Genetic control of Endosperm Balance Number (EBN): three additive loci in a threshold-like system

Summary The genetic control of Endosperm Balance Number (EBN) was investigated by a complete diallel of four exceptional diploid Solanum commersonii-S. chacoense hybrids (1 1/2 EBN) and backcrosses to their species parents, S. commersonii (1 EBN) and S. chacoense (2 EBN). Crosses in which the female parent had a higher EBN value than the male, S. chacoense (2 EBN)XF₁ (11/2 EBN) and F₁ (11/2 EBN)X S. commersonii (1 EBN), produced viable seed to aborted seed ratios of 11.1 and 11.3, respectively, and had average to small sized viable seed. Crosses in which the female parent had a lower EBN value than the male, S. commersonii (1 EBN)XF₁ (11/2 EBN) and F₁ (11/2 EBN)XS. chacoense (2 EBN), produced viable seed to aborted seed ratios of 1 7.9 and 1 6.7, respectively, and had average to large sized viable seeds. The results of these crosses appear to be consistent with the relative EBN values of the male and female parent. A model is proposed for the system regulating endosperm development. The assumptions of this model are: (1) three unlinked loci control the system; (2) the loci are homozygous within a species; (3) the genes have additive effects and are of equal strength within a species; (4) the genes within S. chacoense have twice the effect with respect to endosperm regulation as those within S. commersonii; and (5) a slight excess maternal dosage will produce the qualitative effect of small but viable seed. This model, in which quantitative genes operate in a dosage dependent system bears many similarities to classical, threshold-type genetic models.