Hybridization and its impact on the ontogenetic allometry of skulls in macaques

The importance of hybridization in morphological diversification is a fundamental topic in evolutionary biology. However, despite accumulated knowledge concerning adult hybrid variations, how hybridization affects ontogenetic allometry is less well understood. Herein, we assessed the effects of hybridization on the postnatal ontogenetic allometry in skulls of a putative hybrid population of introduced Taiwanese macaques (Macaca cyclopis) and native Japanese macaques (M. fuscata). Genomic analyses indicated that the population consisted of individuals with various degrees of admixture, formed by male migration of Japanese to Taiwanese macaques. No marked ontogenetic trajectory angle dissociation was observed in the overall skull shape of hybrids. Transgressive variations in the overall skull shape were observed mainly in non-allometric components. Hybridization collapsed an unknown mechanism found in Japanese macaques that restricts the growth rate of the maxillary sinus (the hollow space in the face), producing hybrids with a mosaic pattern, i.e., the maxillary sinus is as large as that in Taiwanese macaques, although the overall skull shape is intermediate. Our findings suggest that the transgressive variations can be caused by prenatal shape modifications and the lifting of the genetic restriction on the regional growth rate, highlighting the complex genetic and ontogenetic bases underlying hybridization-induced morphological diversification.

Methods
This study investigated the genetic and morphological variation of Macaca cyclopis, M. fuscata and their hybrids. Genetic variation was assessed using single nucleotide polymorphisms obtained by double-digested restriction site-associated DNA sequencing. Morphological variation was assessed using landmark-based geometric morphometrics. See the README and related paper for details.