Abstract
We have examined the worldwide distribution of a Y-chromosomal base-substitution polymorphism, the T/C transition at SRY-2627, where the T allele defines haplogroup 22; sequencing of primate homologues shows that the ancestral state cannot be determined unambiguously but is probably the C allele. Of 1,191 human Y chromosomes analyzed, 33 belong to haplogroup 22. Twenty-nine come from Iberia, and the highest frequencies are in Basques (11%; n=117) and Catalans (22%; n=32). Microsatellite and minisatellite (MSY1) diversity analysis shows that non-Iberian haplogroup-22 chromosomes are not significantly different from Iberian ones. The simplest interpretation of these data is that haplogroup 22 arose in Iberia and that non-Iberian cases reflect Iberian emigrants. Several different methods were used to date the origin of the polymorphism: microsatellite data gave ages of 1,650, 2,700, 3,100, or 3,450 years, and MSY1 gave ages of 1,000, 2,300, or 2,650 years, although 95% confidence intervals on all of these figures are wide. The age of the split between Basque and Catalan haplogroup-22 chromosomes was calculated as only 20% of the age of the lineage as a whole. This study thus provides evidence for direct or indirect gene flow over the substantial linguistic barrier between the Indo-European and non-Indo-European-speaking populations of the Catalans and the Basques, during the past few thousand years.
Highlights
The presence of different polymorphic systems with different mutation rates and processes is a particular strength of the Y chromosome and allows us to use Y-chromosome markers “genealogically,” defining lineages (“haplogroups”) with slowly mutating biallelic polymorphisms such as base substitutions, which can be regarded as unique events in human evolution, and examining diversity within these haplogroups, using polymorphisms that mutate more rapidly, such as microsatellites (Kayser et al 1997) and the minisatellite, MSY1 (Jobling et al 1998)
A T/C transition polymorphism 2,627 bp 5 of the start codon of the SRY gene has been described in previous reports (Bianchi et al 1997; Veitia et al 1997); the T allele defines a Y-chromosomal haplogroup that we term “haplogroup 22” and can be conveniently typed in a PCR-RFLP assay, since the T allele alone creates a BsiHKAI site and the C allele alone creates a BanI site
We compared the human sequence with its homologues in the great apes, in an attempt to determine ancestral state definitively
Summary
We have examined the worldwide distribution of a Y-chromosomal base-substitution polymorphism, the T/ C transition at SRY-2627, where the T allele defines haplogroup 22; sequencing of primate homologues shows that the ancestral state cannot be determined unambiguously but is probably the C allele. The presence of different polymorphic systems with different mutation rates and processes is a particular strength of the Y chromosome and allows us to use Y-chromosome markers “genealogically,” defining lineages (“haplogroups”) with slowly mutating biallelic polymorphisms such as base substitutions, which can be regarded as unique events in human evolution, and examining diversity within these haplogroups, using polymorphisms that mutate more rapidly, such as microsatellites (Kayser et al 1997) and the minisatellite, MSY1 (Jobling et al 1998) This alleviates the problem of recurrent mutation at these loci and allows attempts to be made to date haplogroup origins. This constitutes evidence for substantial recent male-mediated gene flow over a major linguistic barrier
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