Reaction norms for metamorphic traits in natterjack toads to larval density and pond duration

Abstract

The evolution of environmentally-induced changes in phenotype or reaction norm implies both the existence at some time of genetic variation within a population for that plasticity measured by the presence of genotype x environment interaction ( G x E), and that phenotypic variation affects fitness. Otherwise, the genetic structure of polygenic traits may restrict the evolution of the reaction norm by the lack of independent evolution of a given trait in different environments or by genetic trade-offs with other traits that affect fitness. In this paper, we analyze the existence of G x E in metamorphic traits to two environmental factors, larval density and pond duration in a factorial experiment with Bufo calamita tadpoles in semi-natural conditions and in the laboratory.Results showed no plastic temporal response in metamorphosis to pond durability at low larval density. The rank of genotypes did not change across different hydroperiods, implying a high genetic correlation that may constrain the evolution of the reaction norm. At high larval density a significant G x E interaction was found, suggesting the potential for the evolution of the reaction norm. A sibship ( # 1) attained the presumed "optimal" reaction norm by accelerating developmental rate in short duration ponds and delaying it in longer ponds. This could be translated in fitness by an increment in metamorphic survival and size at metamorphosis in short and long ponds respectively with respect to non-plastic sibships.

Biological factors controlling developmental duration, growth and metamorphosis of the larval green toad, Bufo viridis viridis

“…Opposing the present results, Reques and Tejedo (1997) found that there was no plastic response to pond duration. Similarly, Gervasi and Foufopoulos (2008), noticed the main effect of desiccation in their experiment was on development or differentiation rate, not growth rate.…”

Developmental responses to pond desiccation in tadpoles of the British anuran amphibians (Bufo bufo, B. calamita and Rana temporaria)

“…At low densities in long duration ponds, tadpoles displayed no additional growth, suggesting that there was no advantage in lingering even if potential growth conditions are good (Tejedo & Reques, 1994). By monitoring the development of individual B. calamita sibships from the same population, Reques & Tejedo (1997) found that there was no plastic response to pond duration in tadpoles raised under low larval densities. However, at high densities variability in response between the different sibships was very high.…”

“…Experiments by Tejedo & Reques (1994) and Reques & Tejedo (1997) have shown that developmental plasticity in B. calamita tadpoles may be regulated by a combination of larval density and food supply. Larvae from a mixed cohort of 12 sibships raised at low density displayed faster development than those raised at high density in short duration ponds, but the results from long duration ponds were not consistent with an adaptive explanation of plasticity.…”

Life history plasticity and fitness in a caddisfly in response to proximate cues of pond-drying

“…If pre-reproductive mortality rates increase, which can occur under pond-drying, then individuals should accelerate development and make the transition to the adult stage sooner than animals under permanent hydroperiods. Empirical studies confirm that, under pond-drying, most amphibians accelerate juvenile development to escape pre-reproductive mortality due to desiccation (Wilbur and Collins 1973;Werner 1986;Newman 1992;Denver 1997;Reques and Tejedo 1997;Denver et al 1998; for an exception, see: Brady and Griffiths 2000;Merila et al 2000;Blaustein et al 2001;Altwegg 2002;Loman 2002;Doughty and Roberts 2003;Morey and Reznick 2004;Denoël et al 2005). For example, in the laboratory, western spadefoot toads accelerate development in response to a reduction in water volume (Denver et al 1998).…”