Dumbbell-shaped brains of Polish crested chickens as a model system for the evolution of novel brain morphologies

Abstract

The evolutionary history of vertebrates is replete with emergence of novel brain morphologies, including the origin of the human brain. Existing model organisms and toolkits for investigating drivers of neuroanatomical innovations have largely proceeded on mammals. As such, a compelling non-mammalian model system would facilitate our understanding of how unique brain morphologies evolve across vertebrates. Here, we present the domestic chicken breed, white crested Polish chickens, as an avian model for investigating how novel brain morphologies originate. Most notably, these crested chickens exhibit cerebral herniation from anterodorsal displacement of the telencephalon, which results in a prominent protuberance on the dorsal aspect of the skull. We use a high-density geometric morphometric approach on cephalic endocasts to characterize their brain morphology. Compared with standard white Leghorn chickens (WLCs) and modern avian diversity, the results demonstrate that crested chickens possess a highly variable and unique overall brain configuration. Proportional sizes of neuroanatomical regions are within the observed range of extant birds sampled in this study, but Polish chickens differ from WLCs in possessing a relatively larger cerebrum and smaller cerebellum and medulla. Given their accessibility, phylogenetic proximity, and unique neuroanatomy, we propose that crested breeds, combined with standard chickens, form a promising comparative system for investigating the emergence of novel brain morphologies.

Keywords: Gallus; domestication; endocasts; geometric morphometrics; micro-CT imaging.

FIGURE 1

Characteristic phenotypes of white crested Polish chickens. (a) A photograph of white crested black Polish chicken showing its elaborate cranial feathers (photographed by Todd L. Green). (b) CT reconstruction of the skull (semi‐transparent) and endocast of an adult white Leghorn chicken. (c) CT reconstruction of the skull (semi‐transparent) and endocast of Polish crested chicken (GGP‐003). CT, computed tomography.

FIGURE 2

Landmark scheme used in this study in (a) lateral, (b) dorsal, and (c) ventral views on an endocranial reconstruction of Polish crested chicken (specimen GGP‐001). Red, yellow, and blue points denote discrete, curve, and surface (semi‐)landmarks, respectively. Green, red, blue, and orange areas indicate the cerebrum, optic lobe, cerebellum, and medulla, respectively, as characterized in the study. The encircled numbers correspond to the landmark numbers assigned to discrete landmarks, as listed in Table 2.

FIGURE 3

Ranked centroid size data for (a) log‐transformed endocranial size and proportional centroid size of (b) cerebrum, (c) optic lobe, (d) cerebellum, and (e) medulla.

FIGURE 4

Morphospaces constructed from first two principal components (PC) axes of (a) endocranial shape between PCC and WLC; (b) PCC and crown birds; (c) cerebrum shape; (d) optic lobe shape; (e) cerebellum shape; and (f) medulla shape. Inset point cloud images along axes illustrate shape changes associated with PC 1 and 2 axes, colored by brain regions. Inset images of endocranial reconstructions are not to actual scale. PCC, Polish crested chicken; WLC, white Leghorn chicken.