“Female density-dependent chemical warfare underlies fitness effects of group sex ratio in flour beetles”

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Imroze Khan, Arun Prakash, Swastika Issar, Mihir Umarani, Rohit Sasidharan, Jagadeesh N. Masagalli, Prakash Lama, Radhika Venkatesan, and Deepa Agashe (Mar 2018)

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Females (not males) drive fitness effects of biased sex ratio via secreted toxins for resource (not sexual) competition

Female Tribolium castaneum beetle secreting quinones (yellow mass) from thoracic gland.
(Credit: Deepa Agashe and Gaurav Agavekar, copyright NCBS)


In animals, skewed sex ratios can affect individual fitness either via sexual (e.g. intersexual conflict or intrasexual mate competition) or non-sexual interactions (e.g. sex-specific resource competition). Because most analyses of sex ratio focus on sexual interactions, the relative importance of sexual vs. non-sexual mechanisms remains unclear. We tested both mechanisms in the flour beetle Tribolium castaneum, where male-biased sex ratios increase female fitness relative to unbiased or female-biased groups. Although flour beetles show both sexual and non-sexual (resource) competition, we found that sexual interactions did not explain female fitness. Instead, female fecundity was dramatically reduced even after a brief exposure to flour conditioned by other females. Earlier studies suggested that secreted toxins might mediate density-dependent population growth in flour beetles. We identified ethyl- and methyl-benzoquinone (EBQ and MBQ; “quinones”), as components of adult stink glands that regulate female fecundity. In female-biased groups (i.e. at high female density), females upregulated quinones and suppressed each other’s reproduction. In male-biased groups, low female density and associated low quinone levels maximized fecundity. Thus, females appear to use quinones as weapons for female-specific, density-dependent interference competition. Our results underscore the importance of non-sexual interference competition that may often underlie the fitness consequences of skewed sex ratios.