Estimation of the population sex ratio in butterflies infected with bacterial male-killers: a case study

Abstract

Female-biased population sex ratios in butterflies arise as a consequence of infection by maternally-transmitted endosymbionts that manipulate host sex ratio in order to maximize their transmission down the generations. A critical step toward understanding the host/male-killer relationship as well as the evolutionary consequences of male-killing is to provide accurate estimations for the population sex ratios in the host species. There are two ways to perform this: first, through counting the numbers of males and females in wild collections, and second, through the molecular screening of collected females for bacterial infection, using the PCR. In this paper, we compare the estimations of the two methods in the case of the queen butterfly Danaus chrysippus at Uganda and Sudan. We concluded that molecular screening is the most accurate approach to assess the population sex ratio. Moreover, a theoretical argument has been presented at the end of this article to explain why the bacterial prevalence approach avoids the inherent collection bias that is suffered by the sex ratio method.