To be,or not to be,related: how female guppies bias sperm usage

In the Descent of Man, Darwin wrote “the power to charm the female has sometimes been more important than the power to conquer other males in battle” (Darwin 1871 ). Since his pioneering work, the field of sexual selection has exploded as biologists strive to understand how females bias fertilization towards preferred males. In the context of genetic relatedness between potential mates, two main hypotheses exist to explain female mating preferences. First, a female may bias fertilization towards genetically dissimilar males if she gains evolutionary fitness through the production of genetically diverse offspring – a model known as dissortative mating or inbreeding avoidance. Second, a female may favour genetically similar males if her offspring are more likely to inherit coadapted gene complexes – a model known as assortative mating or outbreeding avoidance. In this issue of Molecular Ecology, Gasparini et al. (2015) demonstrate that female guppies bias fertilization towards males which are more related to them at major histocompatibility (MHC) class IIB genes. Amazingly, this bias occurs after insemination of sperm from two different males.