Euphytica 94: 227–235, 1997. 227 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Crossability relationships among wild potato species with different ploidies and Endosperm Balance Numbers (EBN) Ricardo W. Masuelli 1 & Elsa L. Camadro 2 1 Departamento de Ciencias Biol´ ogicas, Facultad de Ciencias Agrarias, U.N. de Cuyo C.C. 7 Chacras de Coria (5505), Mendoza, Argentina; 2 Unidad Integrada Balcarce, C.C. 276 (7620), Balcarce, Buenos Aires, Argentina Received 8 February 1996; accepted 14 November 1996 Key words: crossability, Endosperm Balance Number (EBN), interspecific crosses, potato, Solanum Summary Three wild potato species with different ploidies and Endosperm Balance Numbers (EBN) were crossed in a complete diallel design and the development of the embryo and endosperm as well as the type of seeds produced were analyzed. The compatible crosses – intraspecific intra-EBN and interspecific intra-EBN – produced more than 89% plump seeds, whereas in the incompatible crosses – intraspecific inter-EBN and interspecific inter-EBN – more than 85% of the seeds were not as well developed or were shrunken. The histological analysis revealed that inviable seeds had less developed or collapsed endosperms and thicker endotheliums than viable ones. A gradation of crossabilities was observed among species. The self-compatible species Solanum acaule had good performance as a female but not as a male parent. Among the self-incompatible species, Solanum gourlayi was the best male parent but had a poor performance as a female, whereas Solanum commersonii had an intermediate behavior. Differences in crossabilities among genotypes within species were also observed. These results can not be solely explained by the EBN hypothesis. It is, therefore, suggested that the EBN may be part of a more complex system of interspecific barriers acting at the pre- and post-zygotic levels. Introduction There are around 200 tuber-bearing species in the genus Solanum, among which gene flow is limited by internal hybridization barriers. These barriers can be classified into two groups: (1) pre-zygotic, like pollen-pistil incompatibility (Grun & Aubertin, 1966; Camadro & Peloquin, 1981) and (2) post-zygotic, among which the dysfunction of the hybrid endosperm is one of the most important (see Johnston et al., 1980). In order to circumvent the barriers at the endosperm level, strategies have been developed which involve ploidy level manipulations (haploidization and poly- ploidization) (Peloquin, 1981). Various hypotheses have been proposed to explain the failure of endosperm development in interploid crosses in higher plants (see Johnston et al., 1980). According to Lin (1984) a 2:1 maternal to paternal genomic relation has to be met in the endosperm for normal development of this tissue in intraspecif- ic crosses in maize; in fact, almost all the results of interploid, intraspecific crosses can be explained on the basis of this genomic relation. However, the results of many interspecific crosses in other species can not be accounted for by this hypothesis, appar- ently because the genes involved in the development of the hybrid endosperm may have different values or ‘strength’ among species, regardless of ploidies. In this respect, and on the basis of interspecific cross- ing results, Johnston & Hanneman (1980) assigned values, called Endosperm Balance Number (EBN), to various Solanum species according to their behav- ior in regard to endosperm development and inde- pendent of their ploidies. According to their hypoth- esis, normal development of the hybrid endosperm requires a 2:1 maternal to paternal EBN ratio in this tissue. Crosses are compatible at the endosperm level between species with the same EBN, but not between species with different EBN. The EBN hypothesis is similar to the hypothesis put forward by Nishiyama