Patterns of Isoenzyme Variation in Plectritis (Valerianaceae)

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Abstract
Samples from populations of all four North American species of Plectritis from throughout their range in the United states were assayed for isoenzymes of 14 enzyme systems presumably coded by 29 loci. Analysis of the levels and organization of isoenzyme diversity in the genus indicated that the four species form two groups with regard to population genetic structure. Plectritis congesta is a relatively highly outcrossed species, with high levels of polymorphism and observed heterozygosity. It has significantly less of its total diversity attributable to interpopulational differentiation than any of the other three species studied. Plectritis brachystemon, P. macrocera, and P. ciliosa have lower outcrossing rates, levels of polymorphism and heterozygosity, and a higher proportion of total diversity due to interpopulational differentiation. Average genetic distances among conspecific Plectritis populations were well above that reported for other species by Gottlieb. Patterns of genetic distances among Plectritis populations do not appear to be strongly associated with either geographic patterns or with established systematic relationships. It is clear that it may not always be appropriate to use isoenzyme data to make taxonomic decisions. Nevertheless, the details of population biology and population genetics that such data reveal may clarify the sources of problems in some "problem" taxa, for instance by confirming a taxon as highly selfed, with consequent interpopulational differentiation.