Is it just a coincidence that aposematic polymorphism and sex ratio distortion co-occur in a tropical butterfly?

Abstract

Background: The cosmopolitan butterfly Danaus chrysippus is polymorphic only within the geographic zone where it is infected by the male-killing bacterium Spiroplasma. Hypothesis: The different colour forms of D. chrysippus represent incipient species that have undergone hybridization as a result of Spiroplasma invasion, because many females of the frequently infected forms are forced to mate with the males of the less infected forms. Prediction: Some forms are more susceptible to Spiroplasma infection than others. Times and places: Uganda and Sudan during 2005, 2006, and 2007. Analytical method: We collected D. chrysippus butterflies in the wild. We recorded their colour pattern and sex. We then used polymerase chain reaction (PCR) to determine whether they were infected with Spiroplasma. We estimated the morph ratio, the sex ratio, and the prevalence of Spiroplasma in different populations and regions. The association between colour pattern and Spiroplasma infection was subjected to statistical analysis. Conclusion: We found no significant difference in the sex ratio or the prevalence of Spiroplasma between different forms. Colour forms do not vary in their susceptibility to Spiroplasma infection.