Heterosis versus breakdown in fish hybrids revealed by one-parental species-associated viral infection

Abstract The F1 generation of hybrids exhibits heterosis in vigour-related traits, with their expression often superior to that of both parents. Hybrid breakdown is documented for post-F1 generations, these exhibiting many disadvantageous traits. Hypothesizing that pathogen load reflects host vigour, we used spring viremia of carp virus (SVCV), associated with common carp, to examine heterosis versus breakdown in hybrids of the fish species common carp and gibel carp. Common carp was highly susceptible to SVCV, whilst gibel carp was less susceptible. F1 generations exhibited heterosis advantage, with the expression of SVCV similar to that in less susceptible species. Paternal backcrosses and F2 generations, burdened by high cyto-nuclear incompatibility, tended to exhibit SVCV dynamics similar to that of highly susceptible species. However, maternal backcrosses of female hybrids and male C. gibelio showed low susceptibility to SVCV. Cyto-nuclear incompatibility and the direction of cross-breeding seem to play a role in determining the susceptibility of hybrids to viral infection.