A General Model of Distant Hybridization Reveals the Conditions for Extinction in Atlantic Salmon and Brown Trout

Abstract

Interspecific hybridization is common in nature but can be increased in frequency or even originated by human actions, such as species introduction or habitat modification, which may threaten species persistence. When hybridization occurs between distantly related species, referred to as “distant hybridization,” the resulting hybrids are generally infertile or fertile but do not undergo chromosomal recombination during gametogenesis. Here, we present a model describing this frequent but poorly studied interspecific hybridization to assess its consequences on parental species and to anticipate the conditions under which they can reach extinction. Our general model fully incorporates three important processes: density-dependent competition, dominance/recessivity inheritance of traits and assortative mating. We demonstrate its use and flexibility by assessing population extinction risk between Atlantic salmon and brown trout in Norway, whose interbreeding has recently increased due to farmed fish releases into the wild. We identified the set of conditions under which hybridization may threaten salmonid species. Thanks to the flexibility of our model, we evaluated the effect of an additional risk factor, a parasitic disease, and showed that the cumulative effects dramatically increase the extinction risk. The consequences of distant hybridization are not genetically, but demographically mediated. Our general model is useful to better comprehend the evolution of such hybrid systems and we demonstrated its importance in the field of conservation biology to set up management recommendations when this increasingly frequent type of hybridization is in action.

Modelling interspecific hybridization with genome exclusion to identify conservation actions: the case of native and invasive Pelophylax waterfrogs

“…For this aim, we adapted (see Appendix S1 for details) the model of distant species hybridization without genetic introgression between parental species that we have recently developed (Quilodrán et al. ). In our simulations, to assure that native frogs reach a stable equilibrium before the invasion of P. ridibundus, we let the native L / E system evolve for 200 time steps (years).…”

“…Assuming equal sex ratio, we used eqn to calculate the weighted number of breeding events leading to offspring of type L ( n L ), based on eqn in Quilodrán et al. (): where φ L ( t ) is a normalization factor such that the proportion of homotypic and heterotypic mating of L is equal to 1 (Appendix S1).…”

“…The parameter γ represents the interbreeding success rate, as introduced in Currat et al () and Quilodrán et al. (). When γ is equal to zero, there is a complete reproductive isolation whereas a value of 1 corresponds to a panmictic reproduction between both species.…”

“…The parameter ω represents the fitness of the new hybrids (Quilodrán et al. ), in which zero represents sterile individuals, whereas a value of one corresponds to individuals as fertile as the old P. esculentus E . We assumed that the old and the new hybrids, the fertile and the sterile ones, have the same survival parameters and mating preferences.…”

“…To this aim, we adapted and implemented a model of interspecific hybridization without genetic introgression that we have recently developed (Quilodrán et al. ). The case study of biological invasion reinforced by nonintrogressive hybridization that we investigated here is the colonization of Western Europe by the waterfrog Pelophylax ridibundus coming either from Central or Southern Europe (Holsbeek and Jooris ), apparently mediated by human activities (Luquet et al.…”