Germ Line Versus Soma in the Transition from Egg to Embryo

Abstract

With few exceptions, all animals acquire the ability to produce eggs or sperm at some point in their lifecycle. Despite this near universal requirement for sexual reproduction, there exists an incredible diversity in germ-line development. For example, animals exhibit a vast range of differences in the timing at which the germ line, which retains reproductive potential, separates from the soma, or terminally differentiated, non-reproductive cells. This separation may occur during embryonic development, after gastrulation, or even in adults, depending on the organism. The molecular mechanisms of germ line segregation are also highly diverse, and intimately intertwined with the overall transition from a fertilized egg to an embryo. The earliest embryonic stages of many species are largely controlled by maternally supplied factors. Later in development, patterning control shifts to the embryonic genome and, concomitantly with this transition, the maternally supplied factors are broadly degraded. This chapter attempts to integrate these processes – germ line segregation, and how the divergence of germ line and soma may utilize the egg to embryo transitions differently. In some embryos, this difference is subtle or maybe lacking altogether, whereas in other embryos, this difference in utilization may be a key step in the divergence of the two lineages. Here we will focus our discussion on the echinoderms, and in particular the sea urchins, in which recent studies have provided mechanistic understanding in germ line determination. We propose that the germ line in sea urchins requires an acquisition of maternal factors from the egg and, when compared to other members of the taxon, this appears to be a derived mechanism. The acquisition is early – at the 32 cell stage – and involves active protection of maternal mRNAs, which are instead degraded in somatic cells with the maternal to embryonic transition. We collectively refer to this model as the Time Capsule method for germ line determination.

Functional equivalence of germ plasm organizers

“…Living systems have the unique ability to reproduce copies of themselves. In animals, the reproductive cells or their precursors, the primordial germ cells (PGCs) are specified by two different modes during embryogenesis [reviewed in 1 ]. In the inductive mode, the embryo generates signals, which specify a subset of cells to differentiate into PGCs.…”

Germline development in amniotes: A paradigm shift in primordial germ cell specification

“…The revision of data available on PGC specification in amniotes sheds doubts on the current tendency to divide embryos in two clear‐cut categories, the ones adopting maternal specification (predetermination) versus the ones employing induction (epigenesis), as previously suggested from studies focussing on non‐amniotes 3, 107, 108. The only amniote species where the molecular mechanism of induction has been dissected is the mouse.…”

“…We propose that even when the early segregation occurs, multipotent progenitors (presumably pPGCs) are established in the embryo, regardless of the defined modality of PGC formation adopted by the species. These multipotent precursors can give rise to the germ line either during embryonic development or adult life 3, 107, 108. The question remains as to when and by which mechanism those pPGCs are established (retention of maternal transcripts, germ plasm, induction etc).…”

“…We define here somatic cells as cells that can no longer become pPGCs or PGCs 3. PGCs and pPGCs are distinct from somatic cells because they contain molecules that inhibit gene expression (transcriptional and translational repression) often concentrated in granules 4, 5, 6.…”

“…The function of germ plasm is still not well understood, but it could confer germline identity by either suppressing somatic fate or promoting germline‐specific activity, such as specific metabolism requirements or post‐transcriptional modifications regulating certain classes of RNAs 6. Interestingly, the existence of maternal determinants seems to correlate with the timing of embryonic genome activation 3.…”