Cytogenetic and evolutionary studies in Secale. II. Interrelationships of the wild species

Abstract

One strain of Secale silvestre, S. vavilovii, and S. africanum, two strains of S. montanum, the six possible interspecific hybrids, and a reciprocal hybrid of S. vavilovii and S. africanum were studied. S. silvestre is well set off morphologically from all other species. Similarly, S. vavilovii differs rather strikingly from the remaining taxa. S. montanum and S. africanum are quite similar morphologically. F1 hybrids were in general intermediate between the two parents. Data on the crossability of the wild species have been presented. No difficulty was encountered in producing the hybrid seeds. Meiosis in the parental species was normal and meiotic abnormalities were rare. Meiosis in S. montanum × africanum was also normal. However, in all the hybrids in which S. silvestre was one of the parents, irregularities were quite frequent at all stages of meiosis. The hybrids S. montanum × vavilovii, S. vavilovii × africanum, and S. africanum × vavilovii were characterized by a lower frequency of these abnormalities. It is, therefore, concluded that S. silvestre is genetically and cytologically differentiated from the remaining species. On the other hand, S. montanum and S. africanum are quite similar genetically and cytologically. S. vavilovii is decidedly closer to S. montanum than any other species. Translocation configurations occurred in all these hybrids at metaphase I of meiosis. From these configurations it was inferred that S. montanum, S. africanum, and S. silvestre differ from each other by one small translocation. S. vavilovii differs from S. montanum, S. africanum, and S. silvestre by one and the same large translocation, but an additional small translocation difference is present between S. vavilovii and S. africanum and between S. vavilovii and S. silvestre. Pollen fertility of the F1 hybirds was considerably lowered, and reduction in pollen fertility could not be wholly accounted for on the basis of adjacent separation of translocation multivalents. It was therefore concluded that genic sterility barriers also exist between all the wild species except between S. montanum and S. africanum. The hybrids S. vavilovii × silvestre, S. vavilovii × africanum, and S. africanum × vavilovii were seed sterile. Seed set in S. montanum × vavilovii and S. montanum × africanum was quite high. Only a few florets set seed in S. montanum × silvestre. On the basis of geographical distribution, breeding habit, growth habit, morphology, crossability, cytological, and genetic affinities, S. silvestre differs from the remaining species rather strikingly, while the other species seem to be more closely interrelated. It is therefore suggested that S. silvestre be placed in a section by itself, and the other four species in a second section. On similar grounds it is suggested that S. silvestre differentiated from S. montanum much earlier than the other three species. All species differentiated from S. montanum by fixation of one translocation at a time, except S. cereale which differentiated in two steps.