Differences in population differentiation between an outcrossing and selfing species

Background/Question/Methods   Selfing inhibits gene flow among plant family lines, leading to an increase in population differentiation. Outcrossing populations can emulate these conditions when separated by distances or geographic features large enough to act as reproductive barriers. While selfing populations of plants have a reproductive mechanism facilitating population differentiation at small scales, outcrossing populations could show similar population differentiation among sites far enough away from each other to inhibit most gene flow. Lupinus nanus is an outcrossing species with high levels of morphological differentiation among populations, suggesting acute population differentiation. This study examines microsatellite loci of L. nanus and its close selfing relative, Lupinus bicolor, to determine whether the large distances between populations allow for similar levels of genetic differentiation. Samples from both species were collected from five sites ranging in distance from 683km to 108km apart. Morphological data was also collected to compare with genetic data.

Results/Conclusions   Observed versus expected values of heterozygosity and Fis values supported the selfing nature of L. bicolor, and population structure illustrated discrete among site and between species populations. Fst values showed that L. bicolor has higher among site genetic differentiation than L. nanus, but Mantel tests did not find a correlation between distance among population and among site genetic differentiation or morphological differentiation in either species. Both species do, however, have differing levels of heterozygosity indicating variation in outcrossing rates among populations. This study demonstrates that selfing populations can maintain higher population differentiation than outcrossing populations even at distances large enough to impede pollinator mediated gene flow. It also shows that the link between population differentiation and geographic distance may be weaker than previously thought, suggesting that habitat type and population size might have a greater effect on local adaptation than distance among sites. I am currently implementing a follow-up study examining the link between local adaptation and distance at these sites.