Diploid cultivated species, haploids derived from tetraploid cultivars, and their wildSolanum relatives were used to generate various diploid potato genetic stocks. These stocks have significant value in potato breeding because: 1) genetic diversity in the potato population can be kept variable with the diploid germplasm which confers genetic variation from wild and cultivated species; 2) crossability of this diploid germplasm is facilitated by the function of 2n gametes; and 3) high levels of pest resistances originating from the process of introgression of the wild and cultivated genetic resources can be used to improve the tetraploid cultivated potato genepool.

Genetic diversity in Japanese and North American potato cultivars for processing was evaluated by RAPD analysis. Thirty-six polymorphic DNA segments, or RAPDs , were detected by amplification via Polymerase chain Reaction (PCR) with fourteen decamer primers in eight Japanese and eight North American cultivars. The average numbers of different RAPDs were 14.4 within Japanese cultivars, 13.9 within North American cultivars, and 14.7 between them. Their differences were not significant. Therefore, it is indicated that the genetic diversities considerably similar in Japanese and North American cultivars

In the early 1980s the old A1 mating type population of the potato late blight pathogen, Phytophthora infestans, was displaced by new A1 and A2 mating type isolates in Europe. Analyses of virulence characteristics and DNA fingerprint patterns of a large number of isolates revealed that with the introduction of new P.infestans isolates the level of genetic diversity in the population has increased dramatically. Experiments under controlled conditions and under natural conditions in the field demonstrated that oospores are formed in large numbers after inoculation of potato leaves with a mixture of A1 and A2 mating type isolates of P.infestans. Oospores in soil, exposed to natural weather conditions during the winter, remain viable for at least eight months. It is concluded that after the introduction of the new A1 and A2 mating type population in The Netherlands P.infestans reproduces sexually and forms oospores. It is likely that oospores play an important role in the epidemiology of potato late blight.

More than 300 isolates of the Irish potato famine fungus, Phytophthora infestans, collected in 20 countries on five continents, were analyzed for genetic variation at the mating type and two allozyme loci. A subset of more than 200 isolates was also analyzed for DNA "fingerprint" variation. A surprising result was that a single clonal lineage dominated most populations worldwide. All of the variation within this lineage appeared to have arisen by mitotic recombination or by mutation. In addition to the most common clonal lineage, a number of different, but apparently closely related, lineages occurred in the United States and Canada. The low levels of gene diversity in the derived populations compared to the presumed ancestral population in central Mexico indicate that P. infestans went through extreme genetic bottlenecks during its dispersal. The genetic data are consistent with the hypothesis that the initial migration of P. infestans in the 1840s was from Mexico to the United States and that only a single genetic individual was transported to Europe and subsequently to the rest of the world. If this hypothesis is correct, then the Irish potato famine was caused by a single clonal genotype of P. infestans.

Host-parasite compatibility strongly influences the sex determination of potato cyst nematodes (Globodera rostochiensis and G. pallida). The probability of a juvenile developing into a male may be higher in the case of poor compatibility than in the case of full compatibility. As a result, avirulence toward the resistant host may lead to the production of more males. In contrast, virulence favors the production of females. These conflicting sexual trends imply opposing selection pressures by a resistant host. This may create an equilibrium frequency between the avirulent and the virulent nematodes, thereby preserving host resistance. In this way, parasite populations with environmental sex determination could maintain their genetic diversity at the expense of their reproduction rate. The equilibrium frequency of avirulence and the matching level of durable resistance are derived mathematically for complete, monogenic resistance.