Sexual selection, and conflicts between sex or within sex, can induce morphological variations within species. This may challenge the identification and characterization of diagnostic morphological characters from reproductive organs that are of utmost importance for the morphology-based classification of faunal diversity, particularly on species level. In Lepidoptera, and in some other insects, females have evolved reproductive organs that can accommodate and/or digest nuptial gifts, or so called spermatophores, transferred from the males during copulation. The properties of the spermatophores principally depend on the male’s condition, which is defined by resources acquired and depleted across life stages. In these conditions, the size and/or shape of the female reproductive organs are likely to differ visually between females of different mating status, and with their mates’ condition. The widespread African monarch, Danaus chrysippus, can host an endosymbiotic bacterium, Spiroplasma that induces son-killing in this species. When prevalent, such symbionts distort their host population sex-ratio, which can increase the reproduction load on the few uninfected males, lead to a local excess of unmated females, and resource-depletion in males. Here, we characterized variation in the female genitalia across populations with different levels of Spiroplasma infection. First, virgin females have smaller organs, while mated females show expanded organs; and further, females from highly female-biased and male-killing Spiroplasma infected populations have a larger area of their corpus bursae covered with signa structures. These observations potentially suggest that females from populations that are male depleted may have evolved an efficient mechanical way to optimize the digestion of small nutritious spermatophores.

Background Sexual selection and conflicts within and between sexes promote morphological diversity of reproductive traits within species. Variation in the morphology of diagnostic reproductive characters within species offer an excellent opportunity to study these evolutionary processes as drivers of species diversification. The African monarch, Danaus chrysippus (Linnaeus, 1758), is widespread across Africa. The species is polytypic, with the respective geographical ranges of the four colour morphs only overlapping in East Africa. Furthermore, some of the populations host an endosymbiotic bacterium, Spiroplasma, which induces son-killing and distorts the local host population sex-ratio, creating sexual conflicts between the females seeking to optimize their fecundity and the limited mating capacity of the rare males. Methods We dissected females from Kenya, Rwanda and South Africa, where Spiroplasma vary in presence and prevalence (high, variable and absent, respectively), and conducted microscopy imaging of their reproductive organs. We then characterized the effect of population, female body size, and female mating status, on the size and shape of different genitalia characters of the D. chrysippus female butterflies. Results We showed that although the general morphology of the organs is conserved in D. chrysippus, female genitalia vary in size and shape between and within populations. The virgin females have smaller organs, while the same organs were expanded in mated females. Females from highly female-biased populations, where the male-killing Spiroplasma is prevalent, also have a larger area of their corpus bursae covered with signa structures. However, this pattern occurs because a larger proportion of the females remains virgin in the female-biased populations rather than because of male depletion due to the symbiont, as males from sex-ratio distorted populations did not produce significantly smaller nutritious spermatophores.