Heterosis and Interspecific Hybridization

Abstract

The release of hybrid cultivars is among the main achievements of plant breeding. The exploitation of heterosis led to a significant increase in edible yield in seed crops. There are single-, three-way-, and double-cross hybrids. In the F1 hybrid, the undesirable (often deleterious) recessive alleles from one parent are suppressed by the dominant allele of the other parent. Overdominance is the alternative theory regarding this outbreeding enhancement or hybrid vigor, that is, the heterozygote being superior to either homozygote parent. The biochemical, physiological, and molecular basis of hybrid vigor remain however elusive. Advances in functional genomics, transcriptomics, proteomics, and metabolomics research are helping in understanding heterosis in plants using a system-level approach. Genetic diversity and distance among breeding lines and their correlation with hybrid performance may define heterotic groups and assist predicting hybrid yield. When combining ability information lacks, knowledge on the relationship among genotypes aids to select parents for further crossing. Genetic distance will not be enough to account for heterosis, because population features, genotype by environment interactions, and the trait itself affect hybrid vigor. There are some selfing species with successful F1 hybrid cultivars, for example, rice among cereals and tomato among vegetables. Their use depends on the added value given by heterosis and efficient pollination mechanisms to justify the development and production costs of hybrid seed. Cytoplasmic and genic male sterility provide means for producing hybrid seed in various selfing species. Genomic selection may further enhance the efficiency of hybrid breeding because the inbred parents determine hybrid genotypes. Heterosis in polyploidy species seems to be more complex than in diploid species. Interspecific hybrids result from mating two species and are very often sterile. Translocation lines ensue by incorporating a single chromosome segment from an alien or wild species into a crop, while chromosome doubling in somatic cells or gametes of F1 hybrids lead to amphidiploids that are “bridges” for moving single chromosomes from one species to another or for developing new crops.