Molecular Biology and Evolution, Vol 16, 1599-1606, Copyright © 1999 by Society for Molecular Biology and Evolution
D Garrigan and SV Edwards
Twenty-three sequence haplotypes spanning the boundary of the second exon
and intron of a red-winged blackbird Mhc class II B gene, Agph- DAB1, are
presented. The polymorphism of the exon segment is distributed in two
divergent allelic lineages which appear to be maintained by balancing
selection. The silent nucleotide diversity of the exon (pi = 0.101) is more
than five times that of the intron (pi = 0.018) and decays rapidly across
the exon-intron boundary. Additionally, genealogical reconstruction
indicates that divergence from a common ancestor in the exon sample is over
four times that of the intron. The intron sequences reveal a pattern of
polymorphism which is characteristic of directional selection, rather than
a pattern expected from linkage to a balanced polymorphism. These results
suggest that the evolutionary histories of these two adjacent regions have
been disassociated by recombination or gene conversion. The estimated
population recombination parameter between the exon and the intron is
sufficiently high (4NeC = 8.545) to explain the homogenization of intron
sequences. Compatibility analyses estimate that these events primarily
occur from the exon-intron boundary to about 20-30 bases into the intron.
Additionally, the observation that divergent exon alleles share identical
intron sequence supports the conclusion of disassociation of exon and
intron evolutionary histories by recombination.
ORIGINAL ARTICLE
Polymorphism across an exon-intron boundary in an avian Mhc class II B gene
Department of Zoology, University of Washington, USA. garrigan@asu.edu
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