Insect sex determination: a cascade of mechanisms.

Three conditions are essential for evolution: variation, heredity and time. Most animal species transmit genetic information through sexual reproduction in order to meet the first two conditions. The consequence for sexual reproduction is the need for (at least) two sexes with different reproductive roles. Consequently, evolutionary issues arise, such as differences in morphological and behavioral attributes between the sexes, which are reflected in the plethora of animal sex-determining systems. Indeed, a transition to asexual development may occur, sometimes induced by microbial endosymbionts. All these aspects are represented in the insect sex-determining system, which is the focal topic of this issue of Sexual Development . The first contribution by van Doorn is a theoretical treatise on the evolutionary forces acting on sex determination, including transitions of environmental and genetic sex determination and male and female heterogamety. In the contribution of Bopp, Saccone and Beye, three modules in the insect sexdetermining cascade are derived: the instructive module, the transduction module and the execution module. They argue how the instructive module, as well as the executive module, are primary targets of evolutionary divergence, while the transduction remains conserved. An intriguing example of this is presented by Scott, Pimsler and Tarone, who discuss the difference between the sex determination system of Lucilia cuprina , where sex is determined by a Y-linked male-determining gene (M) and that of Chrysomya rufifacies , in which the maternal genotype involving a dominant factor (F/f) determines sex. The contribution by Geuverink and Beukeboom shows, by phylogenetic analysis, that the transformer part of the transduction module may be subject to multiple independent losses or recruitments during evolution. When sex is determined under haplodiploidy, a transition to asexual reproduction may occur, known as thelytoky. This is addressed by Vorburger, who discusses how diploid male production and other constraints may impede or promote the evolution of thelytoky. A special case, the manipulation Published online: February 13, 2014