Early Embryogenesis and Organogenesis in the Annelid Owenia Fusiformis

Abstract

Background : Annelids are a diverse group of segmented worms within Spiralia, whose embryos exhibit spiral cleavage and a variety of larval forms. While most modern embryological studies focus on species with unequal spiral cleavage nested in Pleistoannelida (Sedentaria + Errantia), a few recent studies looked into Owenia fusiformis , a member of the sister group to all remaining annelids and thus a key lineage to understand annelid and spiralian evolution and development. However, the timing of early cleavage and detailed morphogenetic events leading to the formation of the idiosyncratic mitraria larva of O. fusiformis remain largely unexplored.Results : O. fusiformis undergoes equal spiral cleavage where the first quartet of animal micromeres are slightly larger than the vegetal macromeres. Cleavage results in a coeloblastula approximately five hours post fertilization (hpf) at 19 ºC. Gastrulation occurs via invagination and completes four hours later, with putative mesodermal precursors and the chaetoblasts appearing 10 hpf at the dorsoposterior side. Soon after, at 11 hpf, the apical tuft emerges, followed by the first neurons (as revealed by the expression of elav1 and synaptotagmin1 ) in the apical organ and the prototroch by 13 hpf. Muscles connecting the chaetal sac to various larval tissues develop around 18 hpf and by the time the mitraria is fully formed at 22 hpf, there are FMRFamide + neurons in the apical organ and prototroch, the latter forming a prototrochal ring. As the mitraria feeds, it grows in size and the prototroch expands through active proliferation. The larva becomes competent after ~3 weeks post fertilization at 15 ºC, when a conspicuous juvenile rudiment has formed ventrally.Conclusions : O. fusiformis embryogenesis is similar to that of other equal spiral cleaving annelids, supporting that equal cleavage is associated with the formation of a coeloblastula, gastrulation via invagination, and a feeding trochophore-like larva in Annelida. The nervous system of the mitraria larva forms earlier and is more complex than previously recognized and develops from anterior to posterior, which is likely an ancestral condition to Annelida. Altogether, our study identifies the major developmental events during O. fusiformis ontogeny, defining a conceptual framework for future investigations.

Annelid functional genomics reveal the origins of bilaterian life cycles

“…Tables 2-7). Therefore, O. fusiformis genome contains a more complete gene repertoire that those reported for other annelid lineages, which together with its phylogenetic position and conserved early embryogenesis 25,26 makes it a key lineage to reconstruct the evolution of Annelida and Lophotrochozoa generally. Chromosomal linkage of orthologous genes between deuterostomes and protostomes supports the presence of 22 ancestral linkage groups in bilaterians 29,30 .…”

“…Sexually mature Owenia fusiformis Delle Chiaje, 1844 adults were collected from subtidal waters near the Station Biologique de Roscoff and cultured in the lab as described before 25 . In vitro fertilisations and collections of embryonic and larval stages were performed as previously described 25 .…”

“…Sexually mature Owenia fusiformis Delle Chiaje, 1844 adults were collected from subtidal waters near the Station Biologique de Roscoff and cultured in the lab as described before 25 . In vitro fertilisations and collections of embryonic and larval stages were performed as previously described 25 . Capitella teleta Blake, Grassle & Eckelbarger, 2009 was cultured, grown, and sifted, and its embryos and larvae were collected following established protocols 59 .…”

Annelid functional genomics reveal the origins of bilaterian life cycles

“…To estimate the haploid DNA nuclear content of O. fusiformis, we used a flow cytometer Partex CyFlow Space fitted with a Cobalt Samba green laser (532 nm, 100 mW) as described for the annelid Dimorphilus gyrociliatus 25 and adult individuals of Drosophila melanogaster as reference. Additionally, we used Jellyfish v.2.3 61 to count and generate a 31-mer histogram from adaptor-cleaned, short-read Illumina reads (see section below), and GenomeScope 2.0 62 to obtain an in-silico estimation of the genome size and heterozygosity of O. fusiformis.…”

“…In particular, the idiosyncratic oweniid larva, commonly referred to as "mitraria" 21 , has an enlarged pre-oral region and a single tuft of posterior chaetae, as well as a pair of nephridia and a long monociliated ciliary band alike those of phylogenetically distant larvae of deuterostome lineages, such as echinoderms and hemichordates [22][23][24] . Despite these distinctive larval traits, oweniids exhibit many developmental characters considered ancestral to Annelida, and even Lophotrochozoa as a whole 25,26 , as well as similarities in larval molecular patterns with other trochophore and bilaterian larvae 24,25,27,28 . Consequently, whether the mitraria larva is the result of divergent or convergent evolution is unclear, which makes the comparative analysis of this lineage-restricted larva an excellent case study to investigate how larval traits evolve, and thereby formulate and assess scenarios on the origin of animal life cycles.…”