Using Development to Shed Light on the Evolution of Suckling

Abigail S. Tucker

Nurturing young with milk is a defining feature of mammals. Therian mammals (placental and marsupial) give birth to live young that nurse from a teat until they are developed enough to feed on their own. In contrast, the egg-laying mammals (monotremes) lack nipples, so the hatchlings lap the milk as it is secreted from specialized patches on the mother's abdomen. To suckle effectively, therian mammals must be able to form an anterior and posterior seal. By wrapping the tongue around the teat and pressing it against the hard palate, the anterior seal is formed, while for the posterior seal, synchronized activity is required between the tensor veli palatini (TVP), which stiffens the soft palate and the muscles that elevate the tongue. The TVP is reported as absent in monotremes (echidna and platypus), indicating that the ability to stiffen the soft palate was lost or never developed. Soft palates in therian mammals lie between a structure called the hamulus, which forms within the neural crest derived medial pterygoid, and provides anchorage for the TVP. We have compared development of this region of the soft palate and show that the medial pterygoid is formed by the fusion of endochondral (Sox9-positive) and membranous (Runx2-positive) bones in the mouse. In the echidna, these bones form but do not fuse and remain separate elements. Fusion was normal in muscular dysgenesis mice, but loss of muscle activity led to loss of the bursa that forms around the hamulus, highlighting a requirement for force for correct development. Surprisingly, a hamulus was identified in the platypus neonate, suggesting that this structure is a conserved feature of all mammals. Such comparative developmental biology allows us to identify homologous elements across diverse species and shed light on how novel features might have evolved.