MESODERM EVