Early mesodermal expression of Hox genes in the polychaete Alitta virens (Annelida, Lophotrochozoa)

Construction of a massive genetic resource by transcriptome sequencing and genetic characterization of Megasyllis nipponica (Annelida: Syllidae)

Understanding the processes and consequences of the morphological diversity of organisms is one of the major goals of evolutionary biology. Studies on the evolution of developmental mechanisms of morphologies, or evo-devo, have been extensively conducted in many taxa and have revealed many interesting phenomena at the molecular level. However, many other taxa exhibiting intriguing morphological diversity remain unexplored in the field of evo-devo. Although the annelid family Syllidae shows spectacular diversity in morphological development associated with reproduction, its evo-devo study, especially on molecular development, has progressed slowly. In this study, we focused on Megasyllis nipponica as a new model species for evo-devo in syllids and performed transcriptome sequencing to develop a massive genetic resource, which will be useful for future molecular studies. From the transcriptome data, we identified candidate genes that are likely involved in morphogenesis, including genes involved in hormone regulation, sex determination and appendage development. Furthermore, a computational analysis of the transcriptome sequence data indicated the occurrence of DNA methylation in coding regions of the M. nipponica genome. In addition, flow cytometry analysis showed that the genome size of M. nipponica was approximately 524 megabases. These results facilitate the study of morphogenesis in molecular terms and contribute to our understanding of the morphological diversity in syllids.

Monsters reveal patterns: bifurcated annelids and their implications for the study of development and evolution

During recent decades, the study of anatomical anomalies has been of great relevance for research on development and its evolution. Yet most animal groups have never been studied under this perspective. In annelids, one of the most common and remarkable anomalies is anteroposterior axis bifurcation, that is animals that have two or more heads and/or tails. Bifurcated annelids were first described in the 18th century and have been occasionally reported since then. However, these animals have rarely been considered other than curiosities, one-off anomalies, or monsters, and a condensed but comprehensive analysis of this phenomenon is lacking. Such an analysis of the existing knowledge is necessary for addressing the different patterns of annelid bifurcation, as well as to understand possible developmental mechanisms behind them and their evolution. In this review we summarize reports of annelid bifurcation published during the last 275 years and the wide variety of anatomies they present. Our survey reveals bifurcation as a widespread phenomenon found all over the annelid tree. Moreover, it also shows that bifurcations can be classified into different types according to anatomy (lateral versus dorsoventral) or developmental origin (embryonic versus postembryonic, the latter occurring in relation to regeneration, reproduction, or growth). Regarding embryos, three different types of bifurcation can be found: conjoined twins (in clitellates); Janus embryos (two posterior ends with a single head which shows duplicated structures); and duplicitas cruciata embryos (with anterior and posterior bifurcation with a 90° rotation). In adults, we show that while lateral bifurcation can result in well-integrated phenotypes, dorsoventral bifurcation cannot since it requires the discontinuity of at least some internal organs. The relevance of this distinction is highlighted in the case of the Ribbon Clade, a group of syllid annelids in which some species reproduce by collateral and successive gemmiparity (which involves dorsoventral bifurcation), while others grow by branching laterally. Although most known cases of bifurcation came from accidental findings in the wild or were unintentionally produced, experimental studies resulting in the induction of bifurcation of both embryos and adults are also reviewed. In embryos, these experimental studies show how mechanical or chemical disruption of the zygote can result in bifurcation. In adults, the ventral nervous system and the digestive tract seem to play a role in the induction of bifurcation. Based on the reviewed evidence, we argue that the long-forgotten study of annelid developmental anomalies should be incorporated into the growing field of annelid EvoDevo and examined with modern techniques and perspectives.