| Transpecific Polymorphisms in an Inversion Linked Esterase Locus in Drosophila buzzatii (2003) | |||||||||||||
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| Nucleotide variation was studied in a 1.1 kb section of the coding region of an Esterase gene (Est-A) that maps in the center of the segments rearranged by polymorphic inversions in the cactophilic D. buzzatii. We examine 30 homozygous second-chromosome lines differing in gene arrangement and three D. koepferae isofemale lines as outgroups. Our data show that Est-A is a highly polymorphic gene at both synonymous and replacement sites. Significant departures from homogeneity in the distribution of the ratio of silent polymorphism to divergence predicted by the neutral theory, reveals a local excess of silent polymorphism. This is consistent with the presence of two apparent narrow peaks of elevated silent polymorphism surrounding non conservative amino acid substitutions. These polymorphisms as well as other at synonymous and non synonymous sites are shared with D. koepferae. We suggest that the presence of shared nucleotide polymorphisms is probably due to interspecific gene flow and/or balancing selection acting on replacement variants and/or to a decreased probability of loss of ancestral polymorphisms due to linkage to an adaptive inversion polymorphisms though recurrent mutation and persistence of neutral ancestral polymorphisms cannot be ruled out. The analysis of the distribution of nucleotide variation among the three chromosomal arrangements sampled reveals that derived arrangements (J and JZ3) are less polymorphic than the ancestral ST, and that the widely distributed ST and J arrangements are genetically differentiated. However, an important number of polymorphisms are shared between arrangements suggesting frequent exchange due to either gene conversion and/or double cross overs in heterokaryotypes. Finally, our present results in combination with data of sequence variation at the break points of inversion J suggest that this old gene arrangement has risen in frequency in relatively recent times. | |||||||||||||
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