Gene flow interferes with local adaptation to stressful soils in the very rare plant Calochortus tiburonensis
Abiotic stress tends to have a strong influence on the evolution of plant populations. As such, plants adapted to stressful sites are ideal systems for testing the spatial scale of local adaptation and whether gene flow promotes or restricts it. The Threatened plant Calochortus tiburonensis (Liliaceae) is known from only one hilltop and is confined to disjunct patches of stressful serpentine soils that vary in their nutrient and heavy metal concentrations. Gene flow between populations has the potential to promote local adaption by increasing genetic diversity or to restrict it by introducing maladapted alleles. I conducted a reciprocal pollination experiment between two populations to assess whether the source of the pollen (pollen from within the local population or from the other population) affected seed set in recipient plants. One of the populations is small, isolated and found on an exceptionally stressful patch of serpentine soil; the other is large, growing and found on a patch of soil with higher nutrient concentrations and lower heavy metal concentrations. The two populations are located ≈0.7 km apart along the same ridge.
Results from the two populations were strikingly different. In the large population on less stressful soil, pollen source (pollen from within the population or from the small, isolated population) had no effect on the number of seeds set, percent seed set or seed weight. Plants in the small, isolated population growing on the more stressful soils produced nearly twice as many seeds when pollen came from plants in the same population (mean 32.3 ± 12.8 SD) as opposed to the other population (17.5 ± 13.7), and set a higher percentage of seeds (51% ± 21% vs. 28% ± 20%). These results suggest that local adaptation is occurring at very small spatial scales, at least in the population confined to the more stressful soil. Results also suggest that local adaptation is strong enough to inhibit admixture even at the earliest life stage (fertilization) and serves to reinforce population isolation. I am currently conducting a reciprocal (seed) transplantation experiment in the field to more directly test for local adaption. Results from the pollination experiment suggest that transplanting plants from large growing populations to bolster small vulnerable populations of this very rare plant are ill-advised.