by Gabriele Gusmini and Todd C. Wehner
Department of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7609
Today, watermelon breeders are less interested in studying heterosis and/or general (GCA) and specific (SCA) combining ability, because hybrids have proven their advantage for protection of the parents. Furthernore, seedless cultivars are in high demand and can be produced only as triploid hybrids. However, in the future it might be possible to develop transgenic diploid seedless watermelons. In that case, the question of the advantage in using heterotic hybrids vs. inbred cultivars will still be pertinent and important.
Many Indian scientists in the 1950s and 1960s showed heterosis in watermelon, making hybrid seeds the predominant products in the watermelon seed industry. These studies measured heterosis as well as GCA and SCA in watermelon, but they were based on diallel or top crosses of elite inbreds, not a random set of lines from a population. More recent studies often included only a small number (Nmax=10) of non-randomly chosen elite cultivars as parents, so the results are valid only for those specific crosses and are not generally applicable. Nevertheless, taken as a group, these studies indicate the presence of heterosis in watermelon and the importance of GCA in the choice of parents for hybrid production.
In a more recent study in Brazil, Dr. F.F. de Souza and his group tested seven intercrossing populations with evaluation of reciprocal crosses. They observed significant GCA, SCA, and reciprocal combination effects, along with additive effects. In a further effort, they evaluated GCA and SCA for tetraploid females crossed with diploid males for the production of triploid seeds. They recorded a higher magnitude of GCA effects than SCA effects and strong additive effects for yield components.
Environmental factors such as irrigation or general water availability on yield also seem to be important in contrasting inbred cultivars vs. hybrids. I.e., in Florida in 1985 Dr. B.B. Rhodes observed that watermelon hybrids outyielded inbred cultivars only in irrigated fields, but quality was higher among the inbred cultivars in dry conditions.