M. J. De Maine

An evaluation of the use of dihaploids and unreduced gametes in breeding for quantitative resistance to potato pathogens

In a breeding programme to combine quantitative resistance to two important potato pathogens, Phytophthora infestans causing late blight in foliage and Qlobodera pallida (the cream potato cyst-nematode), tetraploid (4 ×) off spring were obtained by crossing dihaploids, mainly as females, with tetraploids. Glasshouse and field tests in the first and second tuber-years showed that most of the off spring of highly blight-resistant dihaploids had high resistance to P. infestans . Two such dihaploids, PDH 182 and PDH 247, crossed with a tetraploid selected for high general combining ability for potato cyst-nematode (PCN) resistance, gave off spring all of which were highly blight resistant. About one third also had high PCN resistance. Detached leaflet tests were used for rapid blight resistance screening of other progenies from dihaploid × 4 × crosses in their seedling year. Differences were found in the effects of 4 × parents on the mean resistances of progenies from blight-resistant dihaploids. The results also showed differences between dihaploids in their effectiveness in transmitting blight resistance to 4 × offspring. This could be due to differences in the mode of unreduced female gamete formation, i.e. first division restitution or second division restitution (SDR). If all the unreduced gametes were produced by SDR, the differences could be due to differences in homozygosity between dihaploids with respect to blight resistance genes. The dihaploid × 4 × crosses gave few offspring per pollination. The few seedlings obtained may be offset by a higher frequency with the required characters. Since fewer seedlings would have to be grown in order to find the required combination of characters, savings could be made in planting and resistance-testing facilities over those required in conventional breeding. The tetraploid hybrids obtained can be used in a second cycle of breeding in three ways simultaneously: by crossing with other tetraploids, by crossing with dihaploids selected for complementary characters and to produce dihaploids with combined resistances.

The agronomic value of tetraploid F1 hybrids between potatoes of Group Tuberosum and Group Phureja/Stenotomum

SummarySelected tetraploid F1 hybrids, between Tuberosum cultivars and 2x Phureja clones, were compared with standard cultivars in randomized trials during the period 1980 to 1986 under conditions which simulated British agricultural practice. Hybrids were superior to standard cultivars in both total and marketable yield, and produced more tubers. We suggest that high yields in the 4x F1 hybrids result from the addition of complementary parental characters, high tuber numbers from Phureja and generally larger mean tuber size from Tuberosum. We discuss the relo of heterosis in an interploidy breeding scheme utilising unreduced male gametes and emphasise the value of tetraploid hybrids between Tuberosum and improved Phureja.

Field resistance to late blight and potato root eelworm in group tuberosum dihaploids

SummaryAn examination of the feasibility of obtaining dihaploids with high levels of field resistance to important potato pathogens, showed that dihaploids may be produced which are more resistant to late blight or to potato root eelworm pathotype E than are their tetraploid parents. The significance of this is discussed in relation to the possible types of gene action involved in conferring field resistance.

Disease resistance inSolanum phureja and diploid and tetraploidS. tuberosum × S. phureja hybrids

SummaryDiploidSolanum phureja, diploid and tetraploidS. tuberosum × S. phureja hybrid, and tetraploidS. tuberosum backcross clones were assessed for resistance to commercially important diseases. There was a reduction in the expression ofS. phureja characteristics with increasing dosage ofS. tuberosum genetic material. The generally high resistance of theS. phureja group to common scab, potato leafroll virus and potato virus Y decreased, while resistance to gangrene, foliage and tuber blight tended to increase.Resistance genes were probably well dispersed throughout theS. phureja genotype so that many were lost on hybridisation, butS. phureja could be a useful source of scab and virus resistance in the production ofS. tuberosum cultivars.

Breeding for quantitative resistance to potato cyst nematode (Globodera pallida) in tetraploid potatoes using dihaploids and unreduced gametes

SummaryDihaploids were produced from tetraploids resistant to potato cyst nematode (Globodera pallida (Stone)). High levels of resistance were found in the dihaploids and three were used to produce tetraploid progenies by crossing them with susceptible tetraploid cultivars. One dihaploid, PDH505, produced more highly resistant offspring than the other two, PDHs 417 and 418. The latter gave progenies whose levels of resistance were similar to those obtained from susceptible dihaploids crossed with resistant tetraploids.The differences between the progenies of the resistant dihaploids were probably due to different modes of unreduced gamete formation (PDH505 producing gametes by first division restitution (FDR) and PDHs 417 and 418 by second division restitution (SDR)) although cytological studies would be necessary to confirm this. The methods by which dihaploids could be utilised in a tetraploid potato breeding programme are discussed in relation to the mode of unreduced gamete formation.

