An innovative ovipositor for niche exploitation impacts genital coevolution between sexes in a fruit-damaging Drosophila

Abstract

Limited attention has been given to ecological factors influencing the coevolution of male and female genitalia. The innovative ovipositor of Drosophila suzukii, an invading fruit pest, represents an appealing case to document this phenomenon. The serrated saw-like ovipositor is used to pierce the hard skin of ripening fruits that are not used by other fruit flies that prefer soft decaying fruits. Here, we highlight another function of the ovipositor related to its involvement in genital coupling during copulation. We compared the morphology and coupling of male and female genitalia in this species to its sibling species, Drosophila subpulchrella, and to an outgroup species, Drosophila biarmipes These comparisons and a surgical manipulation indicated that the shape of male genitalia in D. suzukii has had to be adjusted to ensure tight coupling, despite having to abandon the use of a hook-like structure, paramere, because of the more linearly elongated ovipositor. This phenomenon demonstrates that ecological niche exploitation can directly affect the mechanics of genital coupling and potentially cause incompatibility among divergent forms. This model case provides new insights towards elucidating the importance of the dual functions of ovipositors in other insect species that potentially induce genital coevolution and ecological speciation.

Keywords: Drosophila subpulchrella; Drosophila suzukii; coevolution between the sexes; mechanical incompatibility; morphology of genitalia.

Figure 1.

Genital coupling of male and female genitalia in D. suzukii (WT3) and D. subpulchrella (H243). (a) A copulating pair of D. suzukii that was flash-frozen in chilled ethanol 13 min after initiation. (a′) Magnified view of the connected genitalia in (a). (b) A copulating pair of D. subpulchrella that was flash-frozen in chilled ethanol 13 min after initiation. (b′) Magnified view of the connected genitalia in (b). (c) Lateral view of ovipositor (OV) in D. suzukii. (d) Lateral view of OV in D. subpulchrella. (e) Spatial configuration of coupled genitalia of a copulating D. suzukii pair after being cleared. (f) Spatial configuration of a D. subpulchrella pair after being cleared. Arrows indicate parameres in (e,f). (e′) Coloured map of (e). (f′) Coloured map of (f). Red and blue colours indicate female and male genital structures, respectively, in (e′,f′). Scale bar is 50 µm except where otherwise indicated.

Figure 2.

Morphology of male genitalia in D. suzukii (WT3) and D. subpulchrella (H243). (a) Ventral side of paramere in D. suzukii. (b) Ventral side of paramere in D. subpulchrella. (c) Clasper (CL) and anal plate (AP) in D. suzukii. (c) Clasper (CL) and anal plate (AP) in D. subpulchrella. Scale bar is 50 µm.

Figure 3.

Paramere contact during copulation. (a) Copulation duration in D. suzukii (WT3), D. subpulchrella (H243) and D. biarmipes (MYS118), measured from the initial coupling of male and female genitalia until detachment. Bars represent means. (b) Phylogenetic relationship (topology) of the three species [23] is shown. Percentage (%) of paramere contact at 3, 8, 13 and 18 min after copulation initiation in the three species. Data for 18 min were not collected in D. biarmipes (MYS118) because of the short copulation time. Proportions of pairs with paramere contact are indicated in dark grey. The difference to D. suzukii was tested by Fisher's exact test with Bonferroni correction. *p < 0.01, **p < 10⁻³. (Online version in colour).

Figure 4.

Variation in paramere shape in D. suzukii and D. subpulchrella. (a) Trimming position of a paramere shown by the broken red line, based on the two landmarks indicated by red circles. (b) Principal component biplots of PC1 and PC2 of the outline shape descriptors from the elliptic Fourier analysis. Left or right intact parameres were normalized by their area from four individuals belonging to each of the 20 D. suzukii TMUS strains and 20 D. subpulchrella NAR strains. Differences in variance size; PC1: F_79,79 = 0.019, p < 10⁻¹⁶; PC2: F_79,79 = 0.161, p < 10⁻¹³ (all individuals were pooled because of the insignificant strain effect on PC1; D. suzukii: F_19,60 = 1.66, p > 0.05; D. subpulchrella: F_19,60 = 1.35, p > 0.05). (c) Paramere shapes of A–H in (b). Overlapped outlines of (A–D) and (E–H) are shown beneath the diagrams.

Figure 5.

Effects of disrupting paramere contact by surgical treatment on conspecific copulation in D. suzukii and D. subpulchrella. (a) Average moving velocity (mm s⁻¹) of the intact and disrupted conspecific copulating pairs of D. suzukii (TMUS8) and D. subpulchrella (H243), and the heterospecific copulating pairs of D. subpulchrella (H243) females and D. suzukii (TMUS8) males. (b) Copulation duration (min) of the intact and disrupted conspecific copulating pairs of D. suzukii (TMUS8) and D. subpulchrella (H243), and the heterospecific copulating pairs of D. subpulchrella (H243) females and D. suzukii (TMUS8) males. *Data shown also in figure 3a. The difference between intact and disrupted pairs was tested by the exact permutation test. (Online version in colour).