Investigating NASPSim3’s Role in Drive-Associated Reproductive Costs in Mimulus guttatus

Selfish genetic elements (SGEs) can bias their transmission—or “drive”—, despite conferring consequences for the organism’ overall fitness. A natural case of female meiotic drive (FMD) has been observed in the Mimulus guttatus Iron Mountain (IM) population where an expanded satellite array within a large non-recombining haplotype on chromosome 11 (MDL11) is associated with preferential segregation into the functional megaspore. Homozygous carriers of the drive haplotype (DD) exhibit reduced seed set and male fertility. The molecular basis of these costs remains unclear. One candidate gene, NASP—a known chaperone of the centromeric histone variant CenH3—resides within MDL11 and shows both sequence divergence and differential expression between drive and non-drive haplotypes. To test NASP’s role in reproductive fitness and drive-associated costs, we used Arabidopsis thaliana as a model system. T-DNA insertion NASP mutants displayed reduced seed set and pollen counts, suggesting roles in both female and male meiotic development for NASP. Complementation with M. guttatus NASP alleles in nasp mutants revealed that the non-drive (D⁻) allele restored male fertility, while preliminary data suggests the drive (D) allele may not. While further data is needed, this indicates the potential of functional divergence of NASP alleles in M. guttatus. Surprisingly, complementation with the A. thaliana NASP allele in a single transgenic line did not fully restore male fertility and was associated with a stress phenotype, suggesting possible dosage sensitivity. This is consistent with prior observations that NASP is overexpressed in meiotic tissues of homozygous driver carriers, although insertion site effects cannot be ruled out Our findings suggest that NASP may contribute to fitness costs associated with female meiotic drive in Mimulus guttatus. Future studies in M. guttatus using overexpression and targeted genome editing will be essential for testing whether NASP dosage or sequence variation alters CenH3 loading and drive efficiency in its native genomic context.