Abstract

   Anser cygnoides has a spherical crest on the beak roof, which is
   described as knob. However, the mechanisms affecting knob morphology
   are unclear. Here, we investigated the phenotypic characteristics and
   molecular basis of knob-size differences in Yangzhou geese.
   Anatomically, the knob was identified as frontal hump in the frontal
   area of the skull, rather than hump of upper beak. Although the frontal
   hump length, and height varied greatly in geese with different knob
   phenotypes, little was changed in the width. Histologically, knob skin
   in large-size knobs geese have a greater length in the stratum corneum,
   stratum spinosum, and stratum reticular than that in small-size knobs
   geese. Moveover, the 415 differentially expressed genes were found
   between the large knobs and small ones through transcriptome profiling.
   In addition, GO enrichment and KEGG pathway analysis revealed 455
   significant GO terms and 210 KEGG pathways were enriched, respectively.
   Among these, TGF-β signaling and thyroid hormone synthesis-signaling
   pathways were identified to determine knob-size phenotype. Furthermore,
   BMP5, DCN, TSHR and ADCY3 were recognized to involve in the growth and
   development of knob. Our data provide comprehensive molecular
   determinants of knob size phenotype, which can potentially promote the
   genetic improvement of goose knobs.

   Subject terms: Genetics, Zoology, Anatomy

Introduction

   Many birds have a distinctive frontal hump on their heads and most
   birds have a frontal hump on their cranium. Some birds show helmet-like
   casques on the dorsal surface of the neurocranium (e.g., Casuarius
   spp.), and some birds display inflated bulges in the frontal area,
   immediately caudal to the frontal-naso hinge (e.g., Balearica). Only a
   few birds present protuberances of the upper beak (e.g., Pauxi
   unicornis)^[42]1. Both Chinese geese (Anser cygnoides) and African
   geese (Anser cygnoides) possess a spherical crest across the beak roof,
   which is described as a knob. However, whether the knob is a hump of
   the upper beak or a frontal crest requires further investigation.

   Although all domesticated Chinese and African geese have a
   characteristic knob-like protuberance across the base of the bill near
   the forehead, a small protuberance is found in wild specie (swan goose,
   Anser cygnoides). The protuberance has been greatly exaggerated by
   artificial selection in Chinese and African goose breeds^[43]2, and
   then forms the knob, which indicates incompletely dominant
   inheritance^[44]3. Generally, the knob size is relatively larger in
   males than that in females, and in adults than in juveniles. In
   additon, knob sizes vary among different breeds and within same breeds.
   For example, Shitou geese have an average knob size of 40 mm, with a
   maximum of 55 mm and a minimum of approximately 30 mm, while the
   average knob size of Magang geese is approximately 28 mm. However, the
   genetic basis underlying differences in knob size phenotype is not
   fully understood.

   The knob, as an ornamental trait, is well developed by the time of
   sexual maturity and provides an identifier of sexual maturity.
   Moreover, the knobs influence first impressions of customers in China
   when making purchase decisions, with a large knob phenotypic size
   generally being preferred. However, the morphological structures of
   knobs and the mechanisms underlying phenotypic variation remain
   unclear. In this study, adult Yangzhou geese with large or small knob
   phenotype were selected, and knob morphologies and histologies were
   observed. Furthermore, the genetic basis of knob-size differences was
   investigated by RNA sequencing technology. These results provide an
   alternative strategy for the genetic improvement of goose knobs to meet
   consumer preferences.