Post-zygotic gametic selection due to endosperm balance number explains unusual chromosome numbers of 3x x 2x progeny in Solanum

Crosses between triploid and diploid genotypes are usually the best sources of trisomics in potato as well as in several other crop species. However, 3x x 2x crosses between triploid (2n=3x=36; 2EBN) Solanum commersonii-S. tuberosum hybrids and diploid (2n= 2x=24; 2EBN) genotypes gave progenies with a high number of extra chromosomes, 29-36, suggesting that only eggs with 17-24 chromosomes produced embryos that reached full development. Our hypothesis is that although triploids produce eggs with a range of chromosome numbers, 3x x 2x crosses involving a 2x(2EBN) parent favor eggs with a high chromosome number. These eggs have higher probabilities of possessing the same endosperm balance number (EBN) value (i.e. 1) of gametes produced by the 2EBN diploid parent to give the required 2:1 maternal to paternal EBN ratio in the hybrid endosperm. Under this model, trisomics are produced only if the diploid parent has an EBN of 1. Based on our results and those reported in the literature, it is proposed that in 3x(2EBN) x 2x(2EBN) crosses the endosperm balance number exercises negative selection for gametes with a low chromosome number, and a corresponding low EBN, and positive selection for gametes with a high chromosome number and EBN.