Selection on a genetic polymorphism counteracts ecological speciation in a stick insect

Area of Science:

• Evolutionary Biology
• Speciation Research
• Population Genetics

Background:

• Speciation can be driven or constrained by the interplay between natural selection and genetic architecture.
• While drivers of speciation are well-studied, constraints on speciation are less understood.
• The stick insect Timema cristinae exhibits partial reproductive isolation linked to host plant adaptation and pattern polymorphism.

Purpose of the Study:

• To investigate how selection on a color polymorphism (green/melanistic) constrains speciation in Timema cristinae.
• To determine the genetic architecture underlying color and pattern polymorphisms.
• To model the role of the melanistic phenotype in gene flow and its impact on local adaptation and speciation.

Main Methods:

• Quantification of genetic architecture for pattern and color polymorphisms using genetic crosses and genome-wide association mapping.
• Development of an individual-based model to simulate gene flow and speciation dynamics.
• Analysis of selection pressures on color phenotypes across different host plants.

Main Results:

• Selection on the melanistic color phenotype counteracts speciation by increasing gene flow between host-associated populations.
• Melanistic stick insects exhibit crypsis on both host plants, resistance to a fungal pathogen, and a mating advantage.
• Both pattern and color polymorphisms are controlled by simple genetic mechanisms.

Conclusions:

• The melanistic phenotype acts as a 'genetic bridge', facilitating gene flow and constraining local adaptation and speciation.
• Variation in selection and trait genetic architecture can impose significant constraints on evolutionary trajectories.
• Understanding these constraints is crucial for comprehending the complex processes of speciation.