Gastrulation occurs in multiple phases at two distinct sites in Latrodectus and Cheiracanthium spiders

Abstract

BackgroundThe longstanding canonical model of spider gastrulation posits that cell internalization occurs only at a unitary central blastopore; and that the cumulus (dorsal organizer) arises from within the early deep layer by cell–cell interaction. Recent work has begun to challenge the canonical model by demonstrating cell internalization at extra-blastoporal sites in two species (Parasteatodatepidariorum and Zygiella x-notata); and showing in Zygiella that the prospective cumulus internalizes first, before other cells are present in the deep layer. The cell behaviors making up spider gastrulation thus appear to show considerable variation, and a wider sampling of taxa is indicated.ResultsWe evaluated the model in three species from two families by direct observation of living embryos. Movements of individual cells were traced from timelapse recordings and the origin and fate of the cumulus determined by CM-DiI labeling. We show that there are two distinct regions of internalization: most cells enter the deep layer via the central blastopore but many additional cells ingress via an extra-blastoporal ring, either at the periphery of the germ disc (Latrodectus spp.) or nearer the central field (Cheiracanthium mildei). In all species, the cumulus cells internalize first; this is shown by tracing cells in timelapse, histology, and by CM-DiI injection into the deep layer. Injection very early in gastrulation labels only cumulus mesenchyme cells whereas injections at later stages label non-cumulus mesoderm and endoderm.ConclusionsWe propose a revised model to accommodate the new data. Our working model has the prospective cumulus cells internalizing first, at the central blastopore. The cumulus cells begin migration before other cells enter the deep layer. This is consistent with early specification of the cumulus and suggests that cell–cell interaction with other deep layer cells is not required for its function. As the cumulus migrates, additional mesendoderm internalizes at two distinct locations: through the central blastopore and at an extra-blastoporal ring. Our work thus demonstrates early, cell-autonomous behavior of the cumulus and variation in subsequent location and timing of cell internalization during gastrulation in spiders.Electronic supplementary materialThe online version of this article (doi:10.1186/s13227-015-0029-z) contains supplementary material, which is available to authorized users.

A novel role forEts4in axis specification and cell migration in the spiderParasteatoda tepidariorum

“…It has been suggested that the cumulus cells of many higher spiders are specified during early embryogenesis and are not secondarily induced after the formation of the germ-disc [13]. Pt-Ets4 marks the cells of the developing primary thickening (st. 3 and 4) and of the migrating cumulus.…”

“…However it is not clear whether all or only a subset of the cells of the primary thickening give rise to the cumulus, or if cumulus cells arise from subsequent cell invagination at the site of the primary thickening [4, 5]. Cell tracing [11, 13], as well as the expression of the endodermal marker forkhead [14] within the primary thickening/cumulus cells led to the suggestion that the primary thickening/cumulus cells are central endodermal cells [8, 14]. However, these studies could not completely rule out that the labeled cumulus cells develop into cells of the visceral mesoderm [13].…”

“…Cell tracing [11, 13], as well as the expression of the endodermal marker forkhead [14] within the primary thickening/cumulus cells led to the suggestion that the primary thickening/cumulus cells are central endodermal cells [8, 14]. However, these studies could not completely rule out that the labeled cumulus cells develop into cells of the visceral mesoderm [13].…”

A novel role for Ets4 in axis specification and cell migration in the spider Parasteatoda tepidariorum

“…However, it is not clear whether all or only a subset of the cells of the primary thickening give rise to the cumulus, or if cumulus cells arise from subsequent cell invagination at the site of the primary thickening ( Oda and Akiyama-Oda, 2008 ; Akiyama-Oda and Oda, 2003 ). Cell tracing ( Holm, 1952 ; Edgar et al, 2015 ), as well as the expression of the endodermal marker forkhead ( Oda et al, 2007 ) within the primary thickening/cumulus cells led to the suggestion that the primary thickening/cumulus cells are central endodermal cells ( Hilbrant et al, 2012 ; Oda et al, 2007 ). However, these studies could not completely rule out that the labeled cumulus cells develop into cells of the visceral mesoderm ( Edgar et al, 2015 ).…”

“…Cell tracing ( Holm, 1952 ; Edgar et al, 2015 ), as well as the expression of the endodermal marker forkhead ( Oda et al, 2007 ) within the primary thickening/cumulus cells led to the suggestion that the primary thickening/cumulus cells are central endodermal cells ( Hilbrant et al, 2012 ; Oda et al, 2007 ). However, these studies could not completely rule out that the labeled cumulus cells develop into cells of the visceral mesoderm ( Edgar et al, 2015 ).…”