The sex difference observed in this study can be categorised as an example of “graduated steps of sterility”30 from advanced in females to complete in males. Genetic and chromosomal incompatibility is probably amplified in the male hybrids by the well known vulnerability of spermatogenesis to pairing aberrations28. We observed stochastic variation in “degree of sterility” even between oocytes of the same F1 genotype. Those containing “correctly” paired configurations are probably able to produce viable balanced oocytes. The more asynapsed regions an oocyte contain, the larger part of its genome undergoes meiotic silencing of unsynapsed chromatin (MSUC)6465, and the higher the chance for the oocyte to be directed to apoptosis. The results of this study indicate that reproductive isolation based on hybrid sterility may be built up in a gradual mode. A gradual genetic divergence and the sequential fixation of different chromosome rearrangements in isolated populations increase the probability of pairing errors followed by MSUC and apoptosis in the hybrid gametocytes.