R. W. Hougas

The potential of potato haploids in breeding and genetic research

Haploidy offers promise of new, efficient approaches to potato breeding. It further presents a unique opportunity for accelerated genetic and cytogenetic study of the common potato, Solanum tuberosum L. Haploid individuals have been found rarely among numerous plant species. Many of these haploid plants have been examined cytologically and a limited number have been examined genetically. Haploids are of interest to plant breeders primarily because they present a rapid method for obtaining a high degree of homozygosity. Chromosome doubling of monoploids (haploids derived from diploid species) results in completely homozygous individuals. The work with corn, cotton and pepper is well known in this respect (3, 8, 28). The common potato is a tetraploid and is considered an autotetraploid by most investigators (2, 19). Haploids (i.e. diploids) from autotetraploids would seem to offer, in some respects, greater possibilities for research and breeding than would haploids from allotetraploids or diploids. This is particularly the case in genera, such as Solanum, which include diploid species closely related to the autotetraploid species. Relatively little has been done in the study of haploids derived from an autotetraploid species. Recently a method for obtaining haploids (2n=24) of the common potato (2n=48) was reported (11, 13). Such haptoids represent the initial tools with which certain of the anajor difficulties inherent in the breeding and genetic study of an autotetraploid may be overcome. Potato breeders of this country and abroad have found the job of developing new commercial varieties an increasingly difficult task. This is readily understandable since, in the United States alone, upwards of 70 new varieties have been released for commercial production since 1932 (12). The ever-increasing demand for incorporation of additional characters in new varieties (e.g. multiple resistance to diseases, superior chipping quatity, early maturity and adaptability to mechanical harvesting) places a heavy burden on the potato breeder. It nmst be recognized, of course, that similar demands are being made of all plant breeders. Potato breeding, however, is vastly more complicated than that of breeding most other crop plants due to tetrasomic inheritance in the potato. Although asexual propagation simplifies the maintenance of varietal genotypes in the potato, it has indirectly tended to coxnplicate the task of developing superior parents. This occurs since, on the one hand, the breeder nmst generally select for a highly heterozygous individual to meet the requirements of a superior commercial variety; and, on the other hand, he must strive for parental stocks sufficiently homozygous to allow for reasonably expeditious recombinations of the genes conditioning the many inherited characters essential to the modern eolnmercial variety.

Genetic variation among haploids of the common potato

Summary1.Genetic variability among 29 haploids (2n=24) of the common potato (2n=48) is described.2.Plant parts of the haploids are reduced in size, with few exceptions, as compared with those of the parent.3.The tuber yields of the haploid plants are in general, considerably less than those of their parents. One notable exception, however, is the haploid US-W20. which in an initial limited trial outyielded its parent Katahdin in a 124-day growing season.4.Twenty-two of the 23 haploids that flowered are functional as female parents. Two haploids. US-W1 from Katahdin and US-W4 from Minn. 20-20-34, are functional both as male and female parents in interhaploid matings as well as in matings withSolanum phureja (2n=24).5.Certain genetic consequences of haploidy are discussed.

Variability in the expression of self-incompatibility in tuber-bearing diploid Solanum species.

SummaryAn intraspecific assessment of the variability in the expression of self-incompatibility was made since such information could be broadly 14 diploid species were used. Self-pollinations in the field were made on intact plants and in the greenhouse on cut-stems. Considerable variation in the self-incompatibility within families was observed. This is surprising since the majority of the diploid species are considered to be self-incompatible. Field pollinations resulted in some “self-compatibility” in 9 of 11 species tested, while 11 of 12 species tested under greenhouse conditions yielded individuals which set self-seed.

