Regulation and Evolution of Developmental Plasticity

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Developmental plasticity, the ability of individuals to adjust their phenotypes in response to variable environmental conditions, is widespread across taxa. Most organisms exhibit plasticity in one or more traits and in response to one or more environmental factors. Yet, how do organisms integrate environmental information and develop a phenotypic response?
 
My research focuses on polyphenisms, an extreme form of plasticity characterized by alternative morphologies. Specifically, I use two systems Onthophagus horned beetles and Pristionchus predatory nematodes.
 
Male horned beetles exhibit a nutritional polyphenism. Low nutrition males develop into small, hornless males, whereas high nutrition males develop into large, fully horned males. Horns in large males are used as weapons to gain access to females. Hornless males, instead, use a sneaker tactic, bypassing large males to gain access to females. I take candidate gene approaches as well as genome-wide approaches to better understand the mechanisms that regulate horn plasticity and how these mechanisms have evolved.
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I am currently using a second model system for my research: Pristionchus nematodes. Species from this genus are characterized by a polyphenism in mouth morphology. In response to starvation and crowding, individuals develop a wide-mouth morphology (“eurystomatous”, “Eu”) that allows them to prey on other nematodes. In contrast, under high food availability and low population densities individuals develop a narrow-mouth morphology (“stenostomoatous”, “St”) that feeds on bacteria and develops faster than their Eu counterpart. I use this system to understand the evolutionary history and origins of the mechanisms that regulate the polyphenism, and I do so by taking a genome-wide approach.

Pristionchus pacificus polyphenism. Stenostomatous, microbivore morph (A) and eurystomatous, predatory morph (B). (C) The Eu morph can prey on other nematodes.

Diversity of horn plasticity across species. Small, low nutrition males (left) are hornless, whereas large, high nutrition males (right) are horned.

For more information:
Casasa S, Moczek AP. 2018. Insulin signalling’s role in mediating tissue-specific nutritional plasticity and robustness in the horn-polyphenic beetle Onthophagus taurus. Proc Biol Sci. 285:20181631.
Casasa S, Zattara EE, Moczek AP. 2020. Nutrition-responsive gene expression and the developmental evolution of insect polyphenism. Nat Ecol Evol. 4:970–978
​Casasa S, Biddle JF, Koutsovoulos GD, Ragsdale EJ. In press. Polyphenism of a novel trait integrated rapidly evolving genes into ancestrally plastic networks. Molecular Biology and Evolution.

​The Role of Plasticity in Evolution

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Under construction.

For more information:
Casasa S, Moczek AP. 2018. The role of ancestral phenotypic plasticity in evolutionary diversification: population density effects in horned beetles. Anim Behav 137:53–61 
Casasa S, Zattara EE, Moczek AP. 2020. Nutrition-responsive gene expression and the developmental evolution of insect polyphenism. Nat Ecol Evol. 4:970–978.