Origin and development of the germ line in sea stars

Abstract

This review summarizes and integrates our current understanding of how sea stars make gametes. Although little is known of the mechanism of germ line formation in these animals, recent results point to specific cells and to cohorts of molecules in the embryos and larvae that may lay the ground work for future research efforts. A coelomic outpocketing forms in the posterior of the gut in larvae, referred to as the posterior enterocoel (PE), that when removed, significantly reduces the number of germ cell later in larval growth. This same PE structure also selectively accumulates several germ-line associated factors – vasa, nanos, piwi – and excludes factors involved in somatic cell fate. Since its formation is relatively late in development, these germ cells may form by inductive mechanisms. When integrated into the morphological observations of germ cells and gonad development in larvae, juveniles, and adults, the field of germ line determination appears to have a good model system to study inductive germ line determination to complement the recent work on the molecular mechanisms in mice. We hope this review will also guide investigators interested in germ line determination and regulation of the germ line in how these animals can help in this research field. The review is not intended to be comprehensive – sea star reproduction has been studied over 100 years and many reviews are comprehensive in their coverage of, for example, seasonal growth of the gonads in response to light, nutrient, and temperature. Rather the intent of this review is to help the reader focus on new experimental results attached to the historical underpinnings of how the germ cell functions in sea stars with particular emphasis to clarify the important areas of priority for future research.

Regeneration in bipinnaria larvae of the bat star Patiria miniata induces rapid and broad new gene expression

“…It was rapidly cleared though from the cells of the posterior enterocoel (PE) cells. The PE is believed to be the site of primordial germ cell formation in P. miniata (Fresques et al, 2014; Inoue, 1992; Inoue and Shirai, 1991; Wessel et al, 2014) and opposite from what is seen in vertebrates, this activity is excluded from the germ line in the sea star. We do not know if the alkaline phosphatase protein, and/or its mRNA, is rapidly turned over in the PE, or if the PE contains some inhibitory activity for the enzyme, but the speed with which the alkaline phosphatase activity changes in the PE suggests a post-transcriptional mechanism.…”

Complexity of Yolk Proteins and Their Dynamics in the Sea Star Patiria miniata

“…Vtg1 mRNA does not appear to be expressed in embryos up to late larva stage whereas substantial, and differential Vtg2 and MYP mRNA expression was surprising. These results are particularly curious since the MYP transcript is present in tissues that become the major rudiment tissues for larval development and for the germ line (Inoue et al ., 1992, Wessel et al ., 2014), and we do not see any protein made from the Vtg2 mRNA (Figure 3). We also do not see any Vtg2 protein even later in larvae (20 days) so we currently have no explanation for any functional role of the Vtg2 mRNA.…”

Somatic cell conversion to a germ cell lineage: A violation or a revelation?

“…The sea star P. miniata (and its close relative P. pectinifera ) makes a pouch off the invaginated endoderm late in gastrulation. This posterior, endodermal evagination pouch (posterior enterocoel [PE]) is believed to be the source of the germline for the adult (Wessel, Fresques, Kiyomoto, Yajima, & Zazueta, 2014). This conclusion is based on two main experimental results.…”