Transpecific polymorphisms in an inversion linked esterase locus in Drosophila buzzatii

Area of Science:

• Population Genetics
• Molecular Evolution
• Drosophila Genomics

Background:

• Polymorphic inversions in Drosophila buzzatii second chromosomes can influence nucleotide variation.
• The Esterase gene Est-A is located within rearranged segments, making it a candidate for studying these effects.

Purpose of the Study:

• To investigate nucleotide variation within the Est-A gene coding region in relation to chromosomal arrangements.
• To assess the impact of inversions on gene polymorphism and divergence.
• To explore potential evolutionary forces shaping variation at this locus.

Main Methods:

• Sequencing of a 1.1 kb coding region of the Est-A gene.
• Analysis of 30 homozygous second-chromosome lines of D. buzzatii with different gene arrangements.
• Inclusion of three D. koepferae isofemale lines as outgroups for divergence analysis.

Main Results:

• Est-A exhibits high polymorphism at both synonymous and replacement sites.
• A local excess of silent polymorphism was detected, deviating from neutral theory predictions.
• Polymorphisms are shared with D. koepferae, suggesting interspecific gene flow or balancing selection.
• Derived chromosomal arrangements (J, JZ(3)) are less polymorphic than the ancestral ST arrangement.
• Genetic differentiation exists between ST and J arrangements, but many polymorphisms are shared, indicating gene conversion or double crossovers.

Conclusions:

• Nucleotide variation in Est-A is shaped by factors including interspecific gene flow, balancing selection, and linkage to adaptive inversions.
• The ancestral ST arrangement is more polymorphic than derived arrangements.
• Evidence suggests recent increases in the frequency of the old gene arrangement J.