Abstract

In sympatric Drosophila species, songs produced by male wing vibration during courtship are an effective mechanism preventing interspecific matings and maintaining sexual isolation between different species. These songs can vary greatly even between closely related species. The aim of this study was to localise X chromosomal and autosomal genes affecting species differences in male courtship song and to study their interaction in the D. virilis group species. Various genes were probed by in situ hybridisation on the X chromosomes of six species of the group, which enabled us to use localised RFLP markers in QTL studies, as well as to compare gene arrangements of different species.

Genetic analyses of differences between the songs of D. virilis and D. littoralis showed that species-specific song traits are affected both by X chromosomal and autosomal genes. The X chromosomal gene(s) having a major impact on pause and pulse length in male song were found to be located at the proximal region of the chromosome. Precise localisation of the song genes was, however, not possible due to multiple chromosome rearrangements restricting recombination between RFLP markers located on this area. The same problem was faced when studying hybrids between D. flavomontana and D. montana with less diverged X chromosomal gene arrangements.

Interaction between the X chromosomal and autosomal song genes in determining male song traits was studied in four species belonging to the virilis and montana phylads of D. virilis group. The long pauses in courtship song were found to be mainly caused by X chromosomal song genes (or maternal / cytological factors), while pulse length was determined by X chromosomal genes interacting with autosomal genes. This confirms the important role of X chromosomal gene(s) in song evolution in the montana phylad species. The direction of dominance in hybrid songs suggests that the songs of the montana phylad species have been affected by directional selection favouring shorter pulses and longer pauses between sound pulses during their evolution.

The levels and patterns of DNA polymorphism in an X-linked fused (fu) gene was studied in different D. montana populations. These studies revealed that D. montana populations are significantly but not completely isolated, and that a selective sweep at fu (or at a gene linked to fu) may be the reason for the reduced levels and patterns of variability of this gene in Finnish D. montana populations. The methods used in this study will be utilized to study variation in ‘song genes’ in the future.