GENETIC-DISTANCE AND ITS ASSOCIATION WITH HETEROSIS IN PEAS

The relationship between the genetic distance of parents and both the heterosis of F-1 hybrids and the variance of F-5 lines was investigated in 72 crosses of pea (Pisum sativum L.). The genetic distance between each pair of parents was estimated, using isozyme (GD(i)), morphological (GD(m)) or quantitative (GD(q)) markers and finally a combination of isozyme and morphological markers (GD(i+m)). GD(m) was poorly correlated with the other measures of genetic distance, which in turn were strongly correlated with each other. Genetic distance was moderately correlated with the level of heterosis for yield over midparent in the F-1 generation, with the highest correlation obtained from GD(i+m). GD was not significantly correlated with heterosis for yield over the better or best parent but it was significantly correlated with all three measures of heterosis for pods per plant and hundred seed weight. There was no correlation between genetic distance and the level of heterosis for yield and total dry matter in the F-2 generation, but GD(i), GD(i+m) and GD(q) were predictive for the level of inbreeding depression in grain yield and total dry matter. When parents were high in genetic distance, crosses produced highly transgressive segregants for basal branches per plant, hundred seed weight, harvest index and onset of flowering. Genetic distance between parents was thus a useful measure for predicting a portion of hybrid performance and also of the variance of derived inbred lines. It was concluded that when choosing parents for a cross, consideration should be given to their genetic distance as well as their overall adaptation and their yield. There is considerable potential for optimising choice of parental combinations in the development of improved pea cultivars.