STOCHASTIC LOSS OF STYLE MORPHS FROM POPULATIONS OF TRISTYLOUSLYTHRUM SALICARIAANDDECODON VERTICILLATUS(LYTHRACEAE)

Abstract

Despite the theoretical importance of stochastic processes in evolution there have been few empirical studies of the interaction between genetic drift and selection on the maintenance of polymorphisms in plant populations. We used computer models to investigate the interaction between drift and frequency-dependent selection in affecting style morph frequencies in populations of tristylous species. Drift produces a distinct pattern of morph frequency variation involving: 1) the loss of the S morph and, to a lesser extent, the M morph; 2) no consistent bias in frequencies within populations; 3) a restricted pattern of variation involving a deficiency of one morph and equal excesses of the other two. Morph frequencies were surveyed in 137 populations of Lythrum salicaria from both its native range in Europe (N = 35) and recent adventive range in Ontario (N = 102), and 133 populations of Decodon verticillatus from four regions in eastern North America with different glacial histories to assess these theoretical predictions. There was a negative relationship between morph loss and population size in both species; the relationship was weaker in D. verticillatus than in L. salicaria. Morph loss was more frequent in the adventive than native range of L. salicaria, and in populations of D. verticillatus from glaciated northern regions compared with the unglaciated southern portion of its range. Simulations incorporating variation in life history, regeneration strategy and mating patterns revealed that the degree of morph loss was strongly influenced by year to year survival, clonal propagation, self-fertilization and departures from disassortative mating. Comparing the pattern of morph frequency variation between species supported these predictions. Morph loss was lower in self-incompatible L. salicaria (0% in Europe; 23% in Ontario), which reproduces through seed compared to self-compatible, clonal D. verticillatus (52%). A stochastic model provides the most parsimonious explanation for observed patterns of morph frequency variation in both species.

GENETIC DRIFT AND FOUNDER EFFECT IN NATIVE VERSUS INTRODUCED POPULATIONS OF AN INVADING PLANT,LYTHRUM SALICARIA(LYTHRACEAE)

“…In very dense patches where it was not easy to distinguish individuals, flowering spikes were sampled at 1 m intervals (following Haldane 1936;Halkka and Halkka 1974;Eckert and Barrett 1992). In populations with fewer 'than 200 individuals, we censused almost all flowering plants.…”

“…Both areas were characterized by agricultural landscapes, with Lythrum salicaria occurring along irrigation ditches, river banks, and moist creek beds. Populations were selected for sampling following criteria outlined in Eckert and Barrett (1992). All populations were separated by at least 800 m, although most were separated by much larger distances.…”

“…In 102 populations from southern Ontario, Canada, Eckert and Barrett (1992) found a pattern of morph loss consistent with the operation of stochastic processes. These results support the widely held expectation that stochastic processes are particularly prevalent during colonization in invading species.…”

“…In contrast, the scattered data available for European populations reported in the literature (Darwin 1877;von Ubisch 1925;Haldane 1936;Schoch-Bodmer 1938;Fisher and Mather 1943;Halkka and Halkka 1974;Andersson 1994;Agren and Ericson 1995) suggest only minor deviations from equilibrium morph frequencies. As noted by Eckert and Barrett (1992), variation in sampling strategies among these European workers makes comparison with the Ontario sample difficult, weakening any assessment of the relative importance of stochastic processes in the native versus introduced range.…”

“…In tristylous populations, frequencies of the three mating types, or "style morphs," are driven towards a single stable equilibrium by frequency-dependent selection owing to disassortative mating among morphs (Heuch 1979a,b;Heuch and Lie 1985;Barrett et al 1987). Stochastic effects can be recognized because founder effect and genetic drift in small populations produce a characteristic pattern of morph-frequency variation and morph loss (Heuch 1980;Barrett et al 1989;Eckert and Barrett 1992;Husband and Barrett 1992). This involves random deviations from even morph frequencies plus a nonrandom pattern of morph loss owing to the pattern of dominance and epistasis at the two loci controlling the polymorphism.…”