FIGURE 6.

Models of processes of hybrid sterility and its circumvention. (A) A new model of hybrid sterility and plant regeneration by AC during microspore development in WK21/Nip F₁. Microspores of WK21/Nip F₁ do not develop into pollen because of HS due to 1) meiotic aberrations and 2) allelic interactions at HS loci. (B) Higher rates of fertile gametes in tetraploids compared with diploids according to the killer–protector model. Under the killer–protector model of HS, the killer protein has a sporophytic effect on gametes during microspore development after meiosis, but the gamete expressing the protector protein is not killed. In the case of a heterozygous tetraploid plant, which contains two killer–protector alleles and two killed alleles, three-quarters of the gametes possess protector alleles. In the case of a heterozygous diploid plant, only half of the gametes carry a killer–protector allele. In theory, a heterozygous tetraploid thus produces 25% more surviving gametes than does a heterozygous diploid plant.