Convergence in the Feeding Apparatuses of Lophophorates and Pterobranch Hemichordates Revealed by 18S rDNA: An Interpretation

Homology of the feeding apparatus of pterobranch hemichordates and the lophophore of bryozoans, brachiopods, and phoronids has been postulated but never rigorously tested. I employ Patterson's (1, 2) three criteria of conjunction, similarity, and congruence to test this putative homology. Although the conjunction and similarity criteria are satisfied, congruence is not. The congruence test is based on a phylogeny derived from 18s rDNA sequence data which show that pterobranchs and lophophorates are in diRerent metazoan subkingdoms and are not closely related. This finding indicates that a lophophore-like apparatus has evolved at least twice in metazoans even though the gross morphology, feeding mechanics, ultrastructure, and ciliary patterns of these organisms are very similar. The high degree of morphological convergence presumably results from similar selective regimes acting on these taxa. These findings indicate that major clades of organisms can evolve in a correspondingfashion despite independent origins.