The results are given of crossing fifteen Central and South American wild Solanum species included in the series Conicibaccata, Demissa, Longipedicellata and Piurana. Nearly all the 210 possible combinations were examined.

Pollen tube inhibition occurred in 144 cases. It was affected by the particular species used (e.g. pollen of Conicibaccata species was strongly inhibited), by polyploidy and by the breeding system. It was progressively weaker from 2x×2x to 6x×6x crosses: in crosses of parents differing in chromosome number the probability of inhibition was greater the lower the ploidy of the pollen. The effect of different breeding systems was to introduce reciprocal differences such that pollen inhibition occurred when self-incompatible species were crossed as females with self-compatible ones but not in reciprocal crosses. This was so at diploid and tetraploid levels except in crosses involving tetraploid and hexaploid Conicibaccata species where the results were in complete contrast: there are indications that species in this series do not have a simple one-locus self-incompatibility system.

Post-fertilization failure occurred in eighteen cases and breakdown was greatest in crosses between parents of different chromosome number.

Hybrids were obtained in forty-seven combinations. Generally, crossability was highest in crosses that gave even polyploid (2x, 4x, 6x) progeny and lowest where uneven polyploids (3x, 5x, 7x) were obtained. All hybrids except two were vigorous.

Meiosis is described in thirty-seven hybrids and the patterns of chromosome association are interpreted in terms of genomic differentiation. It seems that species in series Demissa and Longipedicellata have at least one common genome, which is also present in S. Uiquerrense of series Piurana. Further, it seems that this genome is similar to that of the diploid species S. verrucosum of series Demissa and consequently it is considered as an important link between the Central and South American potatoes. Both S. fendleri and S. stoloniferum of series Longipedicellata have genomic affinities with S. santolallae of series Conicibaccata; this is evidence of another genomic link across the subcontinent. Of the species in series Longipedicellata, S. fendleri and S. stoloniferum are fundamentally similar whereas S. polytrichon is different from both. S. guerreroense and S. iopetalum of series Demissa are very similar and could be considered conspecific. Evidence suggests that S. X semidemissum is probably a hybrid between S. demissum and either S. fendleri or S. stoloniferum.