Selective Advantage for IIIM Males over YM Males in Cage Competition, Mating Competition, and Pupal Emergence in Musca domestica L. (Diptera: Muscidae)

In the house fly, Musca domestica L. (Diptera: Muscidae), sex is usually determined by a dominant factor, M, located on the Y chromosome. However, there are autosomal male (AM) populations in which the M factor is located on one or more of the five autosomes (I-V), most commonly on the third chromosome. Herein we report the use of isogenic strains to determine the relative fitness of YM versus IIIM males in three different experiments. First, cages were started with 50% YM and 50% IIIM males, and the frequencies of YM and IIIM males were evaluated across generations. Second, mating competition studies were preformed with these isogenic strains. Third, the relative emergence rates of IIIM versus YM male pupae held at three temperatures for 3 d were examined. All three studies indicate that IIIM males have a greater fitness than YM males. In the cage competition studies, >90% of the males were IIIM after seven generations. IIIM males were more likely to mate than YM males, and a higher percent of IIIM males emerged after being held as pupae at 4, 16, or 28°C for 3 d. The implications of these studies to the distribution of IIIM and YM males in field populations are discussed.