Inbreeding and outbreeding depression in male courtship song characters in Drosophila montana

In Drosophila montana, male courtship song frequency is closely associated with male courtship success and offspring survival. Other pulse characters (pulse length and cycle number) may also affect female mate choice, whereas pulse train characters (interpulse interval, pulse number and pulse train length) are not associated with these male fitness components. Inbreeding depression in these song characters was investigated by comparing the songs of inbred and outbred fly strains. The average change in most song characters as a result of inbreeding was only a few percent. However, in male song frequency the average inbreeding depression was about 14%, suggesting that this song character is associated with fitness. Outbreeding depression and the genetic architecture of song characters were investigated with interpopulation crosses and joint scaling tests. For pulse train characters the generation means show only evidence of additivity, and the existence of dominance or epistasis in these characters was strongly rejected in each case. In pulse characters the means of the F-1 males were lower than the average of the parental generations. In pulse length and cycle number this difference was attributable to dominance alone. In frequency there was outbreeding depression also in the F-2 generation, suggesting a break-up of favourable epistatic gene combinations. The outbreeding depression in this character in the F-1 generation was caused by dominance, and in the F-2 also by duplicate epistasis between dominant decreasers. The possible role of outbreeding depression and epistasis in speciation is discussed.