Reduced phenotypic covariation in marsupial limbs and the implications for mammalian evolution

Abstract

As serially homologous structures, mammalian fore- and hindlimbs ancestrally share a common developmental and genetic architecture. As a result, mammalian fore- and hindlimbs are predicted to be highly integrated in the absence of selective pressures to form divergent limb morphologies. Marsupials experience such a divergent selective pressure to form a robust forelimb to power a post-natal crawl to the teat. In this study, phenotypic covariation in marsupials was assessed to determine if specialization for the crawl did indeed reduce integration between their fore- and hindlimbs. To explore the evolution of mammalian limb integration, phenotypic covariation in representative eutherians and monotremes was also examined. Phenotypic covariation in limbs was quantified morphometrically, and analysed with correlational and phylogenetic methods. Results indicate that marsupials generally have relatively high levels of within-limb phenotypic covariation, and low levels between limbs, in contrast to the pattern reconstructed for the mammalian ancestor. Our findings support the hypothesis that pressure to specialize in one limb (either the fore- or the hindlimb) can reduce phenotypic covariation between limbs, and that reduced limb phenotypic covariation is derived in marsupials. Further research is needed to test the effect that these differences in limb phenotypic covariation had on the evolution of the major mammalian groups.