S. J. Peloquin

The significance of 2N gametes in potato breeding

SummaryPhureja-haploid Tuberosum hybrids, which produce 2n gametes in addition to n gametes, were used to obtain diploid progenies in 2x – 2x matings, and tetraploid progeny in 4x − 2x matings. Seven of these diploid clones were intermated in a modified diallel design, and also crossed to seven cultivars, to obtain 21, 16-clone diploid; and 49 (35, 2x X 4x and 14, 4x X 2x) 24-clone tetraploid families, respectively. These progenies were included together with the 14 parental clones, in a tuber yield trial conducted in two locations in Wisconsin. Heterotic responses were striking in 4x X 2x progenies. Three of these entries had mean yields of 5.0, 5.0 and 5.2 lbs/hill, for 24 unselected clones, thus outyielding not only the mid-parent but also the best tetraploid cultivars in the experiment, which are highly selected; Wis 643 (4.8 lbs/hill), and Kennebec (4.8 lbs/hill). As a group, the 14, 4x X 2x families averaged 4.4 lbs/hill, while the mean of all seven cultivars was 4.0 lbs/hill, and the mid-parent was at 3.4 lbs/hill. This unusually high mean performance of 336 unselected clones represented in the 14, 4x X 2x progenies, is interpreted as a manifestation of the capacity of 2n pollen, formed by first meiotic division restitution (FDR), to pass onto the progeny the already heterotic diploid genotype in a largely intact array. Beneficial intra and inter-locus interactions are presumably compounded upon syngamy with an unrelated n egg from the tetraploid parent. -The performance of 2x X 4x progenies was at or below that of the midparent. The failure of these families to perform as well as 4x X 2x families may be a reflection of the incapacity of 2n megasporogenesis to avoid meiotic reassortment as efficiently as FDR does, which would result in inbred 2n gametes. However, the method of 2n megasporogenesis is not known.Hereditary variances were large, both within and among families, and indicated considerable nonadditivity in the action of genes affecting tuber yield at both levels of ploidy. Inbreeding was strongly depressing at both ploidy levels.

Breeding value of 2n pollen (diplandroids) in tetraploid x diploid crosses in potatoes.

SummaryThe relative breeding value of first division restitution (FDR) and second division restitution (SDR) 2n male gametes from diploids, and n (2x) gametes from tetraploids were compared. This was done by measuring tuber yield of 105 4x families obtained from 4x x 2x (FDR), 4x x 2x (SDR), and 4x x 4x crosses at two locations. All tetraploid families obtained from 4x x 2x (FDR) matings exhibitedheterosis (the mean of the F1 exceeded that of the higher yielding parent), and outyielded other tetraploid families and 4x cultivars at both locations. The tuber yield of 4x x 2x (SDR; families was higher than that of the families derived from 4x x 4x matings, although the 2x parents had significantly lower yield than tetraploid parents. Specific combining ability (S.C.A.) was found to be significant when families obtained from each category (4x x FDR, 4x x SDR and 4x x 4x) were analyzed individually. Both general combining ability (G.C.A.) (2x) and S.C.A. were significant when families obtained from: 1) 4x x FDR plus 4x x SDR; 2) 4x x FDR plus 4x x 4x; and 3) all categories, were analyzed. Only S.C.A. was found to be significant when 4x x SDR and 4x x 4x families were analyzed together. Consistent results were obtained at each of, and over, the two locations. These results strongly demonstrated that FDR gametes were superior in breeding value to either SDR or n (2x) gametes, and were more homogeneous, Gametes formed by SDR might be superior to n (2x) gametes although they were as heterogeneous. Distinct yield differences between 4x x FDR and 4x x SDR families plus the significant G.C.A. (2x) detected when these two categories were compared, substantiated the cytological interpretation of genetically distinct modes of diplandroid formation.

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.

