An abundance of X-linked genes expressed in spermatogonia

Spermatogonia are the self-renewing, mitotic germ cells of the testis from which sperm arise by means of the differentiation pathway known as spermato genesis'. By contrast with hematopoietic and other mammalian stem-cell popu lations. which have been subjects of intense molecular genetic investigatio n, spermatogonia have remained largely unexplored at the molecular level. H ere we describe a systematic search for genes expressed in mouse spermatogo nia, but not in somatic tissues. We identified 25 genes (19 of which are no vel) that are expressed in only mate germ cells. Of the 25 genes, 3 are Y-l inked and 10 are X-linked. If these genes had been distributed randomly in the genome, one would have expected zero to two of the genes to be X-linked . Our findings indicate that the X chromosome has a predominant role in pre -meiotic stages of mammalian spermatogenesis. We hypothesize that the X chr omosome acquired this prominent role in male germ-cell development as it ev olved from an ordinary, unspecialized autosome.