Abstract
Meiosis is a key event in the manufacturing of an oocyte. During this process, the oocyte creates a set of unique chromosomes by recombining paternal and maternal copies of homologous chromosomes, and by eliminating one set of chromosomes to become haploid. While meiosis is conserved among sexually reproducing eukaryotes, there is a bewildering diversity of strategies among species, and sometimes within sexes of the same species, to achieve proper segregation of chromosomes. Here, we review the very first steps of meiosis in females, when the maternal and paternal copies of each homologous chromosomes have to move, find each other and pair. We explore the similarities and differences observed in C. elegans, Drosophila, zebrafish and mouse females.
Highlights
The oocyte is the final product of germ cell differentiation in females
Five stages are distinguished during prophase I: leptotene is when homologues start to condense and become visible upon staining; zygotene is when homologous chromosomes start to synapse; pachytene marks the completion of synapsis along the full length of every chromosome pair; at diplotene, the synaptonemal complex (SC) depolymerises and homologues remain linked only by chiasmata; diakinesis is the final stage of prophase I, when chromosomes are condensed enough to make chiasmata detectable
While in mutants for a single pairing centers (PCs)-binding protein, there is no synapsis for the corresponding pair(s) of homologous chromosomes; depletion of all four PC-proteins does not prevent the polymerisation of the synaptonemal complex
Summary
The oocyte is the final product of germ cell differentiation in females. In females, depending on the species, all or only a few germ cells complete meiosis and differentiate as oocytes. The remaining germ cells become support cells for nurturing the oocyte In mammals, such as humans and mice, these early stages of differentiation are limited to fetal ovaries, which makes their study challenging, while in C. elegans, Drosophila and zebrafish, oocytes are produced throughout adulthood. Pairing defines the association of homologous chromosomes It can be initiated by a looser coupling or alignment of homologues. Five stages are distinguished during prophase I: leptotene is when homologues start to condense and become visible upon staining; zygotene is when homologous chromosomes start to synapse; pachytene marks the completion of synapsis along the full length of every chromosome pair; at diplotene, the SC depolymerises and homologues remain linked only by chiasmata; diakinesis is the final stage of prophase I, when chromosomes are condensed enough to make chiasmata detectable
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