Causes of Difference in Success of Reciprocal Interspecific Crosses

Abstract

In crosses between species which differ in chromosome number the cytological situation in the endosperm depends on the direction of the cross. If the species with the larger number is used as the female the chromosomes which it possesses and which the other lacks will be doubled in the endosperm, whereas in the reciprocal cross they will be single. In wheat the endosperm in which the excess chromosomes are doubled is normal, whereas that in which they are single is shriveled. Consequently when the species with the larger number is female the cross is more successful than when it is male. A review of the literature shows that as a rule the results are similar in other genera of plants: in cases in which reciprocal interspecific crosses differ in success the more successful is generally the one in which the species with the larger chromosome number is used as the female. There are also other causes of a difference in success, such as a difference in the length of the styles or possibly a faulty interaction of the cytoplasm of one parent and the genes of the other. The writer is very grateful to Professor E. B. Babcock for the use of the facilities of the division of genetics, University of California, where the investigation was carried on during sabbatical leave from the University of Saskatchewan.