Seed-type dependent geographic distance effect
Even after controlling for seed mass, we saw a significant geographic
distance effect on cleistogamous germination at the HD common garden. We
found that cleistogamous germination peaked at approximately 125km from
the HD garden, while chasmogamous germination declined slightly with
increasing distance from the source population, although this effect was
not significant. This corroborates our prior finding that non-local
cleistogamous seeds germinate at a higher rate than non-local
chasmogamous seeds at the HD garden, suggesting this pattern may be
biologically significant. These results are surprising for the genetic
and seed weight differences discussed previously, in addition to
differences in the dispersal ability of the two seeds. In nature,
cleistogamous seeds germinate almost exclusively within the length of a
single stem of the maternal plant (Holsinger 2000), making them
extremely unlikely colonizing mechanisms for D. californica . By
contrast, chasmogamous seeds can disperse farther (Schmitt et al. 1985),
making them better candidates to colonize novel environments (Culley &
Klooster 2007, but see Masuda & Yahara 1992).
Contrary to prediction, the observed seed type and geographic distance
effect cannot be explained by genetic differences between cleistogamous
and chasmogamous seeds. Indeed, our observation that cleistogamous
germination success is improved, while chasmogamous germination success
decreases away from the source population fundamentally dissatisfies the
conditions for dimorphic cleistogamy maintenance at the population level
as outlined by Culley & Kooster (2007). Mating system evolution theory
predicts that cleistogamous and chasmogamous seeds with different
dispersal abilities can be jointly maintained if their dispersal
abilities differ, as they do for D. californica (Schoen & Loyd
1984). The near and far dispersal model for the selection of cleistogamy
predicts that resource allocation will first be spent on the production
of cleistogamous seeds - a so-called pessimistic reproductive
strategy - followed by chasmogamous seed production - anoptimistic reproductive strategy (Zeide 1978, Schoen & Loyd).
Our finding that cleistogamous seeds are generally more vigorous than
their chasmogamous counterparts supports theory predicting that their
investment should be prioritized and aligns with other studies that have
found cleistogamous seeds to be generally superior (Dyksterhuis 1945,
Cheplick & Quinn 1982, Schoen & Loyd 1984, Baskin & Baskin 2017). It
is possible that our experimental design, which planted 25 seeds from
the same maternal plant within close proximity, may have unintentionally
favored cleistogamous germination as these seeds are more likely to be
subjected to sibling competition than farther dispersing chasmogamous
seeds (Schoen & Loyd 1984). This may partially explain our finding that
cleistogamous seeds were more vigorous than chasmogamous seeds. Although
density-dependent processes such as intraspecific competition could
influence the expression of local adaptation, this is rarely addressed
in local adaptation studies (Siepielski et al. 2016).