Evolution of developmental strategies that generate diversity in marine chordates



Among the extensive biodiversity of the marine environments, the sub-phylum of tunicates (aka urochordate), the sister group of vertebrates, represents an astonishing example of phenotypic diversity and adaptation within a sole taxa. In particular, benthic tunicates belonging to the class ascidiacea, are widespread around the world, and are characterized by different body shape and colony organizations. Due to their success in rapidly spreading to newly establish eutrophic environments they are also among the most invasive marine species.

Beside the primitive solitary forms, where the sexual reproduction is the sole way of propagation, many ascidians species independently evolved highly different reproductive strategies, namely asexual reproduction via dissimilar 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. By a comparative study of the budding process in four ascidians species belonging to the same family (Styaelide), we aim to link conserved cellular and molecular mechanisms of body (re)patterning to macroevolution events that generated part of this chordate marine biodiversity. 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.

TASK 1. Strengthen the phylogenetic relationships and map character states for budding and regeneration.

Phylogenetic relationships among tunicates have been extensively studied morphologically and molecularly, however, the monophyletic origin of the Styelidae is still not well supported. It is therefore important to first resolve the phylogenetic relationships of our candidate species and to map the characters related to budding strategies. In order to provide a more detailed and solid picture of intra-family phylogenetic relationships, we intend to add additional species to the existing sequence databases of styelids using newly collected specimens from France and Brazil. .

TASK 2. Developmental mechanisms and ecological context of ascidian reproductive strategies

In this task we aim to provide an anatomical and morphological description of potential cells and tissue directly involved on the formation of new adult bodies during the process of budding. Direct in vivo observation will be undertaken, both in laboratory marine-culture conditions and in the field. In addition, the environmental parameters of the collecting sites will be recorded to enable associative studies between reproductive strategies and ecological information.

TASK 3. Molecular characterization of tissues involved in budding: epithelia and stem cells population/s

In this task, the anatomical data collected in Task 2 will be analysed at the molecular level. The tissues and the cells putatively responsible for budding will be isolated and analysed by two complementary approaches: (1) To undertake unbiased high-throughput gene screening in order to define set of transcripts implicated in the different developmental pathways, (2) to analyse the spatial and temporal patterns of expression and function of candidate genes involved on stem cell maintenance (stemness) and cell differentiation.

TASK 4. Understanding the evolution of genomes in solitary and colonial ascidians

In this task we aim to sequence three complete genomes of our representative colonial and solitary species. The genomes will be assembled either de novo or based on the reference genomes already sequenced, i.e. Botryllus schlosseri, Didemnum vexillum, and several solitary species. The transcriptomic data obtained on Task 3 will facilitate and speedup the assembly process.