Early embryogenesis and organogenesis in the annelidOwenia 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 °C. 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 °C, 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 recognised 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.

ERK1/2 is an ancestral organising signal in spiral cleavage

“…By 5 hpf however, the vegetal pole exhibits again a symmetrical arrangement (Figure 2B). At 6 hpf, with the onset of gastrulation 29 , di-P-ERK1/2 signal is enriched in seven vegetal cells forming a bilaterally symmetrical pattern that includes one 4q micromere and two blastomeres of each of the other three embryonic quadrants (Figure 2A, Figure S1C). At this stage, the macromeres of the fourth quartet (4Q) have cleaved synchronously into the 5Q and 5q cells and all but the 4q micromere enriched in di-P-ERK1/2 have divided.…”

“…For both drugs, treatment from fertilisation (~0.5 hpf) to 5 hpf, when di-P-ERK1/2 is enriched in 4d, effectively blocks activation of ERK1/2 (Figure 3B; Figures S2A; Table S2) and causes the loss of bilateral symmetry, posterior structures (e.g., chaetae and hindgut) and larval muscles in a dosage-dependent manner up to 100% of the embryos at a 10 µM concentration (Figure 3C; Figure S2B; Table S3, S4). Compared to control samples, 0.5 to 5 hpf treated embryos lack a fully formed apical tuft and apical organ, showing reduced ectodermal expression of the apical organ marker six3/6 3 and just a few apical cells positive for the neuronal marker synaptotagmin-1 (syt-1) 29 (Figure 3D). In addition, treated embryos lack expression of hindgut (cdx) and trunk mesodermal (twist) markers 3 , exhibit expanded expression of the oral ectodermal marker gene gsc 3 around the single gut opening (Figure 3D; Figure S2C), and retain expression of the midgut endodermal marker GATA4/5/6b 3 (Figure S2C).…”

“…In the lab, animals were kept in arti cial seawater (ASW) at 15°C. In vitro fertilizations were conducted as previously described 3,29 and embryos develop in glass bowls with ltered ASW at 19 ºC until the desired embryonic stage.…”

“…1C). This marine annelid belongs to Palaeoannelida -the sister group to all remaining annelids 30 -and exhibits embryonic traits considered ancestral to Annelida 29 .…”

The non-brain anterior nerve center and tentacle crown structure of Owenia borealis (Annelida, Oweniidae): the evolution of the nervous system and tentacles in Bilateria

“…(2-7). Members of the family Oweniidae have many unusual morphological, ultrastructural, and developmental characteristics (8)(9)(10)(11)(12)(13)(14)(15)(16). Oweniids together with magelonids have been recently placed among the Palaeoannelida, a sister group to all remaining annelids (5,7).…”