Crossability relationships among the wild diploid potato species Solanum kurtzianum, S. chacoense and S. ruiz-lealii from Argentina

The wild tuber-bearing Solanum species represent a genetic pool of enormous diversity, embracing high variability for many agronomic traits. S. kurtzianum (ktz), S. chacoense (chc) and S. ruiz-lealii (rzl) are wild diploid self-incompatible relatives of the common potato, Solanum tuberosum ssp. tuberosum, with resistance to adverse biotic and abiotic factors. Pollen-pistil/style compatibility relations and seed set in intra- and interspecific crosses among ktz, chc and rzl were investigated. Pre- and post-zygotic hybridization barriers were detected in the three species, with variable incidence because the percentage of genotypic combinations that produced seeds was 15-30% and 7-28% in intra and interspecific crosses, respectively. In pollen-pistil/style incompatible combinations, either pollen grains did not germinate or pollen tube growth was arrested at different sites of the style, but mainly in its upper third. Unilateral incompatibility was detected in both intra- and interspecific crosses, as well as bilateral incompatibility in certain combinations. The incompatibility relations observed could be explained by a genetic model with dominant genes in pistils that interact with corresponding dominant genes in pollen, that are independent of the S-locus. The complex arrangement of compatibility relationships found could not be directly related to taxonomic hypothesis or geographic distribution. These results suggest that divergence between the three species has occurred recently, and that internal barriers of reproductive isolation have not been an important factor in their evolution.