Phylogenetic Studies in Alismatidae, II: Evolution of Marine Angiosperms (Seagrasses) and Hydrophily

Aquatic species represent fewer than two percent of all flowering plants, and only 18 aquatic genera have acquired true hydrophily (water-pollination) which is associated with an unusually high incidence of unisexual flowers. From the subset of submersed, hydrophilous angiosperms, only 13 genera have colonized marine habitats. The evolution of hydrophily, unisexuality, and marine habit in angiosperms was explored using estimates of phylogeny obtained by phylogenetic analyses of chloroplast (rbcL) gene sequence data. Despite what might appear to be difficult evolutionary transitions, hydrophiles are highly polyphyletic with independent origins in the monocotyledon subclass Alismatidae in addition to two derivations in the dicotyledon families Ceratophyllaceae and Callitrichaceae. Yet, even in alismatids, hydroph- ily has evolved many 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 hydrophilous, marine plants offer an extraordinary example of convergent evolution in angiosperms.

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