Bats: A Model for Mammalian Craniofacial Diversification

Abstract

Phyllostomids are a diverse family of neotropical bats with approximately 200 described living species. Phyllostomids are united by their distinctive noses, which are shaped like a leaf, hence their common name, the New World leaf-nosed bats. Phyllostomids perform a variety of ecologically important functions as- sociated with their diverse skull morphologies, which remarkably, and perhaps uniquely, span a range encom- passing much of the diversity of eutherian mammals as a whole. Fruit-eating bats, for example, disperse seeds and often have flat, primate-like faces, while nectar-feeding bats pollinate flowers and have elongated faces—among the most elongated of all mammals—whose length is correlated to the stalk length of the flowers they pollinate. Interestingly, these shapes and sizes become specialized during the growth of the same masses of embryonic tissues—the craniofacial prominences. However, the developmental mechanisms connecting the genotype and phenotype in these diverse morphologies remains unknown. In this thesis, I present the first characterization of the developmental and molecular bases for morphological and functional diversity in phyllostomid bat skulls. I explored the patterns of morphology and development across representatives of the family using X-ray micro-computed tomography, 3D geometric morphometrics and phylogenetic comparative methods. I developed a field research program to obtain embryonic tissues from four different bat species with three biological replicates at similar developmental ages, which enabled me to examine the molecular signatures associated with development and morphological variation between species. I performed immunofluorescent labeling to target proliferation, cranial neural crest cells, and the bone morphogenetic protein (BMP) signaling pathway. These molecular markers were compared in an evolutionary context and ancestral patterns during ontogeny were reconstructed to understand the developmental drivers of craniofacial diversity. To further explore development, I examined how modulating the BMP signaling pathway during craniofacial development in mouse produces morphological geometries similar to what is observed between bat species. My analyses comparing evolutionary and developmental patterns in morphology uncovered two distinct heterochronic processes, hypermorphosis and acceleration, in the evolution of fruit, nectar and vampire bats. Ancestral-state reconstruction of proliferation patterns at the molecular level at stage 18, a stage when differences in craniofacial length emerge, supports two models of heterochrony involved in the evolution of phyllostomids: compared to the predicted ancestor, proliferation is held at low levels in long-faced nectar bats (hypermorphosis) and proliferation is accelerated in short-faced fruit bats (acceleration). We found an important signal affecting proliferation, BMP, varied between morphologies such that an increase in BMP matched to low proliferation and a relative decrease in BMP corresponded to elevated proliferation. The amount of cartilage paralleled the amount of BMP (i.e. long faces have more cartilage, which in turn have more BMP). Based on these observations we conclude BMP-responsive regions are varied between species and relate to cartilage growth and facial growth differences. Using mouse genetics, we demonstrated how modulating BMP signal in the BMP responsive regions could function as a major source of morphological variation. Based on these combined data, this thesis provides a much-needed foundation for understanding the developmental mechanisms involved in the adaptive diversity of skull shape and contributes new insights into the evolution of phyllostomid bats.