Evolution of Tunicate lifestyles shaped by Myosin heavy chain gene duplications, losses, and the diversification of tail muscle cell identities in Oikopleura dioica

Abstract

Tunicates offer a suitable system to study the evolutionary mechanisms underlying lifestyle transitions, ranging from the biphasic lifestyle of ascidians alternating swimming larva with sessile adults, to the fully free-swimming appendicularians. The appendicularian Oikopleura dioica, despite having only ten pairs of tail muscle cells, roughly half than in ascidian larvae, exhibits a remarkably rich repertoire of behaviours required for house inflation, swimming, nodding, and the rhythmic water pumping through the house. This apparent paradox suggests that functional diversification might have arose not from increased cell number, but from molecular specialization within a minimal musculature. To address this question, here, we reconstruct the evolution of the Myosin class II heavy-chain (Myh) gene family across tunicates and generated a developmental expression atlas of all O. dioica Myh genes. Phylogenetic analysis reveals that the cardio-paraxial Myh subfamily duplicated into two tunicate-specific subfamilies, followed by independent bursts of paralogue duplications in ascidians and appendicularians. In O. dioica, two distinct Myh-Scp-Tb paralogs show cardiac expression, and surprisingly, combinations of different Myh-Scp-Tb paralogues define multiple muscle cell identities along the tail. The innovation of this anteroposterior muscle regionalization associated to the Myh-Scp-Tb expansion provides a plausible mechanism for fine-scale modulation of contractile properties within a minimal musculature, helping to resolve the paradox of "fewer cells, but more contractile properties" in appendicularian tail movements. Conversely, the loss of the Myh-Sj/bw gene, which in ascidians is expressed with post-metamorphic body-wall and siphon muscles, is consistent with a pattern of regressive evolution associated to the absence of trunk muscles in O. dioica. This finding supports the view that the appendicularian lifestyle is a secondary derived condition, and that the last common ancestor of tunicates likely possessed an ascidian-like biphasic lifestyle. Together, our findings indicate that both gene-family duplication and gene loss have shaped the evolution of appendicularian tail muscle, enabling the emergence of complex tail-driven behaviours, and offering new insights into the ancestral lifestyle of tunicates.