MBE Advance Access published online on April 23, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn100
Research Article |
Genetic exchange vs genetic differentiation in a medium-sized inversion of Drosophila: the A2/Ast arrangements of D. subobscura
1 Centro de Estudos da Macaronésia, Departamento de Biologia, Universidade da Madeira, 9050 Funchal, Portugal
2 Departament de Genética, Facultat de Biologia, Universitat de Barcelona, 08028 Barcelona, Spain
Corresponding author: Carmen Segarra, Departament de Genética, Facultat de Biología, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain, Phone: 34-934021848, Fax: 34-934034420, e-mail: csegarra{at}ub.edu.
Received for publication November 12, 2007. Revision received March 7, 2008. Accepted for publication April 6, 2008.
Chromosomal inversion polymorphism affects nucleotide variation at loci associated with inversions. In D. subobscura, a species with a rich chromosomal inversion polymorphism and the largest recombinational map so far reported in the Drosophila genus, extensive genetic structure of nucleotide variation was detected in the segment affected by the O3 inversion, a moderately-sized inversion at Muller's element E. Indeed, a strong genetic differentiation all over O3 and no evidence of a higher genetic exchange in the center of the inversion than at breakpoints were detected. In order to ascertain, whether other polymorphic and differently sized inversions of D. subobscura also exhibited a strong genetic structure, nucleotide variation in five gene regions (P236, P275, P150, Sxl and P125) located along the A2 inversion was analyzed in Ast and A2 chromosomes of D. subobscura. A2 is a medium-sized inversion at Muller's element A and forms a single inversion loop in heterokaryotypes. The lower level of variation in A2 relative to Ast and the significant excess of low frequency variants at polymorphic sites indicate that nucleotide variation at A2 is not at mutation-drift equilibrium. The closest region to an inversion breakpoint, P236, exhibits the highest level of genetic differentiation (FST) and of linkage disequilibrium between arrangements and variants at nucleotide polymorphic sites (LD). The remaining four regions show a higher level of genetic exchange between A2 and Ast chromosomes than P236, as revealed by FST and LD estimates. However, significant genetic differentiation between the Ast and A2 arrangements was detected not only at P236, but also in the other four regions separated from the nearest breakpoint by 1.2 Mb to 2.9 Mb. Therefore, the extent of genetic exchange between arrangements has not been high enough to homogenize nucleotide variation in the center of the A2 inversion. A2 can be considered a typical successful inversion of D. subobscura according to its relative length. Chromosomal inversion polymorphism of D. subobscura might thus cause the genome of this species to be highly structured and to harbor different gene pools that might contribute to maintain adaptations to particular environments.
Key Words: inversion polymorphism • Drosophila subobscura • nucleotide polymorphism • genetic differentiation • genetic exchange • A2 and Ast arrangements