Germination of Solanum pollen on artificial media

SummaryA solution of 20% sucrose plus 50 ppm boric acid was found to be a suitable medium for germinatingSolanum pollen in vitro. Pollen from 50Solanum species was tested on the artificial medium. In 41 species the per cent of germination and the extent of pollen tube growth were positively enhanced by the presence of boron. Th nine other species exhibited a lesser sensitivity to boron. A condition of low relative humidity around the germinating medium could apparently change the concentration of the solution and thus the percentage of germination. Therefore, the best in vitro germination was obtained under conditions of 100% relative humidity. A population effect was observed when attempting to germinate a small amount of pollen, i.e., for a given amount of medium a minimum number of pollen grains were necessary to enable the pollen to germinate. This effect could be considerably reduced by adding a water soluble extract of pollen to the medium.

Fertility of S. phureja-haploid S. tuberosum F2 hybrids

SummaryThe purpose of this study was to determine female and male fertility, and self-compatibility within 26 families derived from inter-mating the F1 hybrids from the cross,S. phureja-haploidS. tuberosum. The F1 hybrids involved 5S. phureja selections (P.I.’s 195198, 225685, 225696, 243462, 243466) and 2 haploids (US-W 1 from Katahdin and US-W 42 from Chippewa).Fertility tests were made in the field, greenhouse and laboratory. Female fertility tests were carried out in the field. Bulked pollen of five or more plants was used in order to pollinate each plant in the family. Male fertility was tested by pollen shedding, pollen germination and controlled sib-matings in the field. Self-compatibility tests were made in an air-conditioned greenhouse using the decapitation technique.Sixty-three percent of the plants were female fertile and77% were male fertile. Pollen-shed was an efficient method of determining male fertility in these materials. A low level of pseudo self-compatibility occurred.The vigor of most plants, the high frequency of flowering, and the high incidence of female and male fertility indicated that this material will be useful in exploring potato breeding at the diploid level as well as in genetic and cytogenetic studies.

Cytogenetic studies of South American diploid Solanums, section tuberarium

SummaryMeiosis is regular in a majority of the microsporocytes in a number of clones of six South American diploid species and their hybrids. Meiotic abnormalities such as the presence of univalents, quadrivalents, chromatin bridges and fragments, laggards, formation of restitution nuclei, micronuclei and other irregularities are extant in a small percentage of microsporocytes. The presence of occasional quadrivalents in certain clones of some species and hybrids suggests the existence of some structural differences between certain chromosomes. This probability is further supported by the occurrence of a bridge and fragment resulting from inversion heterozygosity and delayed separation of certain bivalents. The presence of univalents may be caused by segmental or genetic differences between homologous chromosomes as well as to other factors and cannot be entirely explained as being due to precocious separation of rod bivalents. However, the fact that the maximum possible number of bivalents are realized in a large number of the pollen mother cells suggests that such structural differences cannot be very extensive. An unusual type of nucleolus formation occurs. The frequency of various types of secondary groupings is variable.

Chromosome pairing at different ploidy levels in the tuber-bearing Solanums

SummaryThe nature of chromosome pairing at meiosis in some tuberiferous Solanums (2n, 3n and 4n) is described. A number of abnormalities have been noted at meiosis in the diploids including the formation of multivalents and univalents. Although the occurrence of univalents as early as diakinesis suggested that chromosomal differences do exist, the evidence from the diploids as to the nature of the basic genome in Solanum is not clear cut. An examination of meiosis in triploid hybrids (4n × 2n) revealed a considerably higher value for the mean number of bivalents plus trivalents per cell at metaphase I (14.2; 13.5; and 13.3) than has heretofore been reported. The maximum number of such associations recorded was 16. It appears that in the hybrid between the induced tetraploidS. chacoense (n=24) andS. neohawkesii (n=12), at least 8 out of the 12 chromosomes from the diploid parent are capable of exhibiting partial homology. The analysis of pairing in the triple hybrids ((6n × 2n) × 4n) is more complicated. However, there are some indications that autosyndetic pairing within the complement of chromosomes derived from the 4n parent may exist. The occurrence of pairing within the haploid set of chromosomes derived from one diploid species appears to be suggestive of the possible existence of two subgenomes within the haploid genome but further studies of the whole genus are required for a verification of this possibility.

Inter-regional potato introduction and preservation project

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