Discovery of a synaptic mutant in potato haploids and its usefulness for potato breeding.

A synaptic mutant was found in haploids (2n=2x=24) extracted from the Mexican potato variety 'Atzimba' (2n=4x=48). The mutant is inherited as a simple Mendelian recessive, designated sy4. Meiotic abnormalities of the mutant during microsporogenesis include: poor synapsis at pachytene; high frequency of univalents at diakinesis; elongated and curved spindles and univalents being scattered over the spindles at metaphase I and anaphase I; abnormal chromosome distribution at anaphase I; and production of sterile pollen, presumably due to unbalanced chromosome complement. The expression of sy4 in megasporogenesis was also detected. The sy4 mutant is very useful for potato breeding when combined with another meiotic mutant, parallel spindles (ps), because haploids homozygous for sy4 and ps produce fertile 2n pollen which transmit almost intact genotypes of the parents to the progenies. Thus, the meiotic mutants provide a powerful breeding method for maximizing heterozygosity and epistasis. They can also provide a very efficient method of transferring diploid germplasm, which has desired characteristics efficiently combined at the 2x level, to tetraploids. Many haploids have been identified with 2n pollen production by ps alone or by sy4 and ps, vigorous growth and good flowering, and a high level of resistance to late blight. The importance of a further search for meiotic mutants and their use for breeding is discussed.