Meiotic mutants in potato. Valuable variants.

been found in many plant and animal species (Kaul their genetic diversity. In addition to simple disomic and Murthy 1985; Hawley 1993; Dawe 1998; Lindsey inheritance, the employment of haploids has the great 1999). A number of comprehensive reviews surveying advantage for improvement of 2x hybrid populations. research activities with these natural and induced muThese 2x hybrids obtained by this approach can be tants in plants emphasize their value in cytological, decrossed to the 4x potato via functioning of 2n gametes velopmental, biochemical, and molecular investigations (gametes with the sporophytic chromosome number) of the meiotic process per se (Golubovskaya and Mashto obtain 4x progeny (Peloquin et al. 1989b). nenkov 1985; Dawe 1998). Strikingly, no reference was The occurrence of postzygotic barriers may sometimes made to the use of meiotic mutants in genetics, germprevent interspecific and/or interploid crosses, causing plasm transfer, breeding, and evolution. The meiotic abortion of hybrid endosperm. This results in sexual variants found in potato (Solanum tuberosum L.) and its isolation of a number of 2x and 4x species. The endomore than 150 tuber-bearing relatives are unique tools sperm balance number (EBN) hypothesis, formulated for investigating crucial problems in both basic and by Johnston et al. (1980), explains why some closely applied research in this crop species (Peloquin 1983). related species fail to produce hybrid offspring. It states This Perspectives considers the application of these inherthat each Solanum species has an effective ploidy, the ited variations affecting microsporogenesis and megEBN, and after interspecific crosses normal endosperm asporogenesis in potato and their utility in genetics, development occurs only when there is a 2:1 maternal cytology, germplasm transfer, breeding, and evolution to paternal EBN ratio in the hybrid endosperm. Thus, of the fourth most important food crop in the world. successful crosses will occur only when the parents have Potato biology: The cultivated potato is a tetraploid identical EBN. Diploid species with EBN 5 1, and 4x (2n 5 4x 5 48) with four sets of similar chromosomes species with EBN 5 2, are, respectively, sexually isolated (where n is the gametic chromosome number and x is from Tuberosum haploids (EBN 5 2) and cultivated the basic number). The main cultivated germplasm for potato (EBN 5 4). long-day conditions is derived from S. tuberosum Group Description and genetic characteristics of meiotic muTuberosum, whereas S. tuberosum Group Andigena is tants in potatoes: The mutants identified in potato affect adapted for short-day environments. The wild tubermajor aspects of nuclear and cytoplasmic events during bearing relatives form a polyploid series with species microand megasporogenesis. Some key characteristics having 24, 36, 48, 60, and 72 chromosomes—more than of meiotic mutants should be noted: (1) Almost all of 70% of them are diploids (2n 5 2x 5 24). these mutant phenotypes are controlled by a single locus The tetrasomic inheritance of the cultivated potato is (2) the mutations are invariably recessive; (3) sometimes infinitely more complicated than that of 2x relatives. the character associated with a certain genotype fails to Fortunately, haploid plants (2n 5 2x 5 24) of tetraploids manifest—i.e., penetrance is not always 100%; (4) the (2n 5 4x 5 48) can be obtained with ease from the number of meiocytes that express the parental genotype