Heritability of plumage colour morph variation in a wild population of promiscuous, long-lived Australian magpies.

Colour polymorphisms have evolutionary significance for the generation and maintenance of species diversity. Demonstrating heritability of polymorphic traits can be challenging for wild populations of long-lived species because accurate information is required on trait expression and familial relationships. The Australian magpie Cracticus tibicen has a continent-wide distribution featuring several distinct plumage morphs, differing primarily in colour of back feathers. Black or white-backed morphs occur in eastern Australia, with intermediate morphs common in a narrow hybrid zone where the two morphs meet. This study investigated heritability of back colour phenotypes in a hybrid zone population (Seymour, Victoria) based on long-term observational data and DNA samples collected over an 18 year period (1993-2010). High extra-pair paternity (~ 36% offspring), necessitated verification of parent-offspring relationships by parentage analysis. A total of 538 birds (221 parents and 317 offspring) from 36 territories were analysed. Back colour was a continuous trait scored on a five-morph scale in the field (0-4). High and consistent estimates of back colour heritability (h2) were obtained via weighted mid-parent regression (h2 = 0.94) and by animal models (h2 = 0.92, C.I. 0.80-0.99). Single-parent heritability estimates indicated neither maternal nor paternal non-genetic effects (e.g., parent body condition) played a large role in determining offspring back colour, and environmental effects of territory group and cohort contributed little to trait heritability. Distinctive back colouration of the Australian magpie behaves as a quantitative trait that is likely polygenic, although mechanisms responsible for maintaining these geographically structured morphs and the hybrid zone where they meet are unknown.

Sequence variation in the melanocortin-1 receptor (MC1R) does not explain continent-wide plumage color differences in the Australian Magpie (Cracticus tibicen).

The genetic basis of plumage color variation has already been determined for many model species; however, the genetic mechanisms responsible for intraspecific color variation in the majority of wild-bird species are yet to be uncovered. The Australian magpie (Cracticus tibicen) is a large black and white passerine which is widely distributed across the Australian continent. The proportion of melanized back plumage varies between regionally delineated subspecies; where back-color forms overlap, intermediate color phenotypes are produced. This study examined the majority (861 bp) of the coding region of the melanocortin-1 receptor (MC1R), a candidate gene for plumage color differentiation in 98 magpies from across the Australian continent, to determine if the gene is associated with magpie back-color variation and explore phylogeographic signal within the gene. Neutrality and selection tests (Tajima's D, Fu's F (S), MKT) indicate the gene is unlikely to be currently under selection pressure and, together with other lines of evidence, suggest a past demographic expansion event within the species congruent with the results of previous mitochondrial phylogeographic work on this species. None of the 15 synonymous and four nonsynonymous substitutions within MC1R were found to be associated with plumage variation. Our results suggest that genes or regulatory elements other than MC1R may determine back-color variation in C. tibicen.