Sexual reproduction is an ancient feature of life on earth, and the familiar X and Y chromosomes in humans and other model species have led to the impression that sex determination mechanisms are old and conserved. In fact, males and females are determined by diverse mechanisms that evolve rapidly in many taxa. Yet this diversity in primary sex-determining signals is coupled with conserved molecular pathways that trigger male or female development. Conflicting selection on different parts of the genome and on the two sexes may drive many of these transitions, but few systems with rapid turnover of sex determination mechanisms have been rigorously studied. Here we survey our current understanding of how and why sex determination evolves in animals and plants and identify important gaps in our knowledge that present exciting research opportunities to characterize the evolutionary forces and molecular pathways underlying the evolution of sex determination.

Horizontally transmitted parasites are broadly predicted to be more virulent, or costly to host fitness, than those with vertical transmission. This is mainly because vertical transmission, from host parent to offspring, explicitly links the reproductive interests of both parties. Underlying this prediction is a general assumption that parasite transmission success is positively correlated with its virulence. We report results where infection of larval yellow fever mosquitoes Aedes aegypti with the microsporidian Edhazardia aedis was experimentally manipulated. The parasite's complex life cycle allowed comparisons between estimates of horizontal and vertical transmission on host fitness. Our measure of virulence was the fluctuating asymmetry (FA) of adult female wings. Hosts harbouring spores showed higher FAs than controls. Horizontally transmitting spores were associated with higher FAs than vertically transmitting spores. Furthermore, within hosts FA correlated positively with the number of horizontally transmitting spores, while no relation was seen with the number of vertically transmitting spores. A developmental mechanism uncoupling the relationship between vertical transmission and virulence is proposed.

The vast majority of eukaryotic organisms reproduce sexually, yet the nature of the sexual system and the mechanism of sex determination often vary remarkably, even among closely related species. Some species of animals and plants change sex across their lifespan, some contain hermaphrodites as well as males and females, some determine sex with highly differentiated chromosomes, while others determine sex according to their environment. Testing evolutionary hypotheses regarding the causes and consequences of this diversity requires interspecific data placed in a phylogenetic context. Such comparative studies have been hampered by the lack of accessible data listing sexual systems and sex determination mechanisms across the eukaryotic tree of life. Here, we describe a database developed to facilitate access to sexual system and sex chromosome information, with data on sexual systems from 11,038 plant, 705 fish, 173 amphibian, 593 non-avian reptilian, 195 avian, 479 mammalian, and 11,556 invertebrate species.