2n gametes in potato species and their function in sexual polyploidization

SummaryThe extent and pattern of polyploidy in the tuber-bearing Solanums varies among the many taxanomic series that have been identified in this subsection of Solanum. While several series appear to be entirely diploid, others exhibit a range of ploidy levels from 2x to 6x, and some contain only polyploid species.In many diploid, triploid and tetraploid species 2n gametes (gametes or gametophytes with the sporophytic chromosome number) have been detected. Both 2n eggs and 2n pollen occur. 2n gametes provide the opportunity for unilateral and bilateral sexual polyploidization. The genetic determination and consequences of sexual polyploidization strongly suggest that 2n gametes have been the major instrument in the polyploid evolution of the tuber-bearing Solanums. Somatic doubling of species and interspecific hybrids appears to be of very limited importance.New evidence for the occurrence of 2n eggs and 2n pollen in many species is reported, and data from the literature are added to illustrate the widespread distribution of 2n gametes throughout the subsection. A very high correlation is found between polyploidy and 2n gametes, and its significance is discussed. Proof is presented for the occurrence of alleles governing 2n pollen production in the cultivated tetraploids, providing additional evidence for the hypothesis that 2n gametes have been involved in their origin.Multiple unilateral and bilateral sexual polyploidizations are proposed for the origin of the cultivated tetraploids: this accounts for the large variability encountered in this group, which closely resembles that of the related diploids. Similar evolutionary pathways are hypothesized for the other polyploid complexes in the subsection.A scheme is proposed in which participation of both 2n and n gametes link together all ploidy levels in the tuber-bearing Solanums, thus overcoming the ploidy barriers and providing for gene flow throughout the sympatric species of the subsection.

Bilateral sexual polyploidization in potatoes

SummaryBilateral sexual tetraploidization was achieved by intercrossing diploid Phureja × haploid Tuberosum hybrids. Both parents contribute gametes with the somatic number of chromosomes (diplogynoids and diplandroids). More than 100 tetraploid progeny appeared in nine 2x−2x families. The tetraploids are generally more vigorous and significantly outyield their diploid ‘full-sibs’. The superiority of the tetraploids is interpreted on the basis of the mode of diplandroid (2n pollen) formation by first meiotic division restitution (FDR). Heterozygosis, epistasis and genetic diversity, if present in the parental diploid genotype, are largely maintained in the 2n gametes formed by FDR. and syngamy brings about more opportunities for heterotic responses in conjunction with polyploidization. Differences in tuber yield among tetraploid clones within families were found to be statistically significant (P <0.05). Thus, unlike somatic chromosome doubling, which can merely increase chromosome numbers, or somatic hybridization, which could in addition bring about heterosis, sexual polyploidization is also associated with genetic variability. The degree of inbreeding and the genotypic structure in the population of newly arisen tetraploids is discussed. The relationship among asexual reproduction, preservation of favorable nonadditive genetic effects by FDR. restoration of sexuality, and polyploidization is explored. The possible evolutionary significance of these factors is emphasized.

Distribution and significance of diplandroids among the diploid solanums

Clones of several diploid species capable of generating diplandroids (2n pollen) were identified by screening for the occurrence of 2n pollen. Diplandroid production was detected inSolatium bukasovi, S. canasense,S. cardiophyllum, S. chacoense, S. leptostigma, S. megistacrolobum, S. tarijense, S. tuberosum Group Phureja and Group Stenotomum. Parallel spindles at Ana II was the mechanism producing 2n pollen inS. chacoense, Group Phureja, and Group Stenotomum. The widespread distribution both taxonomically and geographically suggests a possible role for diplandroids in evolution. It also encourages exploration of a proposed breeding method in which tetraploids are generated from 2x × 2x matings.

Origin and evolution of cultivated tetraploid potatoes via 2n gametes.

SummaryA high gene frequency for ps (parallel spindles) is expected in cultivated tetraploid potatoes, S. tuberosum Group Tuberosum, if 2n pollen produced by ancestral diploid plants which were psps was involved in the origin and evolution of the potato. Fifty-six North American cultivars (varieties and advanced selections) were pollinated by diploid clones, either W 5295.7 or W 5337.3 which are homozygous recessive for ps. The segregation ratios in regard to 2n pollen production in derived tetraploid progenies, from 4x×2x crosses, reveal the genotype of ps in the cultivars. Microsporogenesis of 2n pollen producing 4x progeny was observed to avoid an overestimation of the frequency of 2n pollen producing plants due to mechanisms other than parallel spindles. More than 50% of the 56 cultivars are simplex (Pspspsps), since in each of these cultivars about 50% of their progeny produced 2n pollen. The ps gene frequency in the 56 cultivars was estimated as high as 0.69. The high frequency of ps in the tetraploid cultivars clearly supports the hypothesis that 2n pollen produced by plants homozygous recessive for ps have been involved in the origin of cultivated tetraploid potatoes, since a higher frequency of ps in the tetraploid than in the ancestral diploid population can be expected from sexual polyploidization but not from somatic doubling. The importance of meiotic mutants such as ps for the successful evolution of polysomic polyploids is emphasized.

