CLONAL REPRODUCTION AND PATTERNS OF GENOTYPIC DIVERSITY IN DECODON-VERTICILLATUS (LYTHRACEAE)

Most perennial plants combine sexual reproduction with some form of cl onal propagation. These mixed strategies may produce considerable vari ation among populations in levels of clonal diversity in response to e cological factors limiting one or other reproductive mode. Surveys of style morph frequencies in 163 populations of the eastern North Americ an, clonal, tristylous aquatic, Decodon verticillatus (L.) Ell. (Lythr aceae) suggested a wide range of clonal diversity among populations. P opulations consisting of a single style morph were most common at the northern margin of the species' range and could have arisen through se vere founder events followed by exclusive clonal propagation. Here, we test this hypothesis by comparing allozyme variation in populations m onomorphic and polymorphic for style length located in Ontario and Mic higan. Each of the four populations monomorphic for style length were fixed for a single three-locus allozyme genotype while the seven trimo rphic and five dimorphic populations contained an average of 26 multil ocus genotypes each. Measures of genotypic diversity were high in poly morphic populations (average D = 0.93 +/- 0.02 standard error; D = 0.0 0 for all populations monomorphic for style length). Three of the popu lations monomorphic for style length were fixed for a heterozygous gen otype at one of the loci surveyed, suggesting that each consists of a single clone. In contrast, genotype frequencies in polymorphic populat ions conformed to Hardy-Weinberg proportions indicative of sexual repr oduction. The range of clonal diversity found in D. verticillatus is t he largest reported for a clonal plant species, although the literatur e is too limited to determine whether this is truly unusual. Clonal di versity in D. verticillatus is likely to be regulated largely by ecolo gical factors affecting seed production and establishment. However, ge netically based sexual sterility also occurs in some populations.