Genetic differentiation studies and phylogenetic inference in the plant genus Limnanthes (section Inflexae)

An electrophoretic survey of genetic variation in 50 populations representing all ten taxa of section Inflexae of the plant genus Limnanthes is reported here with three objectives: 1) to describe genetic differentiation for testing certain phylogenetic hypotheses on the origin of species and infraspecific relationships, 2) to evaluate the concordance of electrophoretic, morphological and hybridization data within the section, and 3) to discuss models of speciation using Limnanthes as an example. Species, subspecies, and populations, designated on the basis of morphology and distribution, gave decreasing values of genetic distances that were apparently maintained across a wide ecological and geographical range. Using electrophoretic, morphological and hybrid fertility data, we concluded that L. montana, intermediate in range between the two disjunct varieties of L. gracilis, is not likely to be a relictual set of populations from what once was a continuously distributed taxon as hypothesized by earlier workers. Neither L.g. gracilis nor L.g. parishii appear to have been founded by long distance dispersal from one to the other. However, a very close genetic relationship was detected between L. gracilis and L. alba. This genetic pattern suggested that the two disjunct L. gracilis varieties were probably connected by a L. alba-like taxon and perhaps originated from that taxon. Evidence based on allozyme variation did not support the thesis that the inbreeder L. floccosa is a recent derivative from the outbreeder L. alba. Among the remaining five taxa (i.e. two varieties of L. gracilis, two varieties of L. alba, and L. montana), genetic distance and interspecific hybridization data are highly concordant, (r = -0.92, P < 0.001). The agreement of these two approaches with species relationships based on morphological similarity was less certain. Limnanthes species appear to exhibit greater interpopulation differentiation than many plant groups, perhaps a reflection of their distinctive island-like distribution pattern. On the other hand, an unusually high crossability is found in Limnanthes. Speciation in Limnanthes appears largely to follow a model of adaptive geographical divergence but certain other modes cannot yet be ruled out.