Low levels of nucleotide diversity in mammalian Y chromosomes.
Sex chromosomes provide a useful context for the study of the relative importance of evolutionary forces affecting genetic diversity. The human Y chromosome shows levels of nucleotide diversity 20% that of autosomes, which is significantly less than expected when differences in effective population size and sex-specific mutation rates are taken into account. To study the generality of low levels of Y chromosome variability in mammalian genomes, we investigated nucleotide diversity in intron sequences of X (1.1-3.0 kb) and Y (0.7-3.5 kb) chromosome genes of five mammals: lynx, wolf, reindeer, cattle, and field vole. For all species, nucleotide diversity was found to be lower on Y than on X, with no segregating site observed in Y-linked sequences of lynx, reindeer, and cattle. For X chromosome sequences, nucleotide diversity was in the range of 1.6 x 10(-4) (lynx) to 8.0 x 10(-4) (field vole). When differences in effective population size and the extent of the male mutation bias were taken into account, all five species showed evidence of reduced levels of Y chromosome variability. Reduced levels of Y chromosome variability have also been observed in Drosophila and in plants, as well as in the female-specific W chromosome of birds. Among the different factors proposed to explain low levels of genetic variability in the sex-limited chromosome (Y/W), we note that selection is the only factor that is broadly applicable irrespective of mode of reproduction and whether there is male or female heterogamety.
Department of Evolutionary Biology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.
Linda
L
Hellborg
Hans
H
Ellegren
D000818:Animals; D017931:DNA Primers; D005060:Europe; D014644:Genetic Variation; D007438:Introns / Q000235:genetics; D008297:Male; D008322:Mammals / Q000235:genetics*; D009154:Mutation / Q000235:genetics; D009656:North America; D020641:Polymorphism, Single Nucleotide; D017422:Sequence Analysis, DNA; D014960:X Chromosome / Q000235:genetics*; D014998:Y Chromosome / Q000235:genetics*
D017931:DNA Primers