Developmental Plasticity
The role of genetic accommodation in the origins and diversification of feeding strategies
 
What role does the environment play in the origins and diversification of traits?
 
Organisms may harbor a considerable amount of genetic variation that is, under typical environmental conditions, hidden from selection (i.e., cryptic genetic variation). Under certain conditions, this genetic variation becomes phenotypically expressed, creating fitness variation among individuals in a population. If, by chance, any of this variation improves an organism's viability in the inducing environment, and they recurrently experience this environment, then genetic modifiers that favor the expression of such variation will increase in the population (i.e., genetic accommodation).
 
Spadefoot toad larvae are an excellent system for addressing the environmental origins of novel traits because they have the potential to be incredibly environmentally responsive, and members of the genus Spea have evolved a novel suite of traits associated with carnivory and cannibalism (Ledón-Rettig and Pfennig 2011).
Right: Members of the spadefoot genus Spea have the ability to develop as a typical omnivorous anuran larva that is small-jawed with smooth mouth parts and a long, coiled gut (top) or as a carnivorous and cannibalistic larva that is large-jawed with serrated, keratinized mouth parts and a short, relatively uncoiled gut (bottom). The expression of this novel, carnivore morph is strongly influenced by shrimp consumption.
I have addressed whether plasticity in response to a novel diet for tadpoles (shrimp) facilitated the novel gut morphology in ancestral Spea, and whether the functionality of this novel morphology has been enhanced in descendant species that specialize on this resource. To answer these questions, I have characterized ancestral patterns of larval gut plasticity (by measuring the response of a species with the ancestral feeding strategy on a novel diet) and how that plasticity has evolved under specific diet regimes (by measuring the response of species with derived feeding strategies on their current diets).
Left: Typical, diet-induced variation in Spea bombinfrons larval guts after having been fed either detritus or shrimp for 24 hours. 
My results indicated that ancestral Spea produced generalist larvae that possessed diet-induced plasticity in gut morphology. Furthermore, certain descendent lineages have become more developmentally plastic when they have the opportunity to utilize many diets, and more canalized when they can only utilize one. Lastly, the novel gut morphology was functionally improved in descendent lineages that expressed the trait, often (functionality was assessed using immunohistochemistry to measure cell proliferation). All these results are consistent with a model of evolution whereby traits originate with an environmental stimulus (i.e., genetic accommodation; Ledón-Rettig et al. 2008) 
Measuring cryptic genetic variation within populations
 
The phenotypif variation produced by an environmental stimulus may be immediately adaptive, but these phenotypes will not become heritable across generations until selection for genetic modifiers stabilize the novel variation. Selection on these modifiers can produce a population that either expresses a novel phenotype constitutively (genetic assimilation), or can reliably respond to changes in environmental conditions (adaptive plasticity).
 
This process can occur most readily if there is already heritable variation the trait that is expressed under those novel or changing environmental conditions (cryptic genetic variation). To determine whether ancestral populations of Speaharbored cryptic genetic variation in traits associated with the carnivorous feeding strategy, I measured the genetic variation in these traits in a species that is likely similar to ancestral populations of Spea. My results indicated that ancestral populations of Spea harbored considerable genetic variation in traits associated with carnivory, and that this genetic variation is exposed when their tadpoles consume shrimp (Ledon-Rettig et al 2010).This provides further evidence that genetic accommodation played a role in the evolution of Spea’s carnivory.
Stress hormones and transitions to novel diets
 
In addition to measuring cryptic genetic variation, I also wanted to identify a mechanism for its expression. Corticosterone (CORT), the major vertebrate stress hormone, is an ideal candidate for this mechanism. In many vertebrate taxa, CORT is upregulated in response to novel or suboptimal conditions. Further, CORT is known to mediate physiological, developmental and morphological plasticity.
With Erica Crespi (Washington State University), I tested the hypothesis that the stress axis is activated in a spadefoot population with the ancestral feeding strategy when they are fed shrimp. I found that this “ancestral” lineage suffered growth and development costs when fed shrimp (while a population with the derived, carnivorous feeding strategy did not) and higher levels of CORT than when they were fed their native diet. Furthermore, by using hormonal manipulations (the addition of exogenous CORT or RU486, a competitive binder for the glucocorticoid receptor), I determined that the increased CORT may be at least partially responsible for the reduced growth and development when these tadpoles were fed the novel diet (Ledón-Rettig et al. 2009).
Right:In tadpoles, the activation of corticosterone (CORT) begins with the environment. Tadpoles receive cues that activate the hypothalamus to produce Corticotropin Releasing Factor (CRF), which in turn activates the pituitary to release Adrenocorticotropic Hormone (ACTH), which activates the interrenal glands to produce CORT. CORT causes negative feed back by suppressing the hypothalamus, but it also effects peripheral tissues through circulation.
Stress hormones as mediators of cryptic genetic variation
 
 
Since ancestral populations of Spea may have elicited higher levels of CORT as they transitioned to a more shrimp- and tadpole-based diet, I tested the hypothesis that shrimp-induced CORT, per se, mediated the expression of cryptic genetic variation in traits associated with the carnivore feeding strategy. I compared the expression of genetic variation expressed by a population of the “ancestral” lineage when tadpoles were fed shrimp (see above) vs. the expression of genetic variation when tadpoles were treated with exogenous CORT.
The range of phenotypic variation expressed in the ancestral lineage was greater when fed the shrimp diet than when fed their native diet, detritus (far left), and was underlain by genetic variation as measured by increased heritability (H2). For some traits, I was able to replicate this phenomenon in certain traits by exposing tadpoles to exogenous CORT, suggesting a role for diet-induced CORT in the expression of cryptic genetic variation (table at left).