Craniofacial morphometric analysis as a differentiation tool in mystax group of Saguinus Hoffmannsegg, 1807 (Cebidae, Callitrichinae): a preliminary test

This study provides a quantitative assessment of the craniofacial variation among recognized species and subspecies in the Saguinus mystax species group and tests the reliability of the proposed method in detecting differences among taxa. Thirty measurements were taken of 66 tamarin specimens. We used non-parametric MANOVA to test for differentiation among species and among subspecies. Interspecific analyses showed significant variation among species, except for S. inustus. We detected significant differentiation between S. mystax mystax and S. m. pileatus, whereas no significant morphometric difference was found between S. imperator subspecies. Measurements presenting some differentiation among species were selected for subsequent analysis of variance. Univariate analyses detected significant differences among species only for three measurements. Our results suggest that the cranial morphometric variation has limited information to discriminate among the taxa in the S. mystax group. However, we cannot disregard the lack of statistical power due to small sampling in some taxa, absence of some subspecies in the analyses, or even lack of informativeness of the chosen measurements. Morphometric analyses was insufficient to demonstrate complete differentiation in Saguinus. The use of morphometric analyses should not be considered as a definitive method for taxa differentiation and delimitation in Saguinus.

A ROR2 coding variant is associated with craniofacial variation in domestic pigeons

SummaryVertebrate craniofacial morphogenesis is a highly orchestrated process that is directed by evolutionarily conserved developmental pathways 1,2. Within species, canalized developmental programs typically produce only modest morphological variation. However, as a result of millennia of artificial selection, the domestic pigeon (Columba livia) displays radical variation in craniofacial morphology within a single species. One of the most striking cases of pigeon craniofacial variation is the short beak phenotype, which has been selected in numerous breeds. Classical genetic experiments suggest that pigeon beak length is regulated by a small number of genetic factors, one of which is sex-linked (Ku2 locus) 3–5. However, the molecular genetic underpinnings of pigeon craniofacial variation remain unknown. To determine the genetic basis of the short beak phenotype, we used geometric morphometrics and quantitative trait loci (QTL) mapping on an F2 intercross between a short-beaked Old German Owl (OGO) and a medium-beaked Racing Homer (RH). We identified a single locus on the Z-chromosome that explains a majority of the variation in beak morphology in the RH x OGO F2 population. In complementary comparative genomic analyses, we found that the same locus is also strongly differentiated between breeds with short and medium beaks. Within the differentiated Ku2 locus, we identified an amino acid substitution in the non-canonical Wnt receptor ROR2 as a putative regulator of pigeon beak length. The non-canonical Wnt (planar cell polarity) pathway serves critical roles in vertebrate neural crest cell migration and craniofacial morphogenesis 6,7. In humans, homozygous ROR2 mutations cause autosomal recessive Robinow syndrome, a rare congenital disorder characterized by skeletal abnormalities, including a widened and shortened facial skeleton 8,9. Our results illustrate how the extraordinary craniofacial variation among pigeons can reveal genetic regulators of vertebrate craniofacial diversity.

Complex genetic architecture of three-dimensional craniofacial shape variation in domestic pigeons

Deciphering the genetic basis of vertebrate craniofacial variation is a longstanding biological problem with broad implications in evolution, development, and human pathology. One of the most stunning examples of craniofacial diversification is the adaptive radiation of birds, in which the beak serves essential roles in virtually every aspect of their life histories. The domestic pigeon (Columba livia) provides an exceptional opportunity to study the genetic underpinnings of craniofacial variation because of its unique balance of experimental accessibility and extraordinary phenotypic diversity within a single species. We used traditional and geometric morphometrics to quantify craniofacial variation in an F2 laboratory cross derived from the straight-beaked Pomeranian Pouter and curved-beak Scandaroon pigeon breeds. Using a combination of genome-wide quantitative trait locus scans and multi-locus modeling, we identified a set of genetic loci associated with complex shape variation in the craniofacial skeleton, including beak curvature, braincase shape, and mandible shape. Some of these loci control coordinated changes between different structures, while others explain variation in the size and shape of specific skull and jaw regions. We find that in domestic pigeons, a complex blend of both independent and coupled genetic effects underlie three-dimensional craniofacial morphology.