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. 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 are, therefore, not sufficient to explain species integrity. In addition, there are no strong evidences indicating that genome differentiation is important in the group. Thus, evidences are presented in this review to support the assertion that interspecific pollen-pistil incompatibiliy, nuclear-cytoplasmic male sterility and 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.