POPULATION-STRUCTURE AND MORPH-SPECIFIC FITNESS DIFFERENCES IN TRISTYLOUS-LYTHRUM SALICARIA

In tristylous plant populations, style-morph frequencies are governed by an interaction between frequency-dependent selection due to disasso rtative mating and stochastic processes. Provided that there are no in herent fitness differences among morphs, frequency-dependent selection should result in equal morph frequencies at equilibrium. Stochastic m odels indicate that the short-styled morph has the highest and the lon g-styled morph the lowest probability of being lost from local populat ions as a result of random processes. We surveyed the morph compositio n of 82 populations of the tristylous, self-incompatible herb Lythrum salicaria in two archipelagos, one in central and one in northern Swed en, located close to the range-margin of the species. To examine wheth er deviations from even morph frequencies can be explained by among-mo rph differences in reproductive success, we quantified flower and seed production in six and three populations in the northern and southern archipelago, respectively, and we recorded segregation ratios in offsp ring produced in six trimorphic populations in the northern area. Seed germination and offspring growth were studied in the greenhouse. Nine ty percent of the populations in the southern archipelago (N = 31) and 69% of the populations in the northern archipelago (N = 35) were trim orphic; the remaining populations were dimorphic (only populations con sisting of at least three flowering plants considered). Dimorphic popu lations were smaller than trimorphic populations, as predicted by stoc hastic models. There was a striking difference in the morph compositio n of L. salicaria populations between the two archipelagos. In the sou thern archipelago, there was a slight excess of the long-styled morph and a corresponding deficiency of the short-styled morph. In contrast, the northern populations were characterized by a marked deficiency of the mid-styled morph: the average frequency of the mid-styled morph i n trimorphic populations was 0.21, and nine of eleven dimorphic popula tions lacked the mid-styled morph. In both archipelagos, the long-styl ed morph (the most common morph) produced about 20% fewer seeds per fr uit than the other morphs. The long-styled morph also tended to produc e fewer seeds per plant. A hand-pollination experiment performed in tw o of the northern populations indicated that seed production per flowe r was pollen-limited in the long-styled morph but not in the other two morphs. Seed germination and offspring size after 24 weeks of growth did not differ among morphs. The mid-styled morph tended to have a hig her representation in the offspring than in the parental generation in all six trimorphic populations studied further indicating that the de ficiency of the mid-styled morph in the northern archipelago does not represent an equilibrium. Taken together, the results do not support t he hypothesis that morph-specific differences in reproductive success can account for deviations from even morph frequencies in L. salicaria . It is suggested that among-morph differences in other components of fitness and historical factors may contribute to the current morph str ucture.