Heterosis

Definition

The superior performance or enhanced fitness of hybrid offspring relative to their inbred parental lines, resulting from the combination of divergent genomes. In arthropods, heterosis manifests as increased fecundity, faster development, larger body size, greater stress tolerance, or improved dispersal capacity compared to parental populations. The effect arises from dominance complementation (masking deleterious recessive alleles) and/or true overdominance at specific loci, and is distinguished from simple additive inheritance by transgressive phenotypes that exceed both parents. Also called hybrid vigor.

Etymology

Greek heteros 'other, different' + -osis 'condition'

Example

In commercial silkworm (Bombyx mori) breeding, crosses between geographically distant strains often exhibit heterosis, with F1 hybrids showing 15–30% higher cocoon weight and improved disease resistance compared to either parental inbred line, enabling more stable silk production than either parent stock alone.

Synonyms

• Hybrid vigor
• outbreeding enhancement

Related Terms

• inbreeding depression
• hybridization
• outbreeding
• F1 hybrid
• heterozygote advantage
• genetic load
• complementation
• transgressive segregation

Usage Notes

Heterosis is typically strongest in F1 hybrids and often declines in F2 and subsequent generations due to segregation and recombination breaking up favorable gene combinations. The term is sometimes used loosely for any hybrid advantage, but specialists reserve it for cases where hybrid performance significantly exceeds the better parent (high-parent heterosis) rather than merely the parental average (mid-parent heterosis). Contrast with outbreeding depression, where hybridization reduces fitness. In entomology, heterosis is exploited in biological control agent production and pollinator breeding programs, but must be weighed against risks of disrupting locally adapted gene complexes in wild populations.