Chloroplast Dna and Isozyme Diversity in Two Mimulus Species (Scrophulariaceae) With Contrasting Mating Systems

Levels of cpDNA and isozyme diversity were contrasted between the mixed‐mating M. guttatus and its highly selfing congener M. micranthus (Scrophulariaceae). Compared to M. micranthus, M. guttatus has two to four times higher diversity for both cpDNA and isozyme variation on a species‐wide level. The selfing M. micranthus also has 1.5 to three times lower within‐population allozyme variation and a greater proportion of its variation distributed among rather than within populations. Chloroplast DNA is here inferred to be uniparentally inherited. Thus the mating system has no effect on the transmission of the chloroplast genome. Since both cpDNA and isozyme variation are similarly reduced in M. micranthus , factors other than the mating system are hypothesized to be responsible for the observed decrease in species‐wide genetic variation in M. micranthus . A recent origin of M. micranthus from a limited number of M. guttatus populations is suggested. Consequently, molecular variation is reduced in M. micranthus due to a bottleneck effect. These data generally demonstrate that levels of cpDNA variation may be high enough in some species to infer evolutionary processes below the species level.