Naisbit et al. [28] state that in the backcross to H. melpomene, ‘sterile females typically laid eggs that did not hatch’ and sterility was associated with Tpi. By contrast, ‘in the backcross to H. cydno, complete sterility was usually manifested as a failure to lay eggs’, and in this cross there was no evidence of a large Z-chromosome effect. This evidence concerns the gene TPI1 and infertility disorder.