of the variation is genetic and how much is a plastic response.
Seed ecology studies focus on two central questions. First, how important is seed dormancy to persistence in seasonally
variable environments? Dormancy is considered a bet-hedging strategy such that some fraction of the seed population
remains dormant even when conditions for germination are optimal (Cohen 1967). This strategy favours species
persistence even after catastrophic loss of the germinating fraction of the seed population. As a general rule, dormancy
is enforced by maternal control over the permeability of the seed coat and this influences the seed’s ability to respond to
environmental cues (Westoby 1981).
Second, how is germination and emergence affected by the biotic factors in the seed’s neighbourhood? The
neighbourhood immediately surrounding a seed may be both intra- and interspecific, and may be comprised of seeds,
seedlings, or mature plants. External influences on germination are similar to the maternal effect in that they are almost
certainly chemical, however, intra-specific factors are expected to be adaptive, but interspecific factors are not.
In this study, we used a rapidly spreading annual grass to investigate variation in germination and emergence across
multiple populations. Our objectives were to determine the strength of two documented influences on germination, a
maternal and a sibling effect, and to determine whether those influences appear fixed or plastic in this species.
2. METHODS
Dry inflorescences of A. triuncialis L. were collected from 12 populations in 4 regions of northern California (Table 1).
Inflorescences usually contained 4-6 spikelets with the lower two often containing two dimorphic seeds. The large seed
was never dormant, but the small seed showed complete germination only when removed from the spikelet. In the
spikelet, dormancy of the small seed is induced by chemicals on the surrounding tissues (maternal effect) and by the
presence of the large seed (sibling effect) (Andrew Dyer, unpublished data).
We used washing and large-seed removal treatments to test the strength of the maternal and sibling effects among the
12 populations. In the four treatment combinations, 200 spikelets (5 groups of 10 spikelets per treatment) from each
population were sown in commercial potting soil in a controlled environment chamber set at 10ºC night and 20ºC day
temperatures. Emergence was monitored over two 10-day watering periods, the second coming after complete drying
of the soil, seeds, and seedlings.
We weighed ~50 pairs of seeds from each population to estimate the slope of the relationship between large and small
seeds produced by the same parent. Percent emergence (arcsine transformed) of small seeds after the first and second
wet periods was analysed in 2-way ANOVA within each population to assess the relative strength of the dormancyinducing factors.
Proceedings of the VIIth International Rangelands Congress
26th July – 1st August 2003, Durban, South Africa
Editors: N. Allsopp, A.R. Palmer, S.J. Milton, K.P. Kirkman, G.I.H. Kerley, C.R. Hurt, C.J. Brown
ISBN Number: 0-958-45348-9
Proceedings produced by: Document Transformation Technologies
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Table 1. Results of 2-way ANOVA on emergence of small seeds for each of 12 populations of Aegilops triuncialis from
northern California, USA. For some analyses, very high emergence resulted in insufficient data (i.d.) for testing
treatment interactions or main effects in the second watering period.
3. RESULTS
Emergence of small seeds varied greatly among populations (Table 1). In the first wet period, the main effect of the
large seed on small seed emergence was significant for all 12 populations (Figure 1, e.g., Hopland-Foster). The main
effect of spikelet rinsing on small seed emergence was significant for 9 populations (Figure 1, e.g., Jepson Prairie). The
interaction between large seed presence and spikelet washing was significant in six of the 10 populations for which
there were sufficient data indicating, usually, that emergence was greatest when both influences on emergence were
reduced. In the second wet period, emergence of the previously dormant seeds did not indicate a strong influence of
either large seed (now dead) presence or maternal effects. The weakness of the effects was in part due to low numbers
of remaining seeds.
