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Germ cell origins

Germ cell origins


The separation of the germ line from the soma is a classic concept in animal biology, and depending on species is thought to involve either fate determination by maternally localised germ plasm (preformation) or induction by intracellular signalling (epigenesis).
 Hydrozoan cnidarians do not fit with this classical picture. The somatic and germ cell lineages are not separated during embryonic development , and there is no dedicated germ line. Instead, germ cells derive from a population of multi-potent stem cells called i-cells (“interstitial cells”), which  generate not only germ cells but also nerve cells, nematocytes (stinging cells ) and gland cells throughout adult life. .
In Clytia, we have found that i-cells in larvae and adult medusae, from which germ cells derive, express a set of conserved germ cell markers: Vasa, Nanos1, Piwi and PL10.  (Leclère et al 2012) In situ hybridization analyses unexpectedly revealed maternal mRNAs for all these genes highly concentrated in a germ plasm–like region at the egg animal pole and inherited by a the i-cell lineage strongly suggestive i-cell fate determination by preformation (Fig. A). On the other hand experimental evidence for parallel epigenetic mechanisms when cleavage stage embryos were separated into single blastomeres or blastula stage embryos into halves,  efficient i-cell formation occurred in both animal and vegetal–derived fragments (Fig. B).

 

 

 

These observations suggest that in Clytia embryos, i-cell-inherited maternal germ plasm and the potential for their formation by epigenesis co-exist, challenging that the common vision of a preformation-epigenesis dichotomy.
We propose that in Clytia as in some annelid and mollusc species, inherited germ plasm favours (but is not essential for) development of a multipotent germ cell lineage, not a dedicated germ line.

 

 

Evelyn Houliston - 26/09/17

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