Potato haploid technologies

The term ‘dihaploid’ is used to describe a plant which contains the haploid number of somatic chromosomes but which contains two genomes per somatic cell. The common potato Solanum tuberosum L. has 48 chromosomes per somatic cell (2n=4x=48), so that the number of chromosomes in the egg cell and in haploid somatic cells is 24. Haploids of S. tuberosum have been the subject of widespread study because they promised a means of investigating the genetic architecture of the world’s most commercially important broad-leaved crop. As the common potato is tetraploid, genetic investigations using it involve much more complex numbers than those arising from the use of diploid plants. Dihaploids offered a means of breeding the common potato at the diploid level. It is important to note, however, that the tetraploid state is generally regarded as the physiologically most efficient ploidy level for potato. Tetraploid S. tuberosum potatoes have commercial yields that far outweigh those of their dihaploid counterparts. It is therefore necessary to have technologies that can both generate haploid plants and regain the tetraploid chromosome level.

Culinary quality of tubers derived from Solanum phureja and S. tuberosum × S. phureja hybrids

Clones from the potato species Solanum phureja and 2x and 4x hybrids between S. phureja and S. tuberosum were tested for the culinary quality of their tubers following steaming, frying and baking. The quality of S. phureja tubers was high but material from the F 1 hybrids showed more disintegration or blackening after cooking than either parental species. Tetraploids produced by backcrossing 4x F 1 hybrids to S. tuberosum cultivars showed less of these defects. Fourteen S. phureja and 2x F 1 hybrid clones selected for their unusual tuber skin pigmentation or shape were found to have a variety of baked flesh colours from orange-yellow to cream with red marbling. Their cooked flesh texture was generally floury and often crumbly (.)

The use of dihaploids in increasing the homozygosity of tetraploid potatoes

SummaryElectrophoresis banding patterns of the tuber proteins of 12 dihaploids of the cultivar Pentland Crown showed that four had types of patatin unlike that of the parent. The patatin types of somatically chromosome-doubled clones derived from three of the dihaploids were identical to those of the dihaploid progenitors. Fourteen dihaploids produced from the chromosome-doubled derivative (PDH40X2) of one dihaploid, which had a variant patatin, had the patatin type of the parent dihaploid.The experiment showed that dihaploids and somatic chromosome doubling could be used to fix variation found in heterozygous tetraploid potatoes. Used together, haploidisation and chromosome doubling can generate highly homozygous tetraploids for use by plant breeders.

Intraspecific somatic and sexual hybridisation between dihaploid lines of Solanum tuberosum L.: evaluation of morphological traits and resistance to late blight Phytophthora infestans (Mont.) de Bary in the foliage

Intraspecific tetraploid somatic and sexual hybrid plants have been resynthesised following protoplast fusion and by sexual crosses between two dihaploid potato (Solanum tuberosum) lines each possessing complementary agronomic traits. The dihaploid PDH 40 possesses good tuber shape and yield but has foliage susceptibility to late blight (Phytophthora infestans). On the other hand, the dihaploid PDH 727 possesses resistance to blight in the foliage but has a low yield of small and irregular shaped tubers. Since it was only possible to use a partial selection strategy based on culture media to facilitate recovery of somatic hybrid plants-further morphological and esterase isozyme based characterisations were performed to identify somatic hybrid plants from amongst the non-hybrid plant material. When the blight resistance of both the intraspecific somatic and sexual hybrid plants was assessed there was no significant difference in the mean resistance value and it was intermediate between those of their parents. However, the range of resistance was much wider among the sexual hybrids than among the plants derived from somatic fusion. An assessment of tuber yield between tetraploid sexual and somatic hybrids showed no significant difference and it was higher than that of either parent value. The implication of these results in the context of potato genetics and breeding is discussed.

Soft-rot resistance combined with other tuber characters in long day-adaptedSolanum phureja

SummaryOffspring were produced from a cross between two long-day-adaptedSolanum phureja clones which carried resistance to tuber soft rot (Erwinia carotovora subsp.atroseptica). In tests carried out on the produce of field-grown plants raised from tubers, over fifty per cent of the 173 offspring were found to be highly resistant. Assessments were also carried out of tuber yield, mean tuber weight, tuber number, shape, regularity, flesh colour, texture of the steamed flesh, fry colour, after-cooking blackening, sprout length after storage and overall dormancy. There were statistically significant differences between clones for all characters (P<0.001).Twelve of the clones were selected on the basis of high resistance, yield, tuber weight, regularity of shape and absence of after-cooking blackening. The value of resistant long-day-adapted diploid material for commercial breeding is discussed.