In vitro sperm-ovum binding assays show no evidence of species-specific zona pellucida binding between wild-derived strains of Mus musculus musculus, M. m. domesticus and M. spretus

The key event in incipient speciation is the emergence of reproductive isolation between diverging populations. Reproductive isolation can occur before or after fertilisation. Postmating prezygotic isolation, or gametic isolation, occurs between insemination and fertilisation and may drive late stages of speciation. This study evaluates gametic isolation in subspecies of the house mouse (Mus musculus musculus – Mmm, M. m. domesticus – Mmd) and a more phylogenetically distant species, M. spretus (Msp). Using an in vitro sperm-ovum binding assay, we quantified sperm-ovum binding in reciprocal crosses among four wild-derived Mmm strains, three Mmd strains, and one Msp strain. Results showed that sperm did not preferentially bind to conspecific ova, but binding rates varied. Mmm sperm bound the fewest ova, followed by Mmd, whereas Msp sperm bound the most, regardless of the origin of the ova. Thus, under standardised in vitro conditions, we detected no evidence for species-specific sperm-ovum binding among the taxa studied. Instead, variation in sperm binding rate may reflect temporal changes in sperm longevity and/or differences in mating-system promiscuity among species, with Msp being the most promiscuous.