Occurrence of 2n pollen and ps gene frequencies in cultivated groups and their related wild species in tuber-bearing Solanums *

SummaryThe gene frequency for parallel spindles (ps) was estimated from the frequency of plants producing 2n pollen in three cultivated groups: 2x Phureja (phu), 2x Stenotomum (stn), and 4x Andigena (adg), as well as in four related wild taxa: 2x Solanum brevicaule (brc), 2x S. sparsipilum (spl), 4x S. gourlayi (grl) and 4x S. gourlayi-S. infundibuliforme hybrids (grl-ifd). Plants with more than 1% large pollen were considered as 2n pollen producers. Observations of meiosis in a sample of 2n pollen-producing plants indicated that parallel spindles is the mechanism of 2n pollen formation. The number of plants with 2n pollen among the total examined was 228 plants (15.5%) of 1,473 in 2x spl, 31 (26.7%) of 116 in 2x brc, 92 (17.4%) of 528 in 2x stn, 665 (22.1%) of 3,008 in 2x phu, 731 (51.4%) of 1,421 in 4x adg, 591 (41.2%) of 1,436 in 4x grl, and 36 (64.3%) out of 56 in 4x grl-ifd. The ps gene frequencies assuming Hardy-Weinberg equilibrium were: 0.393 for 2x spl, 0.462 for 2x brc, 0.417 for 2x stn, 0.470 for 2x phu, 0.847 for 4x adg, 0.801 for 4x grl, and 0.895 for 4x grl-ifd. Twenty-five adg clones were randomly selected from a large population and were crossed with 2x clone W5295.7, which produces 2n pollen by parallel spindles (ps). The 4x progenies from 4x×2x crosses were used to determine the genotypes at the ps locus by screening 10–20 plants in each family for 2n pollen. Based on chromosome segregation at the ps locus, 9, 14, 1, and 1 clones were nulliplex, simplex, simplex or duplex, and duplex, respectively. The frequency of the ps gene in the adg population was estimated to be 0.825 and 0.815 for chromosome and chromatid segregation, respectively. The high frequencies of 2n pollen and the ps gene in cultivated 2x and 4x groups, and in wild taxa closely related to them, provide evidence for sexual polyploidization in the tuber-bearing Solanums.

Sexual polyploidization and depolyploidization: some terminology and definitions

SummarySexual polyploidization (deployploidization) is the process through which a euploid zygote is formed whose chromosome number is increased above (decreased below) the level that would be expected if each parent contributed a gamete carrying one-half the parental premeiotic chromosome number. One or both gametes may contribute (lack) the extra genome (s) so that these processes may be unilateral or bilateral. Sexual polyploidization has the distinctive characteristic of being a genetically innovative mechanism. This is believed to be its most significant difference with respect to somatic doubling. Sexual depolyploidization, which may be conceived to arise in some rare situations, also has the capacity for innovation. Several cytological mechanisms may be envisioned which give rise to polyploidizing gametes. The genetic consequences of these mechanisms can be markedly different in regards to gene reassortment. Short of a mitotic-like method, FDR (first division restitution) appears to be the most appropriate mechanism to attain such a goal. Other modified meiotic processes generally result in more disturbance of the premeiotic genotypes. Under these circumstances, sexual polyploidization is probably associated with mitigated heterotic responses and a more extensive release of genetic variability.As a cytological device, FDR has the unique property of producing balanced 2n gametes even after a numerical nondisjunction has occurred. This simple mechanism may restore fertility to, e.g., monoploids and triploids, so that odd-ploids may possess more evolutionary significance by acquiring a means of producing balanced gametes systematically.Some new terminology is proposed to designate gametes and gametophytes which conserve the parental number of chromosomes, and for polyploids arising in conjunction with their functioning in fertilization.

Gene-centromere mapping by 4x-2x matings in potatoes

SummaryTwo of the four strands of a bivalent are recovered together in tetraploid progeny arising from 4x-2x matings. This provides a method for gene and centromere mapping. The cross pppp x Pp produced 62 nulliplex tetraploid individuals in a total of 951, i.e., 6.5%. The diploid clone was found to produce diplandrous gametes through first division restitution (FDR). The map distance P-centromere was estimated as 13.0 units, the limiting values at the 95% binomial confidence interval being 10.1 and 16.3. The mapping method is explained in detail and a formula is deduced to obtain genotypic series of 2n gametes under particular assumptions. The direction 4x×2x is advantageous, since high seed set diploid clones which give 99% tetraploid progeny, and whose method of diplandroid formation is known are already available. Diploid clones heterozygous for many markers and tetraploids nulliplex for the same markers are needed to fully exploit this method.