The MAPK cascade in equally cleaving spiralian embryos

Abstract

Spiralian development is shared by several protostome phyla and characterized by regularities in early cleavage, fate map, and larva. Experimental evidence from multiple spiralian species implicates cells in the D quadrant lineage as the organizer of future axial development of the embryo. However, the mechanisms by which the D quadrant is specified differ between species with equal and unequal spiral cleavage. Equally cleaving mollusc embryos establish the D quadrant via cell-cell interactions between the micromeres and macromeres at the 24- to 36-cell stage. In unequally cleaving embryos, the D quadrant is established at the 4-cell stage via asymmetries in the first 2 cell divisions. We have begun to explore the molecular mechanisms of D quadrant patterning in spiralians. Previously, we showed that, in the unequally cleaving embryo of the mollusc Ilyanassa obsoleta, the MAPK pathway is activated and functionally required in 3D and also in the micromeres known to require a signal from 3D. Here, we examine the role of MAPK signaling in 4 spiralians with equal cleavage. In 3 equally cleaving molluscs, the chiton Chaetopleura, the limpet Tectura, and the snail Lymnaea, the MAPK pathway is activated in the 3D cell but not in the overlying micromeres. In the equally cleaving embryo of the polychaete annelid Hydroides, MAPK activation was not detected in the 3D macromere but was observed in one of its daughter cells, 4d. In addition, inhibiting Tectura MAPK activation disrupts differentiation of 3D and cells induced by it, supporting a functional role for MAPK in axis specification in equally cleaving spiralians. Thus, MAPK signaling may have a conserved role in the D quadrant organizer cell 3D in molluscs. However, there have been at least 2 evolutionary changes in the activation of the MAPK pathway during spiralian evolution. MAPK function in the Ilyanassa micromeres is a recent cooption and, since the divergence of annelids and molluscs, there has been a shift in onset of MAPK activation between 3D and 4d. We propose that this latter shift correlates with a change in the timing of specification of the secondary embryonic axis.

Development of a feeding trochophore in the polychaete Hydroides elegans

“…Polychaetes lacking larval stages or with non-feeding trochophores develop through unequal cleavage, the D blastomere of the 4-cell embryo being larger than the A, B and C blastomeres (Anderson, 1966). D-quadrant specification in Hydroides involves activation of the MAPK pathway, which is also activated in other equal-and unequal-cleaving spiralians (Lambert and Nagy, 2003). Subsequently, the transcription factor Tbx2/3 is expressed in dorsal cells of all three germ layers (Arenas-Mena, 2013) in a pattern generally similar to the expression of its sea urchin ortholog, which has a dorsal specification role (Gross, 2003).…”

Brefeldin A or monensin inhibits the 3D organizer in gastropod, polyplacophoran, and scaphopod molluscs

“…For all three radialization techniques, there is a loss of bilateral cleavage in the embryo, indicating a similar inhibition of 3D's organizing activity. However, details of U0126-radialized larval development differ between studies of closely related taxa (Lambert and Nagy 2003;Lartillot et al 2002b), and in all species outside of M. muscosa, gastrulation and the body plan remain largely undocumented. A more detailed analysis of BFA-, monensin-, and U0126-radialized development in a single species and across taxa is required to effectively compare the different species and techniques and, ultimately, to understand the organizing function of 3D.…”

“…Discrepancies arise at later stages ; for instance, we did not observe quadri-radially arranged shell gland and stomodea invaginations, but these discrepancies may reflect differences in the larval stages examined and not actual differences in radialized development. It is unclear how BFA-and monensin-radialized development in different molluscs compares to UOl26-radialized development in equal-and unequal-cleaving gastropods, including P. vulgata (Lambert and Nagy 2001;Lambert and Nagy 2003;Lartillot et al 2002b). For all three radialization techniques, there is a loss of bilateral cleavage in the embryo, indicating a similar inhibition of 3D's organizing activity.…”

A twisting story: how a single gene twists a snail? Mechanogenetics

“…Whether the contacts trigger a cell to become 3D or already specified macromere 3D moves to the position is still a matter of debate. This series of events appears to trigger MAPK activation within the 3D macromere (Henry & Perry, 2008), and then MAPK acts upstream of Nodal gene (Lambert & Nagy, 2003). We are currently analysing the molecular processes of these events.…”