Chromosomal and gonadal sex have differing effects on social motivation in mice.

Background: Sex differences in brain development are thought to lead to sex variation in social behavior. Sex differences are fundamentally driven by both gonadal hormones and sex chromosomes, yet little is known about the independent effects of each on social behavior. Further, mouse models of the genetic liability for the neurodevelopmental disorder MYT1L Syndrome have shown sex-specific deficits in social motivation. In this study, we aimed to determine if gonadal hormones or sex chromosomes primarily mediate the sex differences seen in mouse social behavior, both at baseline and in the context of Myt1l haploinsufficiency.

Methods: Four-core genotypes (FCG) mice, which uncouple gonadal and chromosomal sex, were crossed with MYT1L heterozygous mice to create eight different groups with unique combinations of sex factors and MYT1L genotype. A total of 131 mice from all eight groups were assayed for activity and social behavior via the open field and social operant paradigms. Measures of social seeking and orienting were analyzed for main effects of chromosome, gonads, and their interactions with Myt1l mutation.

Results: The FCGxMYT1L cross revealed independent effects of both gonadal and chromosomal sex on activity and social behavior. Specifically, the presence of ovarian hormones led to greater overall activity, social seeking, and social orienting regardless of MYT1L genotype. In contrast, sex chromosomes affected social behavior mainly in the MYT1L heterozygous group, with XX MYT1L mutant mice demonstrating elevated levels of social orienting and seeking compared to XY MYT1L mutant mice.

Conclusions: Gonadal and chromosomal sex have independent mechanisms of driving greater social motivation in females. Additionally, genes on the sex chromosomes may interact with neurodevelopmental risk genes to influence sex variation in atypical social behavior.