Morph-specific artificial selection reveals a constraint on the evolution of polyphenisms

Abstract

Theory predicts that the evolution of polyphenic variation is facilitated where morphs are genetically uncoupled and free to evolve towards their phenotypic optima. However, the assumption that developmentally plastic morphs can evolve independently has not been tested directly. Using morph-specific artificial selection, we investigated correlated evolution between the sexes and male morphs of the bulb mite Large 'fighter' males have a thick and sharply terminating pair of legs used to kill rival males, while small 'scrambler' males have unmodified legs, and search for unguarded females, avoiding fights. We selected on the relative leg width of only the fighter male morph, tracked the evolutionary responses in fighters and the correlated evolutionary responses in scramblers and females that were untouched by direct selection. Fighters diverged in relative leg thickness after six generations; assaying scramblers and females at the ninth generation we observed correlated responses in relative leg width in both. Our results represent strong evidence for the evolution of intraspecific phenotypic diversity despite correlated evolution between morphs and sexes, challenging the idea that male morphs are genetically uncoupled and free to independently respond to selection. We therefore question the perceived necessity for genetic independence in traits with extreme phenotypic plasticity.

Insights from intralocus tactical conflict: adaptive states, interactions with ecology and population divergence

“…Selecting on the relative leg width of only the fighter male ART, they observed a response in relative leg width in not only the fighters, but also a correlated response in the scrambler ART and females. While one conclusion that could be made from these results is that a complete uncoupling between the ARTs is not necessary for tactical dimorphism to evolve (Buzatto et al 2018), these results also demonstrate that the existence of tactical dimorphism does not necessarily indicate that IATC has been resolved ( Fig. 2c).…”

“…When the correlations between traits were compared to correlations across the genetically influenced sexes, which exhibited sexual dimorphism, in both species the correlations across the ARTs were lower. The existence of genetic correlations for traits between ARTs in bulb mites was more recently confirmed using artificial selection (Buzatto et al 2018). Selecting on the relative leg width of only the fighter male ART, they observed a response in relative leg width in not only the fighters, but also a correlated response in the scrambler ART and females.…”

“…In cases where the environment influences the development of the ARTs, this component may need to be included in the design (Buzatto et al 2015, Pike et al 2017. Artificial selection regimes have also been used to indirectly detect genetic correlations (Buzatto et al 2018). Finally, to demonstrate that ARTs are not at their optima for a trait one could also use selection gradients to construct phenotypic landscapes using thin-plate splines to visualize the fitness surface.…”

“…chase and force-copulate females). However, genetic correlations of shared traits across ARTs can constrain ARTs from reaching their optimal states, resulting in intralocus tactical conflict (IATC, Morris et al 2013, Buzatto et al 2018.…”

Life-history consequences of bidirectional selection for male morph in a male-dimorphic bulb mite

“…Buzatto et al (2018) found that bidirectional selection for leg width in R. echinopus fighters resulted in positively correlated responses in leg sizes of scramblers and females. This suggests that the development of leg size is not fully decoupled between fighters and scramblers (also see Pike et al 2017) or females.…”