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Eleven isozyme markers were utilized for quantitative trait loci (QTL) analysis in diploid potato. These markers are distributed among 7 of the 12 chromosomes and therefore give a representative, though sparse, survey of the potato genome. Tuber specific gravity and tuder dormancy were studied. Two segregating diploid populations were constructed from heterozygous self-incompatible parents. These two populations, TRP132 (127 individuals) and TRP133 (110 individuals), have a common maternal parent and combine genomes of Solanum tuberosum (haploid), S. chacoense, and S. phureja. The populations were planted at two locations in Michigan in 1990 using a randomized complete block design with three replications per location. After harvest they were characterized with the isozymes and evaluated for specific gravity and tuber dormancy. To test for QTLs, one-way analyses of variance were conducted for each locus by trait combination. Significant associations between markers and quantitative trait variation were identified, which accounted for a range from 4% to 15% of the phenotypic variation for specific gravity, and from 4.5% to 20.4% for tuber dormancy. Two-way analyses of variance between significant markers were used to identify epistatic interactions between markers. Multiple regression analyses were used to estimate the overall effect of the significant markers on the phenotypic variation for these traits. These values ranged from 15.3% and 32.3% for specific gravity. For dormancy, the significant loci accounted for 8.5% and 36.9% of the total phenotypic variation for each of the populations. We find that isozyme analysis is a useful tool for preliminary QTL studies in potato.

Theoretical models to estimate the coefficient of double reduction in tetraploid organisms and the standard error of this estimate are derived. Using these models, we were able to estimate the coefficient of double reduction for several loci in tetraploid potatoes, Solanum tuberosum L., through examination of segregating isozyme loci in a series of 4x-2x crosses and in haploid progeny derived from six cultivated tetraploid potatoes. Tetraploid x diploid crosses are useful for estimating the frequency of double reduction because of the availability of homozygous diploid tester lines and the large number of tetraploid progeny generated via the functioning of 2n pollen. The strength of haploid analysis is the examination of diploid progeny. However, it is frequently difficult to obtain large numbers of progeny for testing. Based on our results, we conclude that double reduction occurs sporadically in tetraploid potatoes.

Isozyme and ctDNA RFLP patterns were determined for ten historically important potato cultivars (Solanum tuberosum ssp. tuberosuni) in order to relate and confirm their pedigrees. Isozyme polymorphism was detected at 11 of 13 loci examined, whereas only T-type cytoplasm, the predominant ctDNA of S. tuberosum ssp. tuberosum, was observed. Isozyme analysis indicated that potato cultivars previously presumed to be derived from open-pollinated berries of Garnet Chili and Early Rose were in fact the result of hybridizations. In addition, putative pedigrees of Irish Cobbler, White Rose, and Bliss Triumph were not supported. Garnet Chili, the first derivative of Rough Purple Chili, carries allozmyes at Mdh-1 and Pgm-2, which supports the Chilean origin of Rough Purple Chili. The identical ctDNA pattern among the cultivars may indicate a common maternal lineage that traces through Garnet Chili to Rough Purple Chili. The allozyme frequencies estimated from these cultivars provide a base from which subsequent introductions of Solanum species into the ssp. tuberosum gene pool can be assessed.

The role of the x = 7 species, B. adpressa and Diplotaxis erucoides in Brassica evolution, was investigated by hybridizing them to the cultivated species B. nigra (x = 8) and B. oleracea (x = 9). In general, the hybrids displayed a low frequency of bivalents and unbalanced reductional division during meiosis, resulting in poor fertility. This cytological behavior was also observed in hybrids between the x = 7 species, indicating strong genome divergence of both genomes. Among all the hybrids, those of D. erucoides × B. nigra had the highest level of fertility, permitting the generation of D. erucoides-B. nigra alien addition lines. One of the hybrid plants variegated for yellow-white-red petal color and red pigmentation for other organs. The same enzyme activity zones were found to have multiple bands in all the species including those with x = 7 chromosomes, suggesting possible gene duplications. These multiple banded patterns persisted in pollen leachates and were transmitted to the hybrids. The hybrids reported in this study provide additional information on intergenomic relationships among Brassica and related species of allied genera., (© 1988 The American Genetic Association.)

Horizontal starch gel electrophoresis was used to perform genetic analysis to confirm the inheritance of various isozyme loci and report on additional loci in diploid Solanum species. Aps-1, Dia-1, and ldh-1 were identified as new loci, and tetrasomic segregation was reported at the tetraploid level for Mdh-2. Of 15 loci studied, distorted segregations were observed in five diploid test crosses involving the Got-1, ldh-1, Pgi-1, and Adh-1 loci. In these cases, four of the five parents were of interspecific origin; the family segregating for Adh-1 was the exception. Utilizing various clones and interspecific combinations between S. phureja, S. tuberosum, and S. chacoense, further test crosses were made to determine linkage relationships between these isozyme loci and the yellow tuber flesh gene (Y). Two linkages were detected among these markers. Estimates of the ldh-1/Sdh-1 linkage ranged from 10.4 map units (m.u.) to 36.8 m.u., whereas a tight linkage was confirmed for Prx-2/Prx-3 (0.6 m.u.). Comparing putative homologous loci for the ldh-1/Sdh-1 linkage implies some conservation since the divergence of Lycoperison, Solanum, and, to a degree, Capsicum. As a result of codominant expression, these isozyme markers provide new opportunities for further genetic studies of tuber-bearing Solanums., (© 1988 The American Genetic Association.)