Possibility for pathogen escape
Using a subset of seeds collected and processed in the same manner and
from the same source populations as our study, Mackin et al. (2021)
found that epiphytic (seed surface) pathogen community diversity is
significantly higher on chasmogamous than cleistogamous seeds ofD. californica. Although they found higher in vitro germination
with chasmogamous seeds, it is likely that the negative effects of
antagonistic pathogens on seed germination are diminished in a
laboratory setting (Mackin et al. 2021). Seed pathogens can be a
significant barrier to in situ germination, although they are rarely
accounted for in demography studies (Nelson 2017, Mackin et al. 2021).
In general, outcrossed progeny have been shown to have higher infection
scores when compared to self-fertilized progeny, suggesting that
outcrossing can disrupt pathogen resistance (Busch et al. 2004, Koslow
& Clay 2007). There are several theories to explain the maintenance of
both cleistogamous and chasmogamous mating systems (Oakley et al. 2007),
to which seed-type dependent pathogens could contribute.
It is important to note that not all seed-associated fungi are
pathogenic. For example, some species of the Danthonia genus have
a symbiotic relationship with the endophytic clavicipitaceous fungiAtkinsonella hypoxylon which improves host plant vigor (Clay
1984) and exclusively uses cleistogamous seeds for transmission (Clay
1994). The fungus also causes infected plants to abort chasmogamous
inflorescences, further improving the adaptive potential for
cleistogamous seeds (Clay 1984). However, Mackin et al. (2021) did not
find any evidence of clavicipitaceous fungi inside any D.
californica seeds sourced from the populations used in this study.
However, they did find that cleistogamous seeds had fewer fungi
negatively associated with germination than chasmogamous seeds, which
may partially explain the observed higher germination rates of
cleistogamous seeds in our common garden study.
Pathogen escape could explain the higher germination rates of
cleistogamous seeds sourced from populations located an intermediate
distance from the HD common garden. This fitness advantage quickly
dissipated as distance between source and common garden increased,
however, suggesting that some degree of local adaptation may still play
a role in germination. It appears that populations from farther away are
still more maladapted to the common garden site, even if they do benefit
from pathogen escape. Only the seeds from intermediately distanced
populations likely stood to benefit from pathogen escape in our common
garden experiment. Although most transplant experiments occur across
much larger spatial scales (Joshi et al. 2001, Gaillart et al. 2018, van
Boheemen et al. 2019, Zhang et al. 2019), nonlocal advantage at small
scales could be common, especially if environmental heterogeneity is
high (Hereford 2009).