Abstract
Genetic and sexual systems can be evolutionarily dynamic within and among clades. However, identifying the processes responsible for switches between, for instance, sexual and asexual reproduction, or cyclic and non-cyclic life histories remains challenging. When animals evolve parthenogenetic reproduction, information about the sexual mating system becomes lost. Here we report an extraordinary case where we have been able to resurrect sexual adults in a species of beetle that reproduces by parthenogenetic paedogenesis, without the production of adults. Via heat treatment, we were able to artificially induce adult beetles of Micromalthus debilis in order to describe its pre-paedogenetic mating system. Adults showed a highly female biased sex ratio, out-breeding behaviour, and sex-role reversal. Paedogenetic larvae of Micromalthus are infected with the endosymbiotic bacteria Rickettsia and Wolbachia. Clear signs of vestigialization in adults are concurrent with the loss of adults. Our data suggest an ancient female sex ratio bias that predates the loss of adults, perhaps associated with endosymbionts. We propose a model for the transition from a haplodiploid cyclical parthenogenetic life history to parthenogenetic paedogenesis. Paedogenetic development induces a new mechanism of sex ratio bias in midges, wasps and beetles.
Highlights
Insects display an extraordinary richness of genetic and sexual systems
We were able to induce the production of adults in the paedogenetic beetle Micromalthus debilis
Heat treatment was originally suggested by Barber[12]; later Scott[11,18] experimented on colonies and obtained only a few larval males and adult females
Summary
Insects display an extraordinary richness of genetic and sexual systems. At least eight genetic forms of thelytokous parthenogenesis have been described, haplodiploidy is dominant in solitary and social Hymenoptera, hermaphroditism in scale insects, androgenesis is widespread in stick insects, and no less than four variations of hybridogenesis have been reported in ants[1,2]. Many of the species of interest share suites of characters that are often highly correlated (such as inbreeding, skewed sex ratios, or the presence of endosymbionts), making it difficult to identify cause and effect or eliminate third-variable problems such as the combination of two or more selfish genetic elements in one host[7]. Many of these traits make the organisms cryptic or hard to study by conventional means; they are typically not conventional “lab-rat” insects.
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