Phylogeographic analysis of the Y chromosome: new perspectives for the origin of Homo sapiens.

Gruppo di Ricerca: 
Fulvio Cruciani, Beniamino Trombetta, Andrea Massaia, Marco Ippoliti, Eugenia D’Atanasio, Daniele Sellitto, Rosaria Scozzari.

The male specific portion of the human Y chromosome (MSY) is paternally transmitted and haploid. These features together with the absence of recombination make its DNA sequence variation an invaluable tool for the study of modern human evolution. Patrilinearity and haploidy translate into increased levels of population subdivision compared to the autosomes, and the lack of recombination permits the reconstruction of an unequivocal haplogroup phylogeny which can be related to the geographic distribution of the Y haplotypes in an approach known as "phylogeography".

Since the first human Y chromosome phylogenetic tree was published in 2000, the identification of additional variants has lead to increased resolution of the tree (Karafet et al. 2008), but a close reexamination of the backbone and root of the Y chromosome phylogeny has been lacking.

We performed a re-sequencing analysis of the MSY in individuals representative of deep rooting Y haplogroups. We detected a total of 146 biallelic MSY variants, including eight indel polymorphisms and 138 single-nucleotide substitutions, and we used them to reconstruct a new phylogenetic tree (see figure) without taking into account the previous information about phylogenetic relations among the haplogroups we analyzed. There are important changes to the accepted structure of the tree concerning its root and deepest branching. The first split now divides clade A1b from the rest of the tree, with all the remaining haplogroups sharing the derived allele at seven polymorphisms. We dated the most recent common ancestor (MRCA) of the human Y chromosome at 142,000 years ago, which is much earlier than previous estimates. Our dating is easier to reconcile with plausible scenarios of modern human origin. The new structure of the tree also gives a stronger support to the hypothesis of an African origin for modern humans, with four splits leading to African-specific haplogroups (rather than two splits as in the previous phylogeny).

The analysis of 2,204 African Y chromosomes showed that the deepest clades of the revised phylogeny (A1b and A1a, see figure) are only found in the north-western quadrant of Africa, offering new perspectives on the origin of modern humans (Cruciani et al. 2011).

 

References:

Cruciani F. et al. (2011) A revised root for the human Y chromosomal phylogenetic tree: The origin of patrilineal diversity in Africa. Am J Hum Genet 88:814-818.

Karafet T et al. (2008) New binary polymorphisms reshape and increase resolution of the human Y chromosomal haplogroup tree. Genome Res 18:830–838.

Anno del Convegno: 
2011

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