ObjectiveIn this study, we used a systemic Fmr1 knockout in order to investigate both genotype‐ and sex‐specific differences across multiple measures of sociability, repetitive behaviors, activity levels, anxiety, and fear‐related learning and memory.BackgroundFragile X syndrome is the most common monogenic cause of intellectual disability and autism. Few studies to date have examined sex differences in a mouse model of Fragile X syndrome, though clinical data support the idea of differences in both overall prevalence and phenotype between the sexes.MethodsUsing wild‐type and systemic homozygous Fmr1 knockout mice, we assessed a variety of behavioral paradigms in adult animals, including the open field test, elevated plus maze, nose‐poke assay, accelerating rotarod, social partition task, three‐chambered social task, and two different fear conditioning paradigms. Tests were ordered such that the most invasive tests were performed last in the sequence, and testing paradigms for similar behaviors were performed in separate cohorts to minimize testing effects.ResultsOur results indicate several sex‐specific changes in Fmr1 knockout mice, including male‐specific increases in activity levels, and female‐specific increases in repetitive behaviors on both the nose‐poke assay and motor coordination on the accelerating rotarod task. The results also indicated that Fmr1 deletion results in deficits in fear learning and memory across both sexes, and no changes in social behavior across two tasks.ConclusionThese findings highlight the importance of including female subjects in preclinical studies, as simply studying the impact of genetic mutations in males does not yield a complete picture of the phenotype. Further research should explore these marked phenotypic differences among the sexes. Moreover, given that treatment strategies are typically equivalent between the sexes, the results highlight a potential need for sex‐specific therapeutics.

Fragile X Syndrome (FXS) is a neurodevelopmental disorder caused by a trinucleotide (CGG) hyperexpansion in the FMR1 gene, functionally silencing transcription of the fragile x mental retardation protein (FMRP). This disorder is characterized by impaired cognition, communication, and social behavior. The purpose of this study was to investigate the development of ultrasonic vocalization (USV) behavior in a FMR1 deficient mouse model. On postnatal days (PD) 9–14, separate cohorts of FVB/NJ pups were removed from their home cage and isolation-induced USVs were recorded. There were significant genotype- and sex-dependent differences in USV behavior across the different testing days. FMR1 knockout mice showed a significant reduction in vocalizations across all days. There was also a significant difference in vocalizations between male and female mice. We found a significant decrease in total number of calls for KO males on PD9 and PD13, as well as an increase in the total number of calls for KO males on PD12. The KO males also had a significant increase in total call duration on PD12 and a reduction on PD13. The KO female had a significant decrease in the total number of calls on PD9 and PD10. They also had a significant decrease in the total call duration on PD9 and a marginal decrease in total call duration on PD10. These results provide additional evidence for communication deficits in FMR1 deficient mice and provide new insight suggesting sexually dimorphic vocalizations during the neonatal period.