Meiotic abnormalities underlying pollen sterility in wild potato hybrids and spontaneous populations

Wild potato species are widely distributed in the Americas, where they spontaneously grow in very diverse habitats. These species - with low chromosome differentiation - form polyploid series with 2n = 2x, 3x, 4x and 6x (x =12). They are isolated in nature by external and internal hybridisation barriers that can be incomplete, allowing hybridisation in areas of sympatry. Nevertheless, most accessions in germplasm banks, regardless of genetic background of the sampled spontaneous populations, have been assigned specific categories based on morphological characters. To further investigate the extent of hybridisation in the group and for comparative purposes, pollen viability was estimated in (i) artificial hybrids between a commercial cultivar (Calén INTA) of the common potato (tetraploid Solanum tuberosum ssp. tuberosum) and the tetraploid cytotype of the related wild species S. gourlayi, and (ii) samples of plants (accessions) and inflorescences of natural populations from Argentina, tentatively classified as 'presumed hybrids' (S. infundibuliforme-S. gourlayi) and 'species' (S. infundibuliforme, S. gourlayi and S. chacoense). Regardless of origin, 98 out of 103 plants analysed had zero to 70% pollen viability (zero to 40% in eight of them). Pollen grains were of variable size and morphology and, in mostly male sterile plants, the only viable pollen grains were 2n and/or 4n. Furthermore, male sterile plants shared various abnormalities in meiosis I and II (unpaired chromosomes, unequal chromosome distribution, precocious/lagging chromosomes, parallel, tripolar, fused and multiple spindles, unequal size nuclei, dyads, triads and pentads in addition to normal tetrads, among others). These results provide novel evidence to support field observations of early potato botanists on the extent of spontaneous hybridisation in wild Argentinian potato populations, which is not reflected in the current taxonomy and has significant consequences for germplasm conservation and breeding.