The occurrence of hybrid semi-lethals and the cytology ofTriticum machaandTriticum vavilovi

Abstract

1. A semi-lethal gene combination has been found in hexaploid species hybrids involving some strains ofTriticum macha, but not in comparable hybrids involving other strains ofT. macha. The production of these semi-lethal hybrids can be explained by the interaction of two genes designated as maand mb.2. The gene mais carried in some strains ofT. macha. The gene mbis carried in the third chromosome set of all other hexaploid species ofTriticum, inAegilops squarrosaand inAeg. cylindrica.3. The semi-lethal gene combination has been found between species which would otherwise produce fertile hybrids and between species which would otherwise produce sterile hybrids. The semilethal gene combination occurs between species which do not overlap geographically.4. The distribution of the genes maand mbshows that the gene maoriginated inT. machaand that the gene mboriginated inAeg. squarrosaand was introduced from there, during the course of evolution, into the hexaploid species ofTriticum(probably excludingT. macha) and intoAeg. cylindrica.5. Meiosis in semi-lethal plants was comparable to meiosis in normal plants. But the semi-lethal plants showed a reduction in fertility, probably as a result of their poor vegetative growth.6. The chromosomes ofT. machaandT. vavilovihave been no more structurally differentiated than the chromosomes of the other hexaploid species ofTriticum.7. The present study has confirmed that the third chromosome set of the genusTriticumhas been phylogenetically derived fromAeg. squarrosa.