Karyological Evidence of Female Heterogamety in the Mosquitofish, Gambusia affinis

In both sexes of the mosquitofish Gambusia affinis, a constant diploid chromosome number of 48 was observed in squashes of gill epithelium, testes, and spleen. The female karyotype invariably includes a single distinctly large metacentric chromosome, absent in all males examined, which strongly indicates female heterogamety of the WZ type. The smallest acrocentric, also without a homomorph, probably complements this large W; in the male the smallest pair of similar acrocentrics (ZZ?) forms an atypical meiotic bivalent. The cytological expression of digamety in teleostean fishes is briefly discussed.