Patterns of microsatellite variation through a transition zone of a chromosomal cline in Drosophila americana

Chromosomal rearrangements have been considered as important barriers to gene flow and were often used in the delineation of species. The original taxonomic designation of Drosophila americana americana and Drosophila americana texana is based on the presence/absence of a centric fusion between the X- and fourth chromosomes. D. a. americana presents the derived fused state, whereas Drosophila a. texana presents the freely segregating ancestral state. The degree of genetic separation between the two chromosomal forms is still controversial, with different genetic markers yielding contrasting results even when the same populations were analyzed. Using 27 polymorphic microsatellites, we re-evaluated patterns of genetic differentiation between six D. americana populations sampled through a transition zone of both chromosomal forms in the central United States. Our results clearly reject a scenario of two differentiated species forming a hybrid zone in a region of parapatry and indicate that gene flow minimizes genome-wide differentiation associated with the two chromosomal arrangements.