On the sterility of the interracial hybrids in Drosophila pseudoobscura

Offspring from interspecific crosses are frequently equal or superior to their parents in somatic vigor, and are, nevertheless, partially or completely sterile. The sterility is due to a disturbance in the process of gametogenesis usually involving a more or less complete lack of chromosome pairing at meiosis. The known instances of interspecific sterility fall into two groups. To the first group belong cases described by Karpechenko,(1) Clausen,(2) Tschermak and Bleier(3) and many others, who have found that the doubling of the chromosome complement of sterile hybrids results in the appearance of fertile allotetraploids showing a more or less complete chromosome pairing. To the second group belongs the case of the interracial hybrids in Drosophila pseudoobscura. As shown below, the doubling of the chromosomes does not influence here either the lack of chromosome pairing or sterility.