Why Do Hybrids Turn Down Sex?

ABSTRACT Asexual reproduction is ancestral in prokaryotes; the switch to sexuality in eukaryotes is one of the major transitions in the history of life. The study of the maintenance of sex in eukaryotes has raised considerable interest for decades and is still one of evolutionary biology’s most prominent question. The observation that many asexual species are of hybrid origin have led some to propose that asexuality in hybrids results from sexual processes being disturbed because of incompatibilities between the two parental species’ genomes. This proximate theory appears difficult in real life, as it requires fundamental reproductive traits to be profoundly altered without collapsing individuals’ fertility. Repeated failures to produce asexual Amazon Molly in the lab through crossing experiments show that we are still in need of an evolutionary explanation. Here, we present a mathematical model and propose an adaptive route for the evolution of asexuality from previously sexual hybrids. Under smaller reproductive alterations, we show that asexuality can evolve to rescue hybrids’ reproduction. Importantly, we highlight that when incompatibilities only affect the fusion of sperm and egg’s genomes, unreduced meiosis and paternal genome elimination can evolve separately, greatly facilitating the overall evolutionary route.