Phylogeny and evolution of developmental mode in temnopleurid echinoids.

The phylogenetic relationships of 24 nominal species of temnopleurid echinoid were established using molecular and morphological data sets. The analysis combined sequence data from mitochondrial 16S rRNA and cytochrome c oxidase subunit I genes and the nuclear 18S-like small subunit rRNA gene with morphological data concerning coronal, lantern, spine, and pedicellarial traits. All four data sets contain similar phylogenetic information, although each provides support at a different taxonomic level. Two data congruence tests (Templeton's test and the incongruence length difference test) suggested no significant heterogeneity between the data sets, and all data were combined in a total evidence analysis. The resulting well-resolved phylogeny suggests that Microcyphus, Amblypneustes, and Holopneustes are not monophyletic genera, and that Temnopleurus (Temnopleurus) and Temnopleurus (Toreumatica) are not closely related and should not be regarded as subgenera. In contrast to previous morphological analyses, Mespilia is found to be more closely related to Temnotrema and Toreumatica than it is to Microcyphus. The phylogeny was used to test a series of hypotheses about the evolution of developmental patterns. All species of Amblypneustes, Holopneustes, and Microcyphus are lecithotrophic, and many of these taxa are restricted to southern Australia. Planktotrophy is the ancestral condition for the temnopleurids, and the 11 instances of lecithotrophic nonplanktotrophy in this clade can be accounted for by a single developmental transition that occurred an estimated 4.4-7.4 million years ago, apparently before the migration of Microcyphus to southern Australia. The switch to a nonplanktotrophic mode of development is unidirectional with no evidence of reversals.