Genetic disruption of the baculum compromises the ability of male mice to copulate.

The baculum, a bone in the penis of many mammal species, shows an astonishing level of morphological divergence between species. Despite hundreds of years of interest, biologists have been unable to directly test its function. The goal of the current study is to uncover molecular details that could allow selective disruption of the baculum while allowing normal sexual differentiation and skeletal development. We compare patterns of androgen receptor binding and single cell gene expression in the developing penis, forelimbs and hindlimbs of mice. We identified chondrocytes in all three tissue types, but those from the developing penis show several unique features, including a population of chondrocytes that express both Runt-related transcription factor 2 (Runx2) and Androgen receptor (Ar). By combining a Runx2-Cre allele with a floxed Ar allele in mice, we selectively knocked out androgen signaling in late chondrocytes, resulting in a range of defects in baculum morphology. Males with the most disrupted bacula were unable to copulate, and their bacula appears to be disconnected from the corpus cavernosum muscle. Our study provides insights into the diversity of molecular mechanisms leading to bone and offers the first opportunity to directly test the function of the baculum.