What molecular phylogenies tell us about the evolution of larval forms

Abstract

SYNOPSIS. There is interest in assembling a robust hypothesis of animal relationships based on molecular and other character-based evidence, but even if a high degree of phylogenetic resolution is available, there remain challenging problems for postulating ancestral larval traits. This distinction between hypotheses of genealogies and our knowledge of specific traits is illustrated with specific examples of the portion of variable larval traits that are homoplastic (i.e., they require convergences, parallelisms, or character reversals) with respect to specific molecular-based genealogical hypotheses. Corresponding molecular studies suggest (1) maximal incongruity in larval form and metamorphosis for extant echinoderm classes, (2) convergences in larval size and form associated with coloniality in ascidians, (3) multiple losses of the locomotory larval tail in molgulid ascidians, (4) multiple losses of larval feeding and gain of apomictic parthenogenesis within a genus of bivalves, (5) multiple losses of larval feeding in echinoids, (6) alternative explanations of the distribution of feeding and non-feeding larvae among gastropods, and (7) recent modifications in embryonic and larval development of echinoids following prolonged stasis. These examples show that inferences from phylogenetic studies will ultimately be limited by the extent to which homoplasy and polarity can be unambiguously assessed for larval traits. These limitations are illustrated by alternative hypotheses for larval trait synapomorphies among phyla, evolution of feeding with opposed prototrochal and metatrochal ciliary bands, and the retention or reacquisition of the locomotory nauplius of the Euphausiacea and Dendrobranchiata. Inferences on the evolution of larval traits require other sorts of evidence, perhaps including information on the evolution of genes that play important roles in morphogenesis and their sites of expression. ©1994 by the American Society of Zoologists.