Adult female Drosophila pseudoobscura survive and carry fertile sperm through long periods in the cold: populations are unlikely to suffer substantial bottlenecks in overwintering.
To assess whether, while overwintering, natural populations of Drosophila pseudoobscura are likely to experience substantial bottlenecks in their numbers and genotypes, laboratory tests of the cold sensitivities of each stage of the life history and reproduction were undertaken. Three genetically distinctive lineages established from flies caught at high elevation were used for testing in temperatures likely to persist in protected pockets of fermenting deciduous leaf fall in overwintering sites. Sensitivities to cold of each stage in development were measured as frequencies of survival to adulthood following a period in 5 degrees C in a particular stage. The cold sensitivity of adults was measured as the survival in and following cold stays in adulthood. It was found that cold sensitivity decreases as development progresses, but that only adults (females more than males) are able to withstand long periods in the cold. The cold sensitivity of reproductive capacity of males was scored as their success in mating following a two-month cold stay, and of females as the numbers laying fertile eggs following periods of months in the cold. Both males and females maintain reproductive capacity. Of particular significance, however, is that even after six months in the cold females are able to restart production of eggs and these eggs may be fertilized by the sperm of matings prior to their cold stay. Thus, a substantial proportion of overwintering genomes must be those of adult females and those of the sperm carried by females from matings in the previous summer. This simple finding strongly suggests that populations are not likely to suffer substantial bottlenecks while overwintering. Further, it indicates how arrays of genetic variation may be maintained through winters and largely avoid winter selective pressures. Frequent migration between populations is therefore not required to maintain the variation commonly found in populations throughout the species range.
Collett
J I
JI
School of Biological Sciences, University of Sussex, Brighton, United Kingdom.
Jarman
M G
MG
D000818:Animals; D003080:Cold Temperature; D004330:Drosophila / Q000254:growth & development / Q000502:physiology*; D005260:Female; D008297:Male; D012098:Reproduction / Q000502:physiology; D012621:Seasons