PHYLOGENETIC STUDIES IN ALISMATIDAE, II - EVOLUTION OF MARINE ANGIOSPERMS (SEAGRASSES) AND HYDROPHILY

Aquatic species represent fewer than two percent of all flowering plan ts, and only 18 aquatic genera have acquired true hydrophily (water-po llination) which is associated with an unusually high incidence of uni sexual flowers. From the subset of submersed, hydrophilous angiosperms , only 13 genera have colonized marine habitats. The evolution of hydr ophily, unisexuality, and marine habit in angiosperms was explored usi ng estimates of phylogeny obtained by phylogenetic analyses of chlorop last (rbcL) gene sequence data. Despite what might appear to be diffic ult evolutionary transitions, hydrophiles are highly polyphyletic with independent origins in the monocotyledon subclass Alismatidae in addi tion to two derivations in Be dicotyledon families Ceratophyllaceae an d Callitrichaceae. Yet, even in alismatids, hydrophily has evolved man y times. Unisexuality has also evolved repeatedly in the Alismatidae, and is ancestral to the evolution of hydrophiles and marine plants in the Hydrocharitaceae, Marine angiosperms (known only from Alismatidae) have evolved in three separate lineages. The multiple origins of hydr ophilous, marine plants offer an extraordinary example of convergent e volution in angiosperms.