Genomic status of Brassica tournefortii Gouan

The various 20-chromosome cross-compatible taxa of Brassica belonging to the A genome can be put, purely on a morphological basis, into three groups - oleiferous, rapiferous and leafy. B. tournefortii, another 20-chromosome species of Brassica, was crossed with a taxon of each of the above groups with the object of finding its genetic divergence on the basis of meiotic synapsis in the hybrids. All the hybrids were sterile. More bivalents (4 II) were noticed in the hybrid involving the oleiferous taxon than in the rapiferous or leafy ones (2 II). It is inferred, on the basis of genetic equivalence of hybrids dependent upon the pairing relationship in meiotic metaphase, that B. tournefortii is of later origin and has been evolved from the oleiferous group. A new genome, D, has been assigned to B. tournefortii to distinguish it from the other A genome taxa of Brassica because of the low cross-compatibility, hybrid sterility and very little gene flow between the hybrids.