FEATURE ARTICLE
Genetics and the Shape of Dogs
Studying the new sequence of the canine genome shows how tiny genetic changes can create enormous variation within a single species
Elaine A. Ostrander
Sexual Dimorphism
The Georgie Project is remarkable for the number of putative loci that have been discovered by the initial analysis. In addition to loci for head shape, body size, leg length and a host of other traits, loci have also been described that reportedly control differences in size between the sexes, so-called sexual dimorphism. Sexual dimorphism is observed in almost all mammals including, of course, dogs. The mechanisms for maintaining sexualdimorphism are not well understood. It has been shown that the Sry locus on the Y chromosome plays an important role in sex determination and dimorphism, but this is clearly only a small part of the story.
The study of the Portuguese water dog has filled in some additional pieces of this interesting puzzle. This vignette has its roots in the original observation that a locus on chromosome 15, which may or may not be IGF1, interacts with other genes to make males larger and females smaller.
On average, female Portuguese water dogs are 15 percent smaller then males. Chase, Lark and their colleagues observed that in females, a particular haplotype is dominant for small body size. In males, a different set of variants (another unique haplotype) associated with large overall body size is dominant. The locus on CFA15 interacts with another locus on the X chromosome that is known to escape inactivation, meaning that both copies of the genes in this region are turned on (in most locations on the X, only one copy is active).
Females who are homozygous at the X-chromosome locus and who are also homozygous for the large-size CFA15 haplotype are, on average, as large as large males. However, all females that are heterozygous at the X-chromosome marker are small, regardless of their CFA15 genotype. This result suggests several scenarios for how genes interact to affect major complex traits, such as body size, and suggests a mechanism for the evolution of sexual dimorphism.
Two observations from the study must be accounted for in the development of any model to explain canine sexual dimorphism. The explanation must include a discussion of the reversal of dominant haplotypes between males and females associated with CFA15 locus as well as an explanation for the interaction between the CFA15 and X-chromosome loci.
To address the first question, Chase and his colleagues propose the existence of another sex-specific factor. For example, the CFA15 locus might contain two distinct genes associated with two haplotypes; the so-called Ahaplotype acts in both males and females to upregulate size, while the B haplotype and its associated allelesdo not upregulate size but rather contain another gene that suppresses the up-regulator.
The second phenomenon, heterozygote-specific interaction, could be explained by arguing that the activation of haplotype A's critical upregulator gene requires interaction with a protein produced by the X chromosome.
The data of Chase, Lark and their colleagues are consistent with predictions made in the early 1980s that sexual dimorphism evolves because females secondarily become smaller than males as a result of naturalselection for optimal size. Reduction of female size relative to that of males takes place, according to this hypothesis, through an inhibition of major genes that enhance growth, such as the locus on CFA15.
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