Substitutes for genome differentiation in tuber-bearing Solanum: interspecific pollen-pistil incompatibility, nuclear-cytoplasmic male sterility, and endosperm

The cultivated potato, Solanum tuberosum L. (2n=4x=48), has a very large number of related wild and cultivated tuber-bearing species widely distributed in the Americas. These species, grouped in 16 taxonomic series, range from the diploid to the hexaploid level. Polyploid species are either disomic or polysomic, and sexual polyploidization via genetically controlled 2n gametes has played a major role in their evolution. Species are separated in nature by geographical and ecological barriers. However, there are several examples of sympatric species that share the same niches but do not readily cross (i.e., the diploids S. commersonii and S. chacoense in certain areas of Argentina). External barriers alone are, therefore, not sufficient to explain species integrity. In addition, there is no strong evidence indicating that genome differentiation is important in the group. In this review we present evidence supporting the assertion that interspecific pollen-pistil incompatibility, nuclear-cytoplasmic male sterility, and the endosperm are major forces that strengthen the external hybridization barriers allowing, at the same time and under specific circumstances, a certain amount of gene exchange without jeopardizing the integrity of the species.