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Ascidian embryos have been studied since the birth of experimental embryology at the end of the 19th century. They represent textbook examples of mosaic development characterized by a fast development with very few cells and invariant cleavage patterns and lineages. Ascidians belong to tunicates, the vertebrate sister group, and their study is essential to shed light on the emergence of vertebrates. Importantly, deciphering developmental gene regulatory networks has been carried out mostly in two of the three ascidian orders, Phlebobranchia and Stolidobranchia. To infer ancestral developmental programs in ascidians, it is thus essential to carry out molecular embryology in the third ascidian order, the Aplousobranchia. Here, we present genomic resources for the colonial aplousobranch Clavelina lepadiformis : a transcriptome produced from various embryonic stages, and an annotated genome. The assembly consists of 184 contigs making a total of 233.6 Mb with a N50 of 8.5 Mb and a L50 of 11. The 32,318 predicted genes capture 96.3% of BUSCO orthologs. We further show that these resources are suitable to study developmental gene expression and regulation in a comparative framework within ascidians. Additionally, they will prove valuable for evolutionary and ecological studies. [ABSTRACT FROM AUTHOR]

Didemnidae is the largest family of tunicates within Aplousobranchia, with about 578 species. This family comprises eight genera: Atriolum, Clitella, Didemnum, Diplosoma, Leptoclinides, Lissoclinum, Polysyncraton, and Trididemnum. Morphological and molecular data suggest that Didemnidae is monophyletic, but the monophyly of each didemnid genus and their phylogenetic relationships are still poorly understood. This study aimed to evaluate the monophyly of six of the eight didemnid genera and assess their phylogenetic relationships based on mitochondrial (COI) and nuclear (18S) sequences. All genera were recovered as monophyletic except Trididemnum. Didemnum comprises two clades that differ one from the other by the presence of an atrial lip, the number of testicular lobes, and the number of ampullae in larvae. These morphological differences indicate that Didemnum could be split into two genera. Morphological evidence and previous taxonomists have suggested a close relationship between Didemnum and Polysyncraton as well as between Diplosoma and Lissoclinum. We re-evaluate these hypotheses, which are supported by our 18S sequences and concatenated data. [ABSTRACT FROM AUTHOR]

Many sessile, suspension-feeding marine invertebrates mate by spermcasting: aquatic sperm are spawned and gathered by conspecific individuals to fertilize eggs that are generally retained during development. In two phylogenetically distant examples, a cheilostome bryozoan and an aplousobranch ascidian, the receipt of allosperm has previously been shown to alter sex allocation by triggering female investment in eggs and brooding. Here we report experiments demonstrating that two species of cyclostome bryozoan also show restrained female investment in the absence of mating opportunity. In Tubulipora plumosa, the production of female zooids and progeny is much reduced in reproductive isolation. In Filicrisia geniculata, development of distinctive female zooids (gonozooids) begins but halts in the absence of mating opportunity, and no completed gonozooids or progeny result. Reduced female investment in the absence of a mate thus occurs in at least two orders of Bryozoa, but significant differences in detail exist and the evolutionary history within the phylum of the mechanism(s) by which female investment is initiated might be complex. The broadening taxonomic spectrum of examples where female investment appears restrained until allosperm becomes available may signify a general adaptive strategy among outcrossing modular animals, analogous to similarly adaptive sex allocation typical of many flowering plants. [ABSTRACT FROM AUTHOR]