THE REPRODUCTIVE ECOLOGY OF ISLAND POPULATIONS OF DISTYLOUS MENYANTHES TRIFOLIATA (MENYANTHACEAE)

Menyanthes trifoliata L. is a distylous, clonal aquatic macrophyte fou nd in shallow bogs and river margins throughout the boreal ecosystem, including the island of Newfoundland. A combination of long-distance d ispersal and colonization after deglaciation, and pollen limitation re sulting from reduced pollinator diversity and abundance documented on islands, predicts the breakdown of heterostyly to favour the establish ment of self-compatible homostyles on islands. To test if self-fertili zing homostyles have been selected, variation in floral characters and compatibility relationships were examined in M. trifoliata population s from the Avalon Peninsula of Newfoundland. Morph ratio and its effec t on fruit and seed set were examined in nine populations. Of the seve n dimorphic populations, morphs occurred in a 1:1 ratio in four popula tions and deviated significantly from a 1:1 ratio in three populations . The two populations monomorphic for either pin or thrum morphs set f ew fruits or seeds (<15%). A strictly reciprocal arrangement of stigma height and anther length was not observed between pin and thrum morph s in the majority of populations studied. Stigma-anther separation sho wed a bimodal distribution with few intermediate ''homostylous'' flowe rs, rather than the discreet bimodal distribution typical of distylous species. Fruit and seed set were high (>60%) in equal morph ratio pop ulations and were not significantly correlated to stigma-anther separa tion, indicating that there was no selective advantage of being homost ylous. All three populations tested were highly self-incompatible, con firming that there has not been a breakdown of heterostyly on the isla nd of Newfoundland. A reduced pollinator fauna typical of island envir onments may have relaxed stabilizing selection for strict herkogamy be tween floral morphs, resulting in the observed lack of reciprocity.