Polish Crested Chickens: A Promising New Model System with Bizarrely Shaped Brain and Skull

Abstract

Establishment of novel model organisms have accompanied many breakthroughs in our understanding of how major morphological transformations occur. For example, avian models (chicken, quail, duck) have been instrumental in illuminating developmental processes underlying embryogenesis, limb formation, and craniofacial anatomy. However, the capacity of existing model organisms to infer actual evolutionary shifts is limited because they are often separated by extensive phylogenetic gaps. A comparative model system with a pair of closely related, yet phenotypically divergent, species would strengthen the inferential power of “evo-devo” studies. Here, we introduce a promising new model organism—the Polish crested chicken (PCC)—with aberrant brain and skull morphologies. PCCs exhibit cerebral herniations where the forebrain expands dorsally in association with supernumerary foramina and a prominent cranial protuberance on the skull roof. Coupled with typical domestic chickens (TDCs), PCCs form a compelling comparative model system for investigating rapid evolution of novel brain and craniofacial morphologies, brain-skull integration, as well as clinical conditions such as hydrocephalus. In this baseline study, we use computed-tomographic imaging and high-density 3-D geometric morphometric methods to (i) quantify the unique neuroanatomy of PCCs compared to TDCs and other crown birds; and (ii) assess if and how brain and skull shapes are integrated in PCCs and TDCs. Results demonstrate that PCCs possess a truly unique brain morphology among crown birds, with the largest difference in forebrain shape. In addition, brain shape, especially in the forebrain, correlates strongly with not only braincase shape as expected, but also with beak shape and length in both PCCs and TDCs. This phenotypic pattern is consistent with previous developmental studies showing that key molecular signaling from the forebrain is strongly implicated in beak formation (e.g., Fgf, Shh). Beyond phenotypic characterizations of PCC, future investigations will interrogate the developmental processes underlying the evolution of extreme brain and skull morphologies.