D. americana harbours an X/4 fusion that is distributed through a very wide cline along a latitudinal gradient (McAllister 2002; Genome 45: 13-21). It is likely that this cline is maintained by a balance between gene flow and selection on the fusion itself or on associated genes (McAllister and Charlesworth 1999; Genetics 153: 221-233; Vieira et al. 2001; Genetics 158: 279-290). At the base of the X chromosome there is significant differentiation between fusion and non-fusion chromosomes (Vieira et al. 2001; Genetics 158: 279-290; Vieira et al. unpublished results), although there is no DNA sequence differentiation for many autosomal and X-linked genes (Hilton and Hey 1996; Genetics 144: 1015-1025; Hilton and Hey 1997; Genet. Res. 70: 185-194; McAllister and Charlesworth 1999; Genetics 153: 221-233; McAllister and McVean 2000; Genetics 154: 1711-1720; Vieira et al. unpublished results). There is, however, evidence for recombination between X/4 fusion and non-fusion chromosomes at the base of the X and fourth chromosomes (Vieira et al. 2001; Genetics 158: 279-290; Vieira et al. unpublished results). The observed differentiation is thus unlikely to be due to pairing difficulties in individuals heterozygous for the X/4 fusion alone. Nevertheless, this situation allows us to explore the consequences of the divergence of just a portion of the genome; can we find evidence for incompatibilities at the base of the X chromosome between individuals with and without the X/4 fusion ? Could this be a case of an incipient sympatric speciation event ? We are presently studying several other genes located at the base of the X chromosome that can give insight into how the observed differentiation arose and on its maintenance. We are also looking for the causes of the association between the X/4 fusion chromosomes and several chromosomal inversions.
Researchers involved in this project: