Molecular Biology and Evolution, Vol 16, 1633-1640, Copyright © 1999 by Society for Molecular Biology and Evolution
J Jaruzelska, E Zietkiewicz and D Labuda
DNA variability was investigated in the last intron of the Y-chromosome-
specific zinc finger gene, ZFY, and its X homolog on Xp21.3, ZFX. No
polymorphisms were found in the 676-bp ZFY segment in a sample of 205
world-wide-distributed Y chromosomes, other than a solitary nucleotide
variant in one individual (nucleotide diversity pi = 0.0014%). In contrast,
10 segregating sites (pi = 0.082%) were identified within 1,089 bp of the
ZFX sequence in a sample of 336 X chromosomes. Four of these polymorphisms,
which contributed most of the diversity, were located within an Alu insert
disrupting the ZFY-ZFX homology (pi Alu = 0.24%). The diversity in the
homologous portion of the ZFX intron, although higher than that in ZFY, was
lower than that found in genomic segments believed to evolve neutrally;
interspecies divergence in both segments was also reduced. Although this
suggests that the evolution of both ZFY and ZFX homologs may not be
entirely neutral, both Tajima and HKA tests did not reject neutrality. The
lack of statistical significance may be attributed to a lack of power in
these tests (the low divergence and variability values reduce the power of
the HKA and Tajima tests, respectively); furthermore, Homo sapiens has
recently undergone a rapid population growth, and selection is more
difficult to detect in an expanding population. Therefore, the failure to
reject neutrality does not necessarily indicate the absence of selection.
In this context, the phylogenetic argument was given more weight in out
interpretations. The high level of sequence identity in ZFY and ZFX
segments, in spite of their separation 80-130 MYA, reflects a lower
mutation rate as compared with other segments believed to undergo
unconstrained evolution. Thus, the possibility of weak selection
contributing to the low level of nucleotide diversity in the last ZFY
intron cannot be excluded and should be kept in mind in the population
genetics studies based on Y chromosome variability.
ORIGINAL ARTICLE
Is selection responsible for the low level of variation in the last intron of the ZFY locus?
Institute of Human Genetics, Polish Academy of Sciences, Strzeszynska, Poland.
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