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About Cassandra Extavour's Work Dr. Cassandra Extavour Developmental Biology, Harvard University About Cassandra Extavour’s work One of the topics that Cassandra Extavour researches is germ cell development. During a process called embryogenesis, where an animal embryo develops from a fertilized egg cell, only a small percentage of the millions of genetically identical cells created will become germ cells. Germ cells are the cells created in an embryo that contribute their specific genomes to the reproduction of new creatures. The rest of the embryo’s cells become soma cells, which form different parts of the body such as organs, muscle, skin, and bones. These cells reproduce via mitosis, but cannot contribute their specific genomes to the reproduction of new creatures, ending the line of germ cells. A problem in the study of the development of multicellular organisms is then: why do cells that start out with identical genomes do different things in different environments? Cassandra Extavour tries to answer this question by dissecting embryos and ovaries of spiders, crickets, and milkweed bugs, using molecular biology and microscopy tools to map the germline of the cells. The way that a cell is assigned to the germline is caused by a few different ways. In some organisms, before there is an embryo, the molecular content of some cells predetermines them to develop as either germ or soma cells. In other organisms, a cell in the embryo receives chemical signals from neighboring cells that activate or repress genes that turn the cell into a germ cell. Cassandra Extavour is also interested in biological cooperation and the division of labor between germ and soma cells. Biological cooperation is when two or more cells group together to form a new entity, where the cells work together to maximize the reproductive potential of the entity. In the case of multicellular organisms with germ and soma cells, soma cells are called cooperative cells, where they improve the ability of germ cells to transmit their genomes by doing things like producing or preserving energy, at the expense of minimizing their genetic contribution to the next generation. Germs cells, or non-cooperative cells, on the other hand, will use the resources of the soma cells, without contributing its own, in order to preserve its own genome for the next generation. This type of behavior actually benefits the entire organism, and multicellular organisms capitalize on this in order to maximize their reproduction. Publications: Extavour, CG, M Akam "Mechanisms of germ cell specification across the metazoans: epigenesis and preformation" ''Development'' 2004 Dec125, 130 (24) 589-840 Ewen-Campen, Evelyn E Schwager, and Cassandra GM Extavour, ''The molecular machinery of germ line specification'' Molecular and Cellular Reproduction. Volume 77, Issue 1, pages 3–18, January 2010. Dr. Sebastian Groh The Evolution of Crocodylomorpha University College London (UCL) Q&A with Dr. Sebastian Groh: Q. What is your starting point when reconstructing the history and evolution of something like *Crocodylomorpha, and how do you build from there? A. The starting point is to think about what evolutionary history entails, including how the different species in the animal group are related, when different subgroups within the animal group evolved, and where they evolved. Then, peruse previous research and find the best method for your own. Collect data and, lastly, analyze it. One research method he particularly relies on is collecting data by visiting museums with crocodile fossils and writing down every single detail he observes about their anatomy, which usually takes an entire day! Q. Who is a team comprised of and how do you recruit those people? A. The positions usually included in a research team are: ● Principle Investigator (PI)–They supervise the projects going on within the research team and request funding ● Researchers–Usually people with PhDs, they work on their own research projects, teach undergraduate – PhD students, and help the PI with their tasks ● PhD students ● Master’s and Undergraduate students working on their thesis Recruitment is done through advertisements on job sites or within a department. Q. What have you researched/discovered that might add to our understanding of evolution, and do you have any tips for creating a timeline of life on Earth? A. Dr. Groh has researched animals including: ● Harvestmen–closely related to spiders ● Osteostracans–extinct, fish-like creatures that have a very heavy bony headshield and used to live in the oceans and freshwater systems 400 million years ago. He found out that their “variation in head shape isn’t really linked to where they live, but rather to how old they are!” (I.e. the ones that are 430 million years old have a different set of head shields than those that are 350 million years old). ● Crocodiles–He found out that the three main lineages (the ones leading to alligators, crocodiles, and gharials) have existed and been distinct since approximately 100 million years ago (when dinosaurs still roamed the Earth). They used to be much