Evolution of developmental strategies that generate biodiversity in marine chordates – DEVODIVERSITY

Among the extensive biodiversity found in marine environments, the sub-phylum Tunicata (sister group of vertebrates) represents an example of a single taxon with astonishing life history adaptations and phenotypic diversity. In particular, benthic tunicates belonging to the class Ascidiacea, are sessile organisms widespread worldwide that occupy a variety of habitats, including coral reefs, rocky and soft substrates, and a wide diversity of niches. Due to their success in rapidly spreading to newly establish eutrophic environments they are also among the most invasive marine species.
Tunicates (most notably ascidians), together with cnidarians and sponges, show some remarkable examples of how the evolution of alternative modes of reproduction, i.e. sexual and asexual, contribute to the overall diversity of forms and habitat adaptations in marine ecosystems. Ascidians are characterized by different size, colony shape, and organization. Besides solitary forms, in which sexual reproduction is the only way of propagation, colonial ascidians species independently evolved highly different reproductive strategies, namely asexual reproductions via distinct types of budding. Budding can be classified based on two functional types: propagative, which accounts for colony growth, replication and reproduction; and survival or regenerative, i.e. passive forms that ensure colony survival during adverse environmental conditions. In addition to this functional and ecological classification, budding can be categorized based on the developmental origins of budding, i.e. based on the type of putative stem cells or epithelial tissues involved in bud formation.
In this project, we propose to use multiple approaches, commonly used in developmental and cell biology, to investigate the plasticity of asexual development and regeneration mechanisms in colonial ascidians. Our overall goal is to understand the mechanisms that allow the evolution of budding and coloniality in these marine chordates, and how these mechanisms endorsed ascidian species to spread successfully through diverse natural and artificial marine ecosystems.
The implementation of this proposal will allow the two partner teams to consolidate a research consortium and to carry out four major tasks that will encompass evolutionary, developmental, ecological, and genomic studies. In Task 1, we will use additional taxa and new approaches to try to resolve phylogenetic relationships among the family of Styelidae in order to map intermediate states of developmental transitions during the evolution of budding strategies. This study will provide information on the directionality of the reproductive changes. Task 2 will adopt a morphological and ecological approach to describe anatomical and developmental details of budding processes and to test for environmental conditions associated to particular budding modes in each species. Task 3 will be devoted to characterize the cellular and molecular basis of budding, including precursor epithelia and progenitor cell populations. Finally, in Task 4 we will launch a comparative genomic approach in order to provide a better understanding of the evolution of a major life history ‘transitions, specifically from sexual to asexual propagation.
In this comparative and multidisciplinary study we aim to adopt a developmental biology approach to link conserved cellular and molecular mechanisms of body (re)patterning to macroevolution events that generated marine chordate biodiversity and new reproductive adaptations. We expect that the outcomes of this project will generate basic understanding of asexual propagation in tunicates, which may reveal new research directions and targets for effective control of invasive ascidians and that may provide new insights into the origins of stem cell function and regenerative biology in our own phylum, Chordata.