R. Thomson et al.
Recent common ancestry of human Y chromosomes:
Evidence from DNA sequence data.
Proc. Natl. Acad. Sci. US 20 (June 2000) 97: 7360.
Abstract
- During the past 10 years, DNA polymorphisms have been widely used to reconstruct
human evolutionary history. Mitochondrial DNA originally was used for this
purpose, because the high mutation rate of mitochondrial DNA produced numerous
polymorphisms and the absence of recombination facilitated their interpretation.
In male lineages, the Y chromosome shares some of these properties, e.g.,
uniparental inheritance and absence of recombination (in the nonrecombining
part), but until recently, studies of the Y chromosome have been hampered
by the scarcity of DNA sequence polymorphisms.
- The authors analyzed a data set of DNA sequence variation at three Y chromosome
genes (SMCY, DBY, DFFRY) in a worldwide sample of human Y chromosomes. Between
53 and 70 chromosomes were fully screened for sequence variation at each locus
by a high- performance liquid chromatography method. The sum of the lengths
of the 3 genes is 64,120 base pairs. The authors used these data to study
the ancestral genealogy of human Y chromosomes, with a particular focus on
estimating the expected time to the most recent common ancestor and the expected
ages of certain mutations with interesting geographic distributions.
- The geographic variation distribution was found similar to that obtained
for other gene loci, but the expected time to the most recent common ancestor
is remarkably short, on the order of 50,000 years. Thus, although previous
studies have noted that Y chromosome variation shows extreme geographic structure,
the authors suggest that the spread of Y chromosomes out of Africa is much
more recent than previously believed. The authors also suggest that their
data indicate substantial population growth in the effective number of human
Y chromosomes.
- Commenting on this work, Jaume Bertranpetit (Universidad Pompeu Fabra Barcelona,
ES) states as follows: "Inferences from molecules to populations are not straightforward,
and there have been recurrent worries [about] what was being analyzed, either
the genes or genomic regions on one hand, or the individuals, populations,
or species on the other. There have been worries concerning the accuracy of
our knowledge of genome dynamics, worries concerning the ability and power
to detect specific processes and disentangle cases where more than one mechanism
may have produced similar genetic patterns, and worries concerning the appropriateness
of evolutionary models needed for the inference. And finally, there have been
worries from anthropologists who do not perceive the interface between the
evolutionary biology of a species and that of tiny fragments of DNA, usually
in noncoding regions, worries surrounding a fast- developing field, heir to
classical population genetics, with brilliant novelties but also eager to
get headlines."
In this context, the term "polymorphism" refers to a naturally occurring variation
in the normal nucleotide sequence of the genome within individuals in a population.
Variations are denoted as polymorphisms only if they cannot be accounted for by
recurrent mutation and occur with a frequency of at least about 1 percent.
The term "recombination", in this context, refers to the occurrence
of progeny with combinations of genes other than those that occurred in the parents,
the occurrence due to independent assortment or "crossing over".
The Y chromosome is one of the two chromosomes that determine sex in many animals,
including humans, and it carries mostly male-specific genes.
Contact: Marcus W. Feldman <marc@charles.stanford.edu>
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