Reproductive success in species with stylar polymorphisms should be sensitive to plant density and the morph composition of local neighbourhoods. To investigate the spatial ecology of mating, Stehlik et al. (2006) mapped the location of floral morphs in N. assoanus populations and determined their female fertility. They found that pollen transfer and mating was context dependent, with the floral morphs responding differently to the density and morph identity of plants in local patches. Studies of Narcissus demonstrate that biased style morph ratios can be an equilibrium expectation if the strength of negative frequency-dependent selection varies among the morphs because of asymmetrical mating. However, because most heterostylous species possess heteromorphic incompatibility, biased morph ratios usually result from non-equilibrium conditions associated with the ecology of populations.
Biased morph ratios are particularly common in species with prolific clonal propagation (Barrett and Forno 1982; Castro et al. 2007). The signature of founder effects is often preserved over long periods, and progress towards morph-frequency equilibrium depends on the regularity of sexual reproduction and the demographic characteristics of populations (Eckert and Barrett 1995). In species with strong heteromorphic incompatibility, extensive clonal spread can interfere with sexual reproduction, resulting in a deficit of inter-morph cross-pollination and reduced seed set (Thompson et al. 1998; Ishihama et al. 2003; Brys et al. 2007; see also Chap. 3, this volume, with respect to the evolutionary implications of clonal propagation on the evolutionary dynamics of homomorphic SI). Species of Nymphoides, a genus of clonal aquatics, commonly exhibit biased morph ratios or stylar monomor-phism, resulting in pollen limitation of fruit set and restricted sexual recruitment (Ornduff 1966; Shibayama and Kadono 2003; Wang et al. 2005a). By manipulating the frequency of flowers of the L- and S-morphs in an experimental population a 100T — 80 8 60
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