Seasonal variation in life history traits in two Drosophila species

Abstract

Seasonal environmental heterogeneity is cyclic, persistent and geographically widespread. In species that reproduce multiple times annually, environmental changes across seasonal time may create different selection regimes that may shape the population ecology and life history adaptation in these species. Here, we investigate how two closely related species of Drosophila in a temperate orchard respond to environmental changes across seasonal time. Natural populations of Drosophila melanogaster and Drosophila simulans were sampled at four timepoints from June through November to assess seasonal change in fundamental aspects of population dynamics as well as life history traits. D. melanogaster exhibit pronounced change across seasonal time: early in the season, the population is inferred to be uniformly young and potentially represents the early generation following overwintering survivorship. D. melanogaster isofemale lines derived from the early population and reared in a common garden are characterized by high tolerance to a variety of stressors as well as a fast rate of development in the laboratory environment that declines across seasonal time. In contrast, wild D. simulans populations were inferred to be consistently heterogeneous in age distribution across seasonal collections; only starvation tolerance changed predictably over seasonal time in a parallel manner as in D. melanogaster. These results suggest fundamental differences in population and evolutionary dynamics between these two taxa associated with seasonal heterogeneity in environmental parameters and associated selection pressures.

Keywords: Drosophila; demography; life history; seasonal change.

Fig. 1

Seasonal changes in age distribution of wild Drosophila estimated using the deconvolution model. Each graph plots the density of the estimated age distribution of the population in nature vs. age in days. Using this method, the June population of Drosophila melanogaster is inferred to be relatively young and is distinct from the subsequent collections that are more heterogeneous and contain older flies. Not collected in June, Drosophila simulans consistently contained flies of old age classes when the species was present in the orchard.

Fig. 2

Wild flies of both species have a seasonal decline in mean (a) and maximum (b) number of eggs (±SE) laid per day during the first 10 days of captivity. Drosophila melanogaster are indicated in filled circles and Drosophila simulans in empty circles. There is no difference between species in the mean reproductive output; however, D. simulans has a higher maximum fecundity compared to D. melanogaster in July but lower during the rest of the season.

Fig. 3

Mean (±SE) development time in hours for F1 individuals (a and b) and flies that had been in a common-garden laboratory environment (c and d) for Drosophila melanogaster (a and c) and Drosophila simulans (b and d). Females are indicated in filled circles and males in empty circles. The F1 development time includes maternal effects that may reflect environmental quality and oscillates around the same duration for both species across seasonal time. The common-garden development time removes such environmental effects and increases drastically for D. melanogaster; however, it does not have a directional change for D. simulans.

Fig. 4

Mean (±SE) recovery time from chill (a and b), knockdown time from heat (c and d) and survival time without food (e and f) for flies that had been in a common-garden laboratory environment. Drosophila melanogaster (a, c, e) shows a seasonal decline in quality for all of these traits, whereas Drosophila simulans (b, d, f) demonstrates no clear pattern for thermal traits and a seasonal decline in starvation resistance. Females are indicated in filled circles and males in empty circles.