264. Primordial germ cell specification in a marsupial, the tammar wallaby

Abstract

Primordial germ cells (PGCs) are the precursors of the gametes. In the mouse, PGCs are specified within the proximal epiblast in response to signals from the extraembryonic membranes during early gastrulation. Epiblast cells competent to form PGCs express Ifitm3. A subset of these cells then express Blimp1, a marker of PGC precursors. Once lineage-restricted, PGCs express Stella. Germ cells entering the gonads express VASA protein, which is a component of the germ plasm in animals in which germ cells are specified by the inheritance of maternal determinatives. Almost all of the research on mammalian PGC specification has used the mouse as a model and it is tacitly assumed that findings in the mouse will apply to mammals in general. We are using the tammar wallaby as a marsupial model for PGC specification. Eutherians and marsupials diverged 125–148 million years ago, so comparisons between the two will provide insights into the evolution of the control of mammalian PGC specification. There are IFITM clusters in both the human (chromosome 11) and mouse (chromosome 7). In the mouse, IFITM1, 2 and 3 are expressed in PGCs, whereas IFITM4 and 5 are not (1). Only one IFITM member, IFITM5, is annotated in the opossum Ensemble database. We have cloned one tammar IFITM member and identified at least one other putative member in the tammar trace archive database. We have also cloned tammar BLIMP1 and VASA, both of which show high sequence conservation with other mammals. RT–PCR profiles for both genes during tammar gastrulation are similar to those for the mouse. In contrast, no marsupial STELLA orthologueue has been identified in either the opossum or tammar genomes. These findings suggest that some but not all of the signals and mechanisms involved in eutherian PGC specification are also applicable to marsupials.

(1) Lange UC, Saitou M, Western P, Barton SC and Surani MA (2003) BMC Dev. Biol. Epub 2003 Mar 19