Summary
A bioRxiv preprint reports XXY and XXYY females at frequencies as high as 88% in some strains of four Drosophila species. The researchers link the pattern to a bias in the chromosome types carried by eggs.
A bioRxiv preprint reports that XXY and XXYY females occur at high frequencies in wild-type strains of four related fruit-fly species: Drosophila mojavensis, D. arizonae, D. navojoa and D. wheeleri. In some strains, the reported frequency of these female chromosome genotypes reached 88%. The researchers also found that males from the strains were frequently XYY or XYYY.
The study’s proposed explanation is a bias in how XXY females pass sex chromosomes to their eggs. The authors’ mathematical model indicates that this meiotic-drive advantage could account for the high frequencies in the strains.
Biased chromosome transmission in eggs
An XXY individual has two X chromosomes and one Y. In the researchers’ account, an XXY female is expected to produce eggs carrying either XY or X in equal proportions. Instead, the study reports a 57:43 ratio of XY-bearing to X-bearing eggs. A consistent departure from equal transmission can change how often chromosome combinations are inherited across generations; the authors’ model finds the measured bias sufficient to explain the high frequencies they observed.
The result bears on how sex chromosomes evolve. The authors argue that where XXY females are common, female fitness and egg production can also influence the evolutionary history of the Y chromosome, rather than its evolution responding only to selection in males.
Evidence from a natural population
The researchers also report finding one XXY female among 87 D. mojavensis females and one XYY male among 36 males collected from a natural population. These observations show that the chromosome combinations occur outside the sampled strains, though the small counts do not provide a precise estimate of their frequency in the wild. The authors say field collections were limited by population declines and safety concerns.
The work is a preprint posted on bioRxiv on October 23, 2026, and has not yet undergone peer review.