SUMMARYAnimals reared in hypoxic environments frequently exhibit smaller body sizes than when reared under normal atmospheric oxygen concentrations. The mechanisms responsible for this widely documented pattern of body size plasticity are poorly known. We studied the ontogeny of responses of Drosophila melanogaster adult body size to hypoxic exposure. We hypothesized that there may be critical oxygen-sensitive periods during D. melanogaster development that are primarily responsive to body size regulation. Instead, our results showed that exposure to hypoxia (an atmospheric partial pressure of oxygen of 10kPa) during any developmental stage (embryo, larvae and pupae) leads to smaller adult size. However, short hypoxic exposures during the late larval and early pupal stages had the greatest effects on adult size. We then investigated whether the observed reductions in size induced by hypoxia at various developmental stages were the result of a decrease in cell size or cell number. Abdominal epithelial cells of flies reared continuously in hypoxia were smaller in mean diameter and were size-limited compared with cells of flies reared in normoxia. Flies reared in hypoxia during the embryonic, larval or pupal stage, or during their entire development, had smaller wing areas than flies reared in normoxia. Flies reared during the pupal stage, or throughout development in hypoxia had smaller wing cells, even after controlling for the effect of wing size. These results suggest that hypoxia effects on the body size of D. melanogaster probably occur by multiple mechanisms operating at various developmental stages.
The timing and pattern of reproductive barrier formation in allopatric populations has received much less attention than the accumulation of reproductive barriers in sympatry. The theory of allopatric speciation suggests that reproductive barriers evolve simply as by-products of overall genetic divergence. However, observations of enhanced premating barriers in allopatric populations suggest that sexual selection driven by intraspecific competition for mates may enhance species-specific signals and accelerate the speciation process. In a previous series of laboratory trials, we examined the strength of premating and postmating barriers in an allopatric species pair of the endangered Sonoran topminnow, Poeciliopsis occidentalis and P. sonoriensis. Behavioral observations provided evidence of asymmetrical assortative mating, while reduced brood sizes and male-biased F 1 sex ratios suggest postmating incompatibilities. Here we examine the combined effects of premating and postmating barriers on the genetic makeup of mixed populations, using cytonuclear genotype frequencies of first-and second-generation offspring. Observed genotype frequencies strongly reflect the directional assortative mating observed in behavioral trials, illustrating how isolating barriers that act earlier in the reproductive cycle will have a greater effect on total reproductive isolation and may be more important to speciation than subsequent postmating reproductive barriers.
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