Genetic variation in threshold reaction norms for alternative reproductive tactics in male Atlantic salmon,Salmo salar

Abstract

Alternative reproductive tactics may be a product of adaptive phenotypic plasticity, such that discontinuous variation in life history depends on both the genotype and the environment. Phenotypes that fall below a genetically determined threshold adopt one tactic, while those exceeding the threshold adopt the alternative tactic. We report evidence of genetic variability in maturation thresholds for male Atlantic salmon (Salmo salar) that mature either as large (more than 1 kg) anadromous males or as small (10-150 g) parr. Using a commongarden experimental protocol, we find that the growth rate at which the sneaker parr phenotype is expressed differs among pure-and mixed-population crosses. Maturation thresholds of hybrids were intermediate to those of pure crosses, consistent with the hypothesis that the life-history switch points are heritable. Our work provides evidence, for a vertebrate, that thresholds for alternative reproductive tactics differ genetically among populations and can be modelled as discontinuous reaction norms for age and size at maturity.

Plasticity of European flounder life history patterns discloses alternatives to catadromy

“…For example, the plasticity of American eel Anguilla rostrata life history patterns was not related to its status (size, sex or age) prior to migration (Thibault et al 2007) but to environmental conditions, similarly to A. anguilla and Japanese eel A. japonica (Daverat et al 2006). However, the phenotypic reproductive plasticity of Atlantic salmon Salmo salar has a genetic basis, but at the individual level, the environment can still be a driving force (Piché et al 2008).…”

A theoretical muddle of the conditional strategy: a comment on Neff and Svensson

“…Not only has this idea been described previously, it has also been recently reviewed [14,16] and empirically supported [12,13,22 -25]. Interestingly, Neff & Svensson [1] acknowledge and cite previous work that demonstrates that threshold reaction norms have underlying genetic variation and can therefore evolve [14,26,27], but by and large discuss the subject as if this awareness had not penetrated the field as a whole. They also neglect the actual quantitative genetics models for threshold traits, and go as far as proposing a verbal model that fuses alternative strategies and conditional strategies [1].…”

Old wine in new bottles: reaction norms in salmonid fishes

“…On the basis of the ISI literature search, four studies have employed a population-cross comparison to study genetic variability in plastic responses by salmonids to environmental change. Three will be discussed here and the fourth (Piché et al, 2008) later in the section on discontinuous reaction norms.…”

“…The first experimental evidence of population genetic differentiation in threshold reaction norms in salmonids was provided by Piché et al (2008), based on common-garden experiments undertaken on crosses in Nova Scotia (Figure 8b), although Nicieza et al (1994) provided circumstantial evidence to this effect in their comparative study of differences in juvenile growth and patterns of length bimodality in Scottish and Spanish Atlantic salmon. Intriguingly, based on the results of a field-transplant experiment in Japan, Morita et al's (2009) work on whitespotted charr, Salvelinus leucomaenis, suggests that threshold reaction norms may, in themselves, exhibit plasticity; despite a common genetic origin for their charr, thresh- 11.5 10.5 9.5 8.5 7.5 11.5 10.5 9.5 8.5 7.5 11.5 10.5 9.5 8.5 7.5 11.5 10.5 9.5 8.5 7.5 11.5 10.5 9.5 8.5 7.5 11.5 10.5 9.5 8.5 Figure 5 Reaction norms for early life-history traits, as a function of temperature, for grayling, Thymallus thymallus, from three populations in central Norway: Aursjen-cold-water population (dot-dashed lines); Lesjaskogsvatn-intermediate-temperature population (dashed lines); Hårrtjnn-warm-water population (solid lines).…”

“…As the phenotype changes along the positively linear reaction norm, the fitness of the genotype (dashed lines) may change as well, in a linear (w 1 ) or nonlinear (w 2 ) manner, Incidence male parr maturity Probability of maturing Length (mm) Weight (g) Figure 8 Threshold reaction norms for the adoption of alternative life histories: (a) hypothesised genetic variability in reaction norm switchpoints between life histories A and B within a single population (re-drawn from Hazel et al, 1990); (b) genetic differences in reaction norms for parr maturity in male Atlantic salmon, Salmo salar, as a function of growth rate (weight at 7 months) (original source: Piché et al, 2008); and (c) environmental differences in reaction norms for maturity in white-spotted char, Salvelinus leucomaenis, as a function of growth rate (length at 1 þ years) (original source: Morita et al, 2009). or it may remain constant (w 3 ). As Figure 10 is also meant to illustrate, changes in fitness may be realised irrespective of whether the reaction norm has a non-zero slope or not.…”