BASIS FOR A SKEWED SEX RATIO IN DIAPAUSE-DESTINED FLESH FLIES.

The sex ratio of flesh flies (Sarcophaga) overwintering in pupal diapause is heavily biased toward males (Denlinger, 1972a, 1972b). The mosquito Toxorhynchitis rutilus (McCrary and Jenner, 1965) and the blow fly Lucilia caesar (Ring, 1971) show a similar male bias. Although the primary sex ratio in each of these cases is 1:1, males have a lower threshold for response to the seasonal cues of short day-length and low temperature. Thus, in a wild population of Sarcophaga bullata observed in Illinois (Denlinger, 1972b), the natural environmental conditions during August elicited an overall diapause incidence of 20%, but males represented 79% of the diapausing segment. Later in the season when all individuals entered diapause both sexes were equally represented. Both sexes terminate diapause at the same time (adult emergence is in mid-May); thus the net effect of the sexual difference in diapause threshold is that females opt to remain reproductively active later in the autumn and hence enter a diapause of shorter duration. In searching for the basis of the skewed sex ratio, I find that pupal diapause has a deleterious effect on female, but not male, reproductive success. To examine reproductive output, groups of approximately 100 Sarcophaga crassipalpis adults derived from a colony originating in Illinois were caged (1 ft3) and observed at 25 C, 15L:9D (light: dark) for 13 days following eclosion. In this ovoviviparous species, most females ovulate on day 6 and are ready to deposit their larvae by day 11. But, when denied a suitable substrate for larviposition, the female will retain her larvae in utero for a few days. By day 13, all potentially viable progeny have had adequate time to complete embryogenesis. When females were dissected on day 13, numbers of infertile eggs and larvae (fertile eggs) were counted. Each female was considered mated only if some larvae could be found in the uterus. Adult mortality was expressed as the percentage of flies that died during the 13 day observation period. Flies reared under long daylengths (15L:9D) never entered pupal diapause (Denlinger, 1971, 1972a), and as adults, their mortality rate was low, and their mating incidence and fertility rate were high (Table 1). On an individual basis, each surviving female produced a mean of 54.7 eggs, 74.6% of which were fertile, thus yielding a contribution of 40.8 fertile eggs/surviving female (Fig. 1). Diapausing pupae, produced using a short daylength (12L: 12D) for both embryonic (25 C) and larval (20 C) development (Denlinger, 1971, 1972a), were held in diapause at 20 C and 12L:12D for 5, 50, or 100 days and then immersed in hexane for 15 min to stimulate initiation of adult development (Denlinger et al., 1980). Flies that had been in pupal diapause produced a yield of eggs similar to that of non-diapausing flies (Table 1), but the fertility rate dropped sharply as the length of pupal diapause increased. Flies that had been in diapause 100 days produced only 17.6 fertile eggs/female (Fig. 1). To test the impact of natural winter conditions on the reproductive output of the flies, a group of diapausing pupae (100% diapause incidence) were held on the windowsill of an unheated building in Franklin County, Ohio, from the inception of diapause in early October until adult emergence in mid-May, 1980. Males and females survived the diapause period equally well (sex ratio 1 d: 0.92 Y, N = 100). Under the standardized laboratory conditions, fewer than 40% of the females mated, egg production