The development of the marsupial shoulder girdle: A case study in Monodelphis domestica

During their embryogenesis, marsupials develop a unique structure, the shoulder arch, which provides the structural and muscle-attachment support necessary for the newborn???s crawl to the teat. One of the most pronounced and important aspects of the shoulder arch is an enlarged coracoid. After marsupial newborns reach the teat, the shoulder arch is remodeled and the coracoid is reduced to a small process on the scapula. Although an understanding of marsupial coracoid reduction has the potential to provide insights into both marsupial evolution and the origin of mammals, little is known about the morphological and cellular processes controlling this process. To remedy this situation, this study examines the morphological and cellular mechanisms behind coracoid reduction in the gray short-tailed opossum, Monodelphis domestica. Furthermore, it explores the expression patterns and levels of target genes in a comparative study between Monodelphis domestica and Mus musculus. A quantitative morphometric study of shoulder girdle development reveals that the coracoid is reduced in size relative to other aspects of the shoulder girdle by growing at a slower rate. Using a series of molecular assays for cell death, no evidence is found for programmed cell death playing a role in the reduction of coracoid size in marsupials. Although it is likely the case that coracoid growth is reduced through a relatively lower rate of cellular proliferation, differences in proliferative rates in the coracoid and scapula were not great enough to be quantified using standard molecular assays. Gene expression results indicated a change in expression timing between the two species for Hoxc6, a gene associated with coracoid patterning. Pax1, an acromion associated gene, shows similar expression and timing between the two sister groups. Genes associated with scapular blade development were found to be expressed to a later stage in Monodelphis domestica compared to Mus musculus. These results indicate a correlation between Hoxc6 patterning and changes in coracoid morphology in the opossum