Evolution requires both the generation of variation through the process of development and the sorting of that variation by natural selection. The principal investigator and her undergraduate students will use the methodologies and insights of molecular biology, systematics (phylogeny construction), comparative anatomy, and paleontology to address the ways that development has shaped and constrained the morphological diversification of the mammalian vertebral column over evolutionary time. Frequently observed categories of column variation have recently been linked to developmental disruptions of the processes of somite generation and axial pattern assignment. This linkage allows the generation of a database that will enable the researchers to identify the type, frequency, and phylogenetic location of critical developmental events in mammalian column evolution. The key morphological innovations the investigators will address are the differentiation of the column's component series, the introduction of constraint in cervical (neck) morphology, and the dramatic column transformations seen in three mammalian groups (sloths, manatees, whales) with unusual lifestyles. This project's primary significance lies in its contribution to our growing understanding of the role of development in evolution. Its secondary impact lies in its integration of undergraduate students into the process of formulating and investigating questions of animal design during the scope of their summer research program, independent study, and senior thesis research experiences.
The vertebrate axial skeleton is a complex structure with repeated anteroposterior (AP) units. In early aquatic vertebrates, highly variable counts of the repeated ‘vertebra + moveable paired rib’ units were present at every segmental position. In contrast, most mammals exhibit rigidly defined regionalized anatomy and highly conserved counts. A few rare mammals appear to escape from these rigid patterns. This study examined the anatomy of these rare species for clues to both the origins of regional complexity over macroevolutionary time and why that complexity constrains future variation in count. Almost all mammals have seven neck (cervical) vertebrae, whatever the length of their necks. We traced the appearance of this trait in the fossil record, and documented that it appears at the same time as the inferred origin of the diaphragm, a uniquely mammalian trait. Using CT scans, we demonstrated that interactions between axial (somitic) and lateral plate mesoderm tissues, such as the diaphragm, are associated with axial patterning changes. We also proposed a developmental mechanism that explains the connection between axial / lateral interaction and constraint of count. The pygmy right whale, Caperea marginata, appears to break the cervical constraint of count. It lacks a first rib, and therefore displaces the sternum and the thorax posteriorly. The comparison of skeletons of individuals of different ages indicated instead that posterior displacement is the result of the partial loss of the first rib in very early ontogeny and the fusion of its distal parts to the second rib. Loss of lateral plate structures such as the sacrum eliminates axial / lateral plate interaction and also the restriction on vertebral count. We used microCT to search for remnants of the sacrum in an early fetus of the blue whale, Balaenoptera musculus. We found no ossified sacral structures. Non-invasive reconstruction of the internal anatomy of this specimen allowed determination of the developmental sequence of key baleen whale traits during gestation. The study also proposed a model of fetal blue whale growth over time, documenting that the accelerated prenatal growth characteristic of Balaenoptera occurs during fetal, not embryonic, development. This project was carried out at Wellesley College, a liberal arts undergraduate institution with a student body that is both exclusively female and highly diverse with respect to geography, ethnicity, and economics. Undergraduate women were fully integrated into all aspects of this research: hypothesis formulation, travel to natural history museums, data collection, data analysis, oral presentation, and co-authorship in the peer-reviewed publications that it generated.
