RECOMBINATION AND MAMMALIAN GENOME EVOLUTION

Several lines of evidence are presented which suggest that sequence G + C content and recombination frequency are related in mammals: (i) ch romosome G + C content is positively correlated to chiasmata density; (ii) the non-pairing region of the Y chromosome has one of the lowest G + C contents of any chromosomal segment; (iii) a reduction in the ra te of recombination at several loci is mirrored by a decrease in G + C content; and (iv) when compared with humans, mice have a lower varian ce in chiasmata density which is reflected in a lower variance in G C content. The observed relation between recombination frequency and s equence G + C content provides an elegant explanation of why gene dens ity is higher in G + C rich isochores than in other parts of the genom e, and why long interspersed elements (LINES) are exclusive to G + C p oor isochores. However, the cause of the relation is as yet unknown. S everal possibilities are considered, including gene conversion.