Genes with spiralian-specific protein motifs are expressed in spiralian ciliary bands.

Spiralia is a large, ancient and diverse clade of animals, with a conserved early developmental program but diverse larval and adult morphologies. One trait shared by many spiralians is the presence of ciliary bands used for locomotion and feeding. To learn more about spiralian-specific traits we have examined the expression of 20 genes with protein motifs that are strongly conserved within the Spiralia, but not detectable outside of it. Here, we show that two of these are specifically expressed in the main ciliary band of the mollusc Tritia (also known as Ilyanassa). Their expression patterns in representative species from five more spiralian phyla—the annelids, nemerteans, phoronids, brachiopods and rotifers—show that at least one of these, lophotrochin, has a conserved and specific role in particular ciliated structures, most consistently in ciliary bands. These results highlight the potential importance of lineage-specific genes or protein motifs for understanding traits shared across ancient lineages. Spiralians have ciliary bands, used for locomotion and feeding, but defining molecular features of these structures are unknown. Here, the authors report a gene, Lophotrochin, that contains a protein domain only found in spiralians, and specifically expressed in diverse ciliary bands across the group, which provides a molecular signature for these structures.