Seasonal morphotypes of Drosophila suzukii differ in key life-history traits during and after a prolonged period of cold exposure

Abstract

Seasonal polyphenism in Drosophila suzukii manifests itself in two discrete adult morphotypes, the "winter morph" (WM) and the "summer morph" (SM). These morphotypes are known to differ in thermal stress tolerance, and they co-occur during parts of the year. In this study, we aimed to estimate morph-specific survival and fecundity in laboratory settings simulating field conditions. We specifically analyzed how WM and SM D. suzukii differed in mortality and reproduction during and after a period of cold exposure resembling winter and spring conditions in temperate climates. The median lifespan of D. suzukii varied around 5 months for the WM flies and around 7 months for the SM flies. WM flies showed higher survival during the cold-exposure period compared with SM flies, and especially SM males suffered high mortality under these conditions. In contrast, SM flies had lower mortality rates than WM flies under spring-like conditions. Intriguingly, reproductive status (virgin or mated) did not impact the fly survival, either during the cold exposure or during spring-like conditions. Even though the reproductive potential of WM flies was greatly reduced compared with SM flies, both WM and SM females that had mated before the cold exposure were able to continuously produce viable offspring for 5 months under spring-like conditions. Finally, the fertility of the overwintered WM males was almost zero, while the surviving SM males did not suffer reduced fertility. Combined with other studies on D. suzukii monitoring and overwintering behavior, these results suggest that overwintered flies of both morphotypes could live long enough to infest the first commercial crops of the season. The high mortality of SM males and the low fertility of WM males after prolonged cold exposure also highlight the necessity for females to store sperm over winter to be able to start reproducing early in the following spring.

Keywords: Drosophila suzukii; fertility; life history; overwintering; reproduction; seasonal polyphenism; spotted‐wing drosophila; survival.

FIGURE 1

Graphical representation of the experimental setup. The timeline displays the treatments related to temperature, diet and reproductive status undergone by the experimental groups of D. suzukii summer‐morphs (a) and winter‐morphs (b). The flies were either kept virgin or mated before or after the cold‐exposure period (see Table 1). The life‐history traits that were assessed are also indicated

FIGURE 2

Lifespan and mortality of D. suzukii experimental groups (see Table 1). (a) Boxplots representing the lifespan of the flies (N = 100 flies per experimental group upon emergence). The flies were monitored throughout the experiment, from emergence (day 0) until death. The cold‐exposure period (31 days in total from day 12 until day 43) is delimited by the blue dashed lines. Each box represents the interquartile range of lifespan for the experimental group, the line in the middle is the median, the whiskers represent extreme values within 1.5 times the interquartile range, and the dots are the “outliers.’’ (b) Barplots representing the mortality of the flies during the acclimation and cold‐exposure periods. The percentage of flies dying (of those alive at the start of each period) during the 8‐day precold acclimation period, the 31‐day cold‐exposure period, and the 4‐day postcold acclimation period is shown for each group of flies. The number of living flies upon the first day of acclimation is indicated by the letter N

FIGURE 3

Age‐specific log hazard (mortality) rates of D. suzukii morphotypes. (a) Fitted smoothing splines of age‐specific log hazard (mortality) rates (solid lines), and 95% confidence bands (shaded regions) for the different levels of the selected predictors (here, the morphotype, the sex, and their interactions). (b) Difference between SM and WM fitted smoothing splines (see (A)) of age‐specific hazard (mortality) rates (solid lines), and 95% confidence band for each sex. (c) Difference between female and male fitted smoothing splines (see (a)) of age‐specific hazard (mortality) rates (solid lines), and 95% confidence band for each morphotype. For age ranges where the confidence bands do not overlap with zero, we consider that there is a significant difference in hazard between SM and WM, or females and males, at those age ranges

FIGURE 4

Lifetime reproductive output of D. suzukii females. The boxplots represent the total number of offspring produced per living female and per vial in the experimental groups of interest (see Table 1). Letters indicate statistical differences after Tukey's multiple comparison test (p < .05)

FIGURE 5

Age‐specific fecundity of WM and SM D. suzukii females that mated before or after the cold‐exposure period. (a) Fitted smoothing splines of age‐specific reproductive success of D. suzukii females for each morphotype (solid lines), and 95% confidence bands (shaded regions) for the different levels of the selected predictors (here, the morphotype, the moment of mating, and their interactions). (b) Difference between SM and WM fitted smoothing splines of age‐specific reproductive success (solid line), and 95% confidence band for this difference. For age ranges where the confidence band does not overlap with zero, there is a significant difference in reproductive success between SM and WM at those age ranges. (c) Difference between post‐cold and pre‐cold mated female fitted smoothing splines of age‐specific reproductive success (solid lines), and 95% confidence band for this difference and for each morphotype. For age ranges where the confidence band does not overlap with zero, there is a significant difference in reproductive success between postcold and precold mated females at those age ranges

FIGURE 6

Fertility of D. suzukii males after cold exposure. After the cold‐exposure period, the WM and SM males were mated with SM females that had not been cold‐exposed, and we measured the lifetime reproductive output of these females. The boxplots represent the total number of offspring produced per vial and per living female. Letters indicate statistical differences (pairwise comparison, p < .05)

FIGURE 7

Age‐specific fertility of WM and SM D. suzukii males that underwent a period of cold exposure. The fitted smoothing splines of age‐specific fertility of D. suzukii males (solid lines), and 95% confidence bands (shaded regions) for the different levels of the selected predictors (here, the morphotype) are represented here. Male fertility is expressed as the reproductive success of the noncold‐exposed laboratory‐reared females that were mated with the experimental males (see Table 1) after the cold‐exposure period