Meeting Abstract
Currently, the phylogenetic positions of Xenacoelomorpha as a sister group to Nephrozoa (deuterstomes, ecdysozoans, and spiralians), and of many phyla within the spiralian clade including chaetognaths are being scrutinized. Our previous study suggested that the tektin gene complements could add to this debate. Tektins are a family of ciliary coiled-coil proteins. Phylogenetic inferences of tektin evolution suggested a series of ancient duplications that generated one, two, four, and five tektin genes in the common ancestor of holozoan (choanoflagellates + metazoans), metazoan, nephrozoan, and spiralian, respectively. Many extant deuterostomes like echinoderms and cephalochordates, and some ecdysozoans like priapulids retained four genes, tektin-2, -1, -4, and -3/5. Several spiralian species including some annelids, molluscs, and brachiopods retain an ancestral spiralian tektin gene complement of five tektin genes including two distinct tektin-3/5A and 3/5B genes from an ancient duplication of the nephrozoan tektin-3/5 gene at the base of the spiralian clade. Do xenacoelomorphs and chaetognaths have a metazoan, nephrozoan, or spiralian tektin gene complement? To answer this question, we surveyed RNA-seq data of dozens of invertebrate taxa including 12 xenacoelomorph and 10 chaetognath species. All xenacoelomorph species share a unique bilaterian tektin complement of three tektin genes (-2, -1, and 4/3/5) with no indication for distinct nephrozoan signature tektin-4 and tektin-3/5 genes, lending support to their position as sister group to nephrozoans. The tektin gene complements in chaetognaths contain the unique spiralian tektin genes 3/5A and 3/5B, thus supporting their inclusion within a spiralian clade.
