DLO-Centre for Plant Breeding and Reproduction Research (CPRO-DLO), PO Box 16, 6700 AA Wageningen, The Netherlands

Background and objectives
South American Solanum species are promising sources of resistance to Phytophthora infestans [1]. They offer high levels of resistance, together with a higher chance of durability hypothesized from the fact that they evolved in a geographic region different from that of the pathogen, which is of Mexican origin. So far, the genetics of resistance in the diploid South American species have only been elucidated for highly resistant S. microdontum, in which both major and minor genes have been found, and for moderately resistant S. arnezii x hondelmannii, in which only minor genes were found [2]. For S. berthaultii, studies of F1 progenies from crosses with susceptible diploid S. tuberosum were inconclusive, as all progeny were highly resistant with little genetic variation [2]. In order to gain insight in the inheritance of the resistance of S. berthaultii, backcrosses with susceptible genotypes were performed and analysed.

Materials and methods
Four resistant F1 genotypes were used for backcrossing. They originated from crosses of two different S. berthaultii BGRC 10.063 genotypes with the susceptible S. tuberosum diploid SH 77-114-2988. The F1 genotypes represented the most and least resistant extremes within their families. These genotypes were crossed with the susceptible S. tuberosum diploid SH 83-114-660. Progenies of about 150 individuals were raised from seed, multiplied in the greenhouse and tested in the field against a complex isolate of P. infestans in two different years as described in [1]. For comparison, BC progeny of resistant F1 genotypes originating from S. microdontum BGRC 24.981 and S. arnezii x hondelmannii BGRC 27.308 were also produced and tested. All parents were included in the tests.

Results and conclusions
The four S. berthaultii backcross populations all segregated in a wide range of susceptible and resistant genotypes. The two most resistant F1 parents yielded progenies that were clearly more resistant than the progenies of the two least resistant F1 parents. This confirms the small, but statistically significant genetic variation in the F1 families. One BC family exhibited a bell shaped curve, indicating a normal distribution of resistance. This family originated from the most resistant of the four F1 parents. The other three BC families were skewed towards susceptibility but with some highly resistant individuals. These results strongly suggest polygenic inheritance.

The BC families of S. microdontum lineage confirmed the presence of major genes in this material, whereas for S. arnezii x hondelmannii polygenic inheritance was confirmed. In the latter progeny the distribution was skewed towards susceptibility. These results make the S. berthaultii case the first example of high level partial resistance to P. infestans, close to full incompatibility, that appears to be inherited in a polygenic fashion.

1. Colon LT, Budding DJ, 1988. Euphytica Supplement 77-86.
2. Colon LT, Jansen RC, Budding DJ, 1995. Theoretical and Applied Genetics 90, 691-698.