The <b><i>PMEL</i></b> Gene and Merle in the Domestic Dog: A Continuum of Insertion Lengths Leads to a Spectrum of Coat Color Variations in Australian Shepherds and Related Breeds

Abstract

Merle is a distinct coat color and pattern found in numerous species, including the domestic dog, characterized by patches of diluted eumelanin (black pigment) interspersed among areas of normal pigmentation. In dogs, this variegated pattern is caused by an insertion of a SINE element into the canine PMEL gene. Although variation in the length of the SINE insertion - due to a variable-length poly(A) tail - has been observed to be associated with variation in merle coat color and patterning, no systematic evaluation of this correlation has been conducted and published in the scientific literature. We performed high-resolution analysis of the SINE insertion lengths in 175 dogs (99 Australian shepherds, 45 miniature Australian shepherds, and 31 miniature American shepherds) and compared the genotypes with the coat phenotypes (when available). SINE insertion lengths varied from 201 to 277 bp, indicating that merle insertion variants can occur in virtually any size along the entire continuum. Genotype-phenotype correlation of 126 dogs with only a single SINE insertion (m/M) identified at least 4 major phenotypic clusters designated as “cryptic,” “atypical,” “classic,” and “harlequin” merle. However, we found several phenotypic outliers that did not cluster within these major groupings, suggesting that insertion size is not the only factor responsible for merle phenotypic variability. In addition, we detected 25 dogs with 2 SINE insertions (M/M) and 24 dogs with more than 2 PMEL (merle) alleles, indicating mosaicism. Genotype-phenotype correlation of M/M dogs suggests that cryptic merle alleles often act like non-merle (m) alleles when combined with atypical, classic, and harlequin-sized alleles. The finding of mosaicism has important implications for the dog's phenotype and the ability to potentially transmit various alleles to its offspring. Furthermore, we identified examples of the SINE insertion poly(A)-tail expansion and contraction between generations, which also has important implications for breeding practices and determining mating pairs to avoid producing double merle dogs. These data demonstrate that there is a continuum of merle insertion lengths associated with a spectrum of coat color and patterns and that genotype-phenotype exceptions and overlap make it difficult to strictly assign certain insertion sizes with an expected coat color, although some generalizations are possible.

Being Merle: The Molecular Genetic Background of the Canine Merle Mutation

“…Since the mutation is identical by descent in each dog breed, it can be assumed that it originated from a common ancestor prior to the formation of breeds [ 2 , 7 ].…”

“…In recent studies [ 1 , 2 , 3 ], the length of the poly(A) sequence was determined by automated high-resolution fluorescent fragment analysis from several individuals of different breeds. The resulting genotypes were correlated to the phenotypes that were sorted into several subcategories.…”

“…Ballif and colleagues [ 2 ] also set up four phenotypic classes: harlequin (the longest), classic, atypical, and the shortest cryptic categories. Phenotypic ranges make overlapping transition zones, in which any samples of the neighboring ranges may also be included.…”

“…They examined 175 animals from three breeds. Separation of PCR products was performed by the ABI SeqStudio Genetic Analyzer with a single base-pair resolution ( Figure 4 ) [ 2 ].…”

“…Several papers have been published recently regarding the molecular genetics of the merle phenotype ( Figure 1 A–F), including three major articles offering theoretical explanations [ 1 , 2 , 3 ] and two breed-specific papers [ 4 , 5 ]. The aim of our review is to provide a comprehensive analysis of each of these results and hypotheses, as well as compare them with those of previous papers [ 6 , 7 ] which identify the merle mutation at the molecular level.…”