Morph frequencies and floral variation in a heterostylous colonizing weed, Lythtum salicaria

Abstract

Lythrum salicaria (purple loosestrife) is an exotic weed that arrived in North America from Europe during the early 1800s. It is a herbaceous perennial with a trimorphic breeding system. Seventy-four populations of L. salicaria were surveyed from Windsor, Ontario, to the Gaspé Peninsula in Quebec. Fifty of the populations were significantly anisoplethic (i.e., unequal frequencies of the three flower morphs), including 10 populations that were nontrimorphic. Populations with fewer than 100 plants tended to have one or even two morphs missing. Although larger populations rarely lacked a morph, they did show significantly skewed morph frequencies. Indices of clonal size, such as number of ramets per genet and genet diameter, differed significantly among sites, and clonal growth also showed significant interaction between morph and site. One-way analyses of variance indicated that morphs differed in terms of either number of ramets per genet or genet diameter in 16% of populations. Morphometric analyses of flowers from 49 populations showed significant variability in floral traits among genets, flower morphs, and sites. Results indicated frequent reduction in herkogamy (spatial separation between anther and stigma), with variant flowers having very little or no stigma–anther separation. Mean stigma–anther separation was lowest in the mid-morph individuals, followed by the short and long morphs. Correspondingly, the frequency of variant flowers was greatest in mid-morph individuals and least in long-morph individuals. Isoplethic and anisoplethic populations did not differ, for any morph, in the frequency of occurrence of these variant flowers. Key words: Lythrum salicaria, purple loosestrife, isoplethy, morph frequency, heterostyly, variant flowers, stigma–anther separation.