High nucleotide sequence variation in a region of low recombination in Drosophila simulans is consistent with the background selection model |
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Authors: | Hamblin MT; Aquadro CF |
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Institution: | Section of Genetics and Development, Cornell University, Ithaca, New York 14853, USA. mth3@cornell.edu |
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Abstract: | We surveyed nucleotide sequence variation at glucose dehydrogenase (Gld),
in a region of low recombination on chromosome 3R, from a population sample
of Drosophila simulans. The levels of nucleotide variation were
surprisingly high. There was no departure from the expectation of a neutral
model for the level of polymorphism, indicating no evidence of a selective
sweep in this region. There was a significant deficiency of singleton
polymorphisms according to the Fu and Li test, although Tajima and Hudson,
Kreitman, and Aguade (HKA) tests do not provide evidence of a significant
elevation of variation due to balancing selection. Genetic map data for the
D. simulans third chromosome were used to calculate expected values of pi
for Gld under a current model of background selection, varying the values
for the parameter sh (selection coefficient against deleterious mutations).
We show that the recombinational landscape of D. simulans is sufficiently
different from that of D. melanogaster that we expect higher variation
under the background selection model, even when effective population sizes
are assumed to be equal. The data for Gld were tested against the
predictions using computer simulations of the distribution of the number of
segregating sites conditioned on pi. Background selection alone can explain
our observations as long as sh is larger than 0.005 and species-level
effective population size is assumed to be several- fold larger than in D.
melanogaster. Alternatively, the deleterious mutation rate may be smaller
in D. simulans, or balancing selection may be acting nearby, thereby
reducing the effect of background selection.
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