1. ¹Institut für Tierzucht und Genetik, Josef Baumann Gasse 1, A-1210 Wien, Austria
2. ²Department of Biological Sciences, 143 Biology Building, University of Iowa, Iowa City, IA 52242-1324, USA

Correspondence: C Schlötterer, Institut für Tierzucht und Genetik, Josef Baumann Gasse 1, A-1210 Wien, Austria. E-mail: christian.schloetterer@vu-wien.ac.at

Received 13 March 2006; Accepted 1 June 2006; Published online 5 July 2006.

Abstract

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.