CRITICAL DISCUSSION ON ABORTIVE INTERSPECIFIC CROSSES IN BRASSICA

In most interspecific hybridization among diploid Brassica species (n = 8, 9, 10), few or no hybrids are obtained, and the three crosses of tetraploid (n = 17, 18, 19) by diploid species are usually only partially successful, especially in one of the reciprocal crosses. Such a cross-incompatibility complex is explained in this paper by a polar-nuclei activation (PNA) hypothesis which proposes two reversible functions, a normal reproduction and a sexual barrier, of double fertilization in higher plants. The first, i.e. the relative activating value (AV) of diploid species, was estimated as 1.0-3.5, whereas that of tetraploid species was 2.7-5.2. In addition, crosses with a polar nuclei activation index (AI) of about 15-87 % were compatible but those beyond the two limits were incompatible, 50 % of those showing normal seed development after self-pollination.