Sex-ratio distorting Spiroplasma infection affects the morphology of female reproductive organs in Danaus chrysippus (Lepidoptera)

Many maternally inherited bacterial infections in insect species can have negative or positive effects on their hosts. One of the diverse phenotypes these bacteria can induce in their hosts is the selective death of the male offspring at early development stages. Male-killing (MK) symbiotic bacteria have been reported in diverse insect species, and these symbionts have been shown to play important roles in shaping the ecology and evolution of diverse host species. One such species is the African Monarch butterfly: Danaus chrysippus, the target species of my study. There are four subspecies of D. chrysippus, which are geographically loosely isolated in different parts of Africa. However, they do interbreed in a hybrid zone located in central Africa. The MK phenotype in D. chrysippus is caused by a Spiroplasma bacterium, which is highly prevalent in the host hybrid zone, where the MK phenotype is so strong that females can only reproduce thanks to a few males that migrate from the surrounding subspecies. Other studies have found a correlation between the size of the female (wingspan) and infection rate. In my thesis the main objective is to study parts of the internal female structures, aiming to investigate if the size of the female reproductive organs correlates with infection as well. My hypothesis is that the female reproductive organs are smaller in highly infected areas because the rare males produce small and resource-depleted spermatophores. This study considered if D. chrysippus females collected from different populations (with different infection rates and sex-ratios) showed a difference in the size of their reproductive organs (corpus bursae and signa). Some females were also dissected to count the amount of spermatophores, and to measure their size. My study includes photographs of these particular organs (corpus bursae and signum) because those have not been published before. The results showed that (A) Danaus chrysippus has two signa, one on the ventral- and one on the dorsal side of the corpus bursa, and they are covered with spike-like structures; (B) spermatophores varied in size between females, but that population, female infection status and population sex-ratio did not significantly affect spermatophore count nor spermatophore size, and (C) in Rwanda, where the sex-ratio is slightly female-biased, the signa covered a significantly smaller portion of the females’ corpus bursa than observed in females from other populations showing either no or a strong female sex-ratio bias.