Strong stabilizing selection on timing of germination in a Mediterranean population of Arabidopsis thaliana

Premise Timing of germination can strongly influence plant fitness by
affecting seedling survival and by having cascading effects on later
life-history traits. In seasonal environments, the period favorable for
seedling establishment and growth is limited, and timing of germination is
likely to be under stabilizing selection because of conflicting selection
through survival and fecundity. Moreover, optimal germination time may
vary among genotypes because of inherent differences in later life-history
traits.   Methods To examine how germination time affects
survival, fecundity and the relative fitness of two genotypes differing in
time to first flower, we conducted a field experiment in an Italian
population of the winter annual Arabidopsis thaliana, in which seedling
establishment occurs mainly in November. We transplanted seedlings of the
local genotype and of a Swedish genotype monthly from August to December
and monitored survival and fecundity.   Key Results Only
seedlings transplanted in November and December survived until
reproduction, and fitness of the November cohort was 35 times higher than
that of the December cohort, indicating strong stabilizing selection.
There was no evidence of conflicting selection: seedling survival, adult
survival and fecundity were all highest in the November cohort. Moreover,
the relative fitness of the two genotypes did not differ significantly
between cohorts.   Conclusions The very narrow window of
opportunity for seedling establishment was related to rapid seasonal
changes in soil moisture and temperature, suggesting that rate of seasonal
change is an important aspect to consider for understanding
spatio-temporal variation in selection on phenological traits.