# Craniofacial shape transition across the house mouse hybrid zone: implications for the genetic architecture and evolution of between-species differences

## TL;DR

This study explores how craniofacial shape changes between two subspecies of house mice and finds that these differences likely have a highly polygenic genetic basis.

## Contribution

The study provides the first evidence that between-species craniofacial variation in mice has a highly polygenic genetic basis.

## Key findings

- Craniofacial shape transitions continuously along a hybridization gradient between two subspecies of house mice.
- Quantitative changes in genome composition correlate with quantitative changes in craniofacial shape.
- Developmental stability remains unaltered despite hybridization and reproductive isolation.

## Abstract

Craniofacial shape differences between taxa have often being linked to environmental adaptation, e.g. to new food sources, or have been studied in the context of domestication. Evidence for the genetic basis of such phenotypic differences to date suggests that within- as well as between-species variation has an oligogenic basis, i.e. few loci of large effect explain most of the variation. In mice, it has been shown that within-population craniofacial variation has a highly polygenic basis, but there are no data regarding the genetic basis of between-species differences. Here, we address this question using a phenotype-focused approach. Using 3D geometric morphometrics, we phenotyped a panel of mice derived from a natural hybrid zone between M. m. domesticus and M. m. musculus, and quantify the transition of craniofacial shape along the hybridization gradient. We find a continuous shape transition along the hybridization gradient, and unaltered developmental stability associated with hybridization. This suggests that the morphospace between the two subspecies is continuous despite reproductive isolation and strong barriers to gene flow. We show that quantitative changes in genome composition generate quantitative changes in craniofacial shape; this supports a highly polygenic basis for between-species craniofacial differences in the house mouse. We discuss our findings in the context of oligogenic versus polygenic models of the genetic architecture of morphological traits.