Histone H2A.Z expression in two indirectly developing marine invertebrates correlates with undifferentiated and multipotent cells.

The embryos of indirect developers generate an intermediate larval stage that nourishes the proliferation of undifferentiated multipotent cell precursors in charge of postembryonic adult formation. Multipotency affects the regulation of many genes and seems to be mediated in part by chromatin modification. Chromatin transcriptional properties are regulated by histone modification and by incorporation of peculiar histone variants. The histone variant H2A.Z is associated with transcriptionally competent chromatin and silent genes primed for activation or permanent repression. However, despite the extensive mechanistic characterizations in unicellular eukaryotes, the essential role of the highly conserved H2A.Z variant during animal embryogenesis remains obscure. We show that the expression of H2A.Z in the larvae of two distant indirectly developing marine invertebrates, a polychaete and a sea urchin, remains high in all their embryonic and postembryonic developmentally competent cell precursors, and declines during their differentiation. In particular, the expression in undifferentiated multipotent adult precursors during feeding larval stages in both organisms provides unique insight about its general association with developmental potential. Our experiments confirm previous reports indicating that the expression of H2A.Z is proliferation (DNA synthesis) independent, in contrast with the DNA synthesis dependence of "mainstream" histones. We suggest that similar H2A.Z transcriptional functions previously identified in unicellular organisms also help to maintain an open chromatin state competent for transcriptional-regulatory transactions during metazoan development.