more diverse than what we currently have—there were plant-eating crocodiles, miniature crocodiles, and more! Dr. Groh highlights the importance of knowing about the evolution of all different kinds of animals in order to create the fullest picture possible of Earth’s timeline. For constructing a timeline of life on Earth, Dr. Groh suggests having a diagram that shows the different relationships between different animal groups in order to keep a better overview. *Crocodylomorpha is the scientific name for the group consisting of crocodilians (crocodiles, alligators, etc.) and their extinct relatives. Publications: Groh, S., Upchurch, P., Barrett, P., & Day, J. J. (2020). The phylogenetic relationships of neosuchian crocodiles and their implications for the convergent evolution of the longirostrine condition. Zoological Journal of the Linnean Society, 188 (2), 473-506. doi:10.1093/zoolinnean/zlz117 Groh, S. (2019). A multi-disciplinary approach to the analysis of crocodylian phylogeny, diversity and biogeographic history in Deep Time (Doctoral dissertation). UCL (University College London). Groh, S., & Giribet, G. (2015). Polyphyly of Caddoidea, reinstatement of the family Acropsopilionidae in Dyspnoi, and a revised classification system of Palpatores (Arachnida, Opiliones). CLADISTICS, 31 (3), 277-290. doi:10.1111/cla.12087 Dr. Mairin Balisi Ecology and Evolutionary Biology La Brea Tar Pits and Museum, Los Angeles, and at the University of California, Merced. About Her Work Dr. Balisi uses the fossil record from the last million years to understand the causes of modern biodiversity, using three main areas of research. 1. The diet and movement of living and extinct animals. Methods: ● Used statistical graphing techniques to show that a Pliocene dog the size of a small wolf and known as the last of the bone-cracking dogs, hunted socially, and that the extinction of the genus could have impacted decomposition and nutrient cycling in North American ecosystems. 2. How ecology and morphology control biodiversity in deep time. Methods: ● Showed that the specialization in both more or less carnivorous traits of primitive canids could have negatively impacted their diversification after the relatively undisturbed first 40 million years of dog evolution. ● Her team estimated body mass using fossil parts, compiled occurrence data, calculated species durations, and formed many statistical tests to look into the relationship of these traits. ● By proving that some of these extinct canids filled niches no longer present or occupied in modern ecosystems, this may be the underlying reason for today’s diminished variety in dog species. 3. Looking for interactions among ecology, the environment, and extinction. Methods: ● Used radiocarbon dating, and studied both the movement and ecomorphological representations of her fossils to see how environmental disturbance shaped the diversity of other small to medium-sized canids to the present. ● Compared patterns of traumatic injury between the saber-tooth cat and dire wolf and helped diagnose the oldest known case of hip dysplasia in a cat, helping us understand causes of more primitive bone injuries in carnivores. Publications: Balisi, MA, AK Sharma, CM Howard, CA Shaw, R Klapper, EL Lindsey. Computed tomography reveals hip dysplasia in Smilodon: Implications for social behavior in an extinct Pleistocene predator. bioRxiv 2020.01.07.897348. Balisi, MA, X Wang, J Sankey, J Biewer, and D Garber. Fossil canids from the Mehrten Formation, late Cenozoic of northern California. Journal of Vertebrate Paleontology 37(6). Wang, X, SC White, MA Balisi, J Biewer, J Sankey, D Garber, and ZJ Tseng. First bone-cracking dog coprolites provide new insight into bone consumption in Borophagus and their unique ecological niche. eLife 7:e34773. Elizabeth Petsios Paleontologist Baylor Department of Geosciences Q&A with Elizabeth Petsios What can fossils tell us about the K-Pg extinction? Fossils are the main way we know that a mass extinction happened. By definition, when a lot of different species go extinct in a relatively short amount of time, that’s called a mass extinction. So if we find a bunch of different fossil species from Cretaceous rocks, and then suddenly those =fossil species are gone in Paleogene rocks, we know a mass extinction happened in between. The K-Pg is famous for wiping out whole groups of animals like non-bird dinosaurs, marine reptiles (like mosasaurs), pterodactyls, and ammonites. How do you analyze fossils? When we collect fossils, we also make observations that can tell us about when, where, and how that animal lived when it was alive. The first observation we make is: How old are the rocks that this fossil was found in? If the rocks are from the Cretaceous Period (meaning they formed in the Cretaceous), then we know that the fossils found in them were alive in the Cretaceous Period (approx. 150 to 65 million years ago). The second observation we make is: In what environment did these rocks form? From looking at the rocks, we can tell if they formed, for example, in the ocean or if they formed on land.
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