Article

[Origin and evolution of mammalian sex chromosomes].

Unité d'immunogénétique humaine, Inserm E21, Institut Pasteur, 25, rue du Dr-Roux, 75724 Paris, France.
Comptes Rendus de l Académie des Sciences - Series III - Sciences de la Vie 02/2001; 324(1):1-11. pp.1-11
Source: PubMed

ABSTRACT Mammals present an XX/XY system of chromosomal sex determination, males being the heterogametic sex. Comparative studies of the gene content of sex chromosomes from the major groups of mammals reveal that most Y genes have X-linked homologues and that X and Y share homologous pseudoautosomal regions. These observations, together with the presence of the two homologous regions (pseudoautosomal regions) at the tips of the sex chromosomes, suggest that these chromosomes began as an ordinary pair of homologous autosomes. Birds present a ZW/ZZ system of chromosomal sex determination where females are the heterogametic sex. In this case, avian sex chromosomes are derived from different pairs of autosomes than mammals. The evolutionary pathway from the autosomal homomorphic departure to the present-day heteromorphic sex chromosomes in mammals includes suppression of X-Y recombination, differentiation of the nascent non-recombining regions, and progressive autosomal addition and attrition of the sex chromosomes. Recent results indicate that the event marking the beginning of the differentiation between the extant X and Y chromosomes occurred about 300 million years ago.

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Keywords

300 million years
 
autosomal homomorphic departure
 
avian sex chromosomes
 
chromosomal sex determination
 
Comparative studies
 
evolutionary pathway
 
heterogametic sex
 
homologous autosomes
 
major groups
 
nascent non-recombining regions
 
present-day heteromorphic sex chromosomes
 
progressive autosomal addition
 
Recent results
 
sex chromosomes
 
two homologous regions
 
X-Y recombination
 
XX/XY system
 
Y chromosomes
 
Y share homologous pseudoautosomal regions
 
ZW/ZZ system
 

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