To develop inbred lines from self-incompatible, cultivated diploid potatoes, an S-locus inhibitor (Sli) gene derived from a self-compatible variant of a wild potato species, Solanum chacoense, was incorporated into various cultivated diploid potatoes. The progeny was selfed twice by the action of the Sli gene to obtain 74 S2 inbred clones belonging to 8 families. More than 40% of them were either non-flowering or pollen sterile. Among the pollen fertile clones, self-compatible clones occurred with a much lower frequency (20.9%) than expected (83.3%). The result demonstrated that self-compatibility was introduced and expressed in the gene pool of cultivated diploid potatoes by an action of the Sli gene, although serious inbreeding depression associated with selfing occurred. The genotypes of S2 inbreds were surveyed using 46 S. chacoense-specific RFLP (restriction fragment length polymorphism) markers covering the whole potato genome. More than half of the markers (67.4%) showed distorted segregation. Particularly, all markers on chromosome 12 were overrepresented in the S2 inbreds. This confirms our earlier finding that the Sli gene locates on chromosome 12 and the alleles linked with this gene are preferentially transmitted because of its essential requirement for selfing.

SummaryVariation, genetic parameters, interrelationships and phenotypic and genetic path analyses for components of field resistance of potatoes to Phytophthora infestuns were studied using detached leaves from 16 potato cultivars. Inter‐genotypic variability was significant for the components and the Area Under Disease Progress Curve (AUDPC). The resistant cultivars generally had a longer latent period and lower lesion size and spore production than the susceptible cultivars. The correlations between AUDPC and infection efficiency, and between AUDPC and spore density were not significant, but latent period, lesion size and sporulation did correlate significantly with AUDPC. Genetic and phenotypic path‐coefficient analyses indicated lesion size to be the most important component of field resistance. The genetic correlation coefficients between the AUDPC and infection efficiency, latent period and spore density arose mainly because of their indirect effects on AUDPC via lesion size. Lesion size and AUDPC had a high genetic coefficient of variation, heritability and genetic advance (genetic gain).

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Although the heritable nature of plant tissue culture responses is now well documented in many species, only a few studies have been conducted to elucidate complete inheritance patterns. Genetic control of in vitro shoot regeneration from leaf explants was investigated inSolanum chacoense using parental, F1 and F2 generations. Broad-sense heritability estimates were high for frequency (percentage) of responsive leaf explants (61-83%) and number of shoots regenerated per responsive explant (53-75%). Consistent with high heritability estimates, a hypothesis involving three genes could be formulated to explain the variability in the response observed in this study. This model implies that homozygous recessive alleles at any two (out of three) loci are required for the highest response, i.e., more than two shoots per explant in more than 40% of the explants. The presence of homozygous recessive alleles at any one of the three loci produces an intermediate response, i.e., fewer than 40% of the explants regenerating fewer than two shoots per explant, and a dominant allele at all the three loci results in non-responsiveness. Additional minor modifier genes, each with a small effect, would also be required to account for the variable intensity of regeneration within groups. Such a relatively simple genetic control of in vitro regenerability suggests that incorporation of this trait should be easy in potato improvement programmes.

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