One of the preeminent biological patterns is the rapid evolution of male genitalia, which often serves as the only means to distinguish closely related species. In many mammals, the penis includes a bony os penis called the baculum. Although the baculum probably functions in the context of male-male competition and/or male-female interactions, functional dissection has been hampered by inadequate methods for studying complex shapes that lack landmarks, as well as a good model system for understanding the genetic basis of size and shape variation in bacula. Here we apply a novel approach to characterize size and shape variation across 50 recombinant inbred lines (the BXD panel). We demonstrate that heritability of baculum shape variation is above 0.40, and we map the genetic basis of this variation to several promising candidate loci. Simulations suggest that many more loci of small effect influence the shape of bacula, but went undetected. This is the first time that candidate genes in baculum shape have been identified, and opens the way to future experimental manipulation for the study of baculum function.?
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