Gene expression during mammalian meiosis.

The expression of a wide variety of genes is developmentally regulated during mammalian meiosis. Drawing mainly on studies in spermatogenesis, this review shows that some of these genes are transcribed exclusively in germ cells, while others are also transcribed in somatic cells. Some of the genes expressed exclusively in spermatogenic cells are unlike any expressed in somatic cells, while others are isologous to genes expressed in somatic cells and are in the same gene family. Some of the developmentally regulated genes also expressed in somatic cells produce spermatogenic cell-specific transcripts, while others produce transcripts that are apparently the same in somatic and germ cells. Possible answers to why so many genes have atypical patterns of expression during meiosis are that: (1) all cell types express certain genes that define their cell type and lineage, (2) spermatogenesis is a developmental process that progresses according to a genetic program directing the sequential and coordinate expression of specific genes, (3) some genes are expressed that encode proteins required for meiosis, (4) some genes are expressed that encode proteins not required until after meiosis, (5) some genes are expressed to compensate for other genes that become inactivated with X chromosome condensation, and (6) it has been suggested that regulation of gene expression becomes leaky during spermatogenesis due to changes in DNA organization, leading to production of irrelevant transcripts. However, it is largely unknown how extrinsic cues from the endocrine system and surrounding somatic cells interact with intrinsic mechanisms of germ cells to activate signal transduction processes regulating transcription during mammalian meiosis.