Figure 1. Effects of washing of spikelets and the presence of large seeds on the emergence of small seeds of Aegilops
triuncialis of 4 of the 12 tested populations representing the 4 sampled regions. The first wet period coincided with the full
germination of the large seeds (where present). Maternal and sibling effects were predicted to be strongest in the first
watering period (filled circles) and weaker in the second period (open circles). Although washing and large seed removal
treatments did have significant effects on emergence, the responses varied strongly with between populations (see Table 1).
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Regression analysis indicated that the relationship between large and small seed mass was very consistent across
populations. Large seeds were 2.0-2.5 times heavier than small seeds and the slope of the relationship was consistently
near 0.35. The range of seed mass was 9.4-25.8 mg for large seeds and 4.6-10.7 mg for small seeds. For 10 of 12
populations, the slope of the regression was 0.27-0.39 (r > 0.69, P < 0.0001).
4. DISCUSSION
In this study, we found very strong evidence that both the maternal and sibling influences on germination of small seeds
in A. triuncialis varied among populations, and across years in the same population (Table 1).
Seed mass was highly variable among populations, but consistently strong treatment effects suggested that, even though
the small seeds of some populations were larger than the larger seeds of other populations, seed size, per se, was not a
primary factor determining dormancy in this species.
The existence of two dormancy-inducing factors suggests a complex evolutionary history for this species. First, the
strong maternal influence supports the prediction that plants living in unpredictable environments should maintain a
dormant seed fraction as a bet-hedging strategy (Cohen 1967). The well-provisioned larger seed germinates easily and
produces a larger seedling. Of the two seed types, the larger is expected to produce seedlings that are more competitive
for resources. In this scenario, the smaller seeds remain dormant as a backup, but germinate later in the season or the
next year as the maternal control weakens.
Second, the strong sibling inhibition by the larger seed supports the prediction that heteromorphic seeds, when
dispersed together, will show an interaction such that one seed has greatly reduced germination in the presence of the
other (Cheplick 1992). Even though the larger seed may be more competitive, any competition between siblings
reduces plant size and the ability to dominate interspecific interactions. Because the smaller seed is slower to emerge
(and presumably slower to germinate), and experiences some control by the maternal tissues, it is necessarily the
suppressed seed in sibling interactions.
Although the maternal and sibling factors appear hierarchical, the strength of each factor varied across populations
suggesting that the environment is an important determining component for both. Habitat productivity varies greatly in
seasonally unpredictable environments due to the amount of precipitation and its distribution throughout the growing
season. The relationship between habitat productivity and seed traits, such as maternal plant mass, seed mass, seed
number, and the strength of the dormancy-inducing factors is as yet unknown. However, a bet-hedging strategy
assumes environmental unpredictability and therefore the maternal effect is not expected to show a correlation with
productivity. In contrast, because increased belowground resource availability implies increased aboveground resource
competition, we predict a positive correlation between productivity (i.e., precipitation) and the strength of the sibling
factor.
Multiple emergence strategies provide greater response flexibility to multiple selection factors. If the maternal
influences on germination provide protection against environmental uncertainty, they are likely to be adaptive genetic
responses to the environment. In contrast, if sibling suppression of germination provides some release from
competition intensity, they are likely to be plastic responses to the growing conditions experienced by the maternal
plant. We suggest that, in highly successful invasive species such as A. triuncialis, suites of traits exist that confer a
competitive advantage under diverse environmental conditions. In A. triuncialis, at least two germination traits
combine to create a higher probability of seedling establishment and population persistence, and these traits have very
likely contributed to its success in northern California.
5. REFERENCES
Baskin CC & Baskin JM 1998. Seeds: Ecology, biogeography, and evolution of dormancy and germination. Academic
Press, San Diego.
Cheplick GP 1992. Sibling competition in plants. Journal of Ecology 80: 567-575.
Cohen D 1967. Optimizing reproduction in a randomly varying environment when a correlation may exist between the
conditions at the time a choice has been made and the subsequent outcome. Journal of Theoretical Biology 16:1-14.
Westoby M 1981. How diversified seed behavior is selected. American Naturalist 118: 822-825.
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