Varroa mites escape the evolutionary trap of haplodiploidy

Genetic diversity is essential for populations adapting to environmental-changes. Following genetic bottlenecks, invasive species often have reduced diversity, yet must rapidly adapt to novel environments. This paradox is especially pronounced in parasites, which face repeated transmission bottlenecks and anthropogenic countermeasures. The challenge of maintaining diversity in the short-term intensifies in haplodiploid species, where males inherit and transmit only half of their mothers genome, limiting effective population size. To examine how such species may maintain adaptability, we studied inheritance in Varroa mites (Varroa destructor), globally invasive honey bee parasite. Using three-generation pedigrees, we found that Varroa is not haplodiploid, as previously thought. Rather, females produce diploid sons who transmit either maternal allele to daughters. This system slows reduced heterozygosity loss under sib-mating, compared to ancestral haplodiploidy, increasing effective population size and adaptability. Our findings reveal a rare reversion from haplodiploidy, long considered an evolutionary end state, and suggest reproductive plasticity underlies the resilience of invasive parasites like Varroa.