Genetic divergence among inbred onion lines and correlation with heterosis and combining ability

Highly heterotic onion hybrids exhibit high yield and superior post-harvest quality. In general, to develop heterotic hybrids, it is first necessary to develop inbred lines with high genetic divergence and good specific combining ability (SCA). We investigated the correlation between SCA, heterosis, and genetic divergence of inbred onion lines. Two groups of inbred onion lines were used as parentals in a partial (6x5) diallel cross. Group I included six S3 male-sterile lines of the "Super Precoce" pedigree (L4827, L4830, L4833, L4835, L4837, and L4839), and group II included five S2 fertile lines of the "Hiper Precoce" pedigree, Granex type with some inter-crosses (L4741, L4742, L4743, L4744, and L4746). Genetic divergence among the parental lines was estimated on the basis of seven morpho-agronomic characters and 10 inter-simple sequence repeat (ISSR) markers. Pearson and Spearmann correlations between morphology-based genetic dissimilarity, ISSR-based genetic dissimilarity, heterosis, and SCA were evaluated. Five characters (number of leaves, bulb yield, bulb diameter, soluble solids content, and dry mass) accounted for 82.8% of the genetic variation present in the genotypes. The greatest genetic dissimilarity, as indicated by both ISSR data (80.4%) and morpho-agronomic data (92.5%), was calculated for L4742 and L4744, which demonstrated that these onion inbred lines were genetically divergent. Based on estimates of SCA and heterosis, L4742 was identified as the most promising lineage for developing superior onion hybrids. None of the correlations between SCA, heterosis, and genetic dissimilarity (morpho-agronomic or ISSR) were significant, thereby indicating that genetic dissimilarity cannot be used directly to select parental lines or to predict the best crosses for all characters. However, the genetic dissimilarities estimated using morpho-agronomic characters and ISSR profiles were useful for identifying the best crosses for bulb yield and bulb diameter.