Single-cell atlases of two lophotrochozoan larvae highlight their complex evolutionary histories

Abstract

Pelagic larval stages are widespread across animals, yet it is unclear if larvae were present in the last common ancestor of animals or whether they evolved multiple times due to common selective pressures. Many marine larvae are at least superficially similar, they are small, swim through beating of ciliated bands and sense the environment with an apical organ structure. To understand these similarities, we have generated single cell atlases for marine larvae from two animal phyla and have compared their cell types. We found clear similarities among ciliary band cells and neurons of the apical organ in the two larvae pointing to possible homology of these structures suggesting a single origin of larvae within the clade analysed here (Lophotrochozoa). We also find several clade specific innovations in each larva, including distinct myocytes and shell gland cells in the oyster larva. Oyster shell gland cells express many novel genes which have made previous gene age estimates for trochophore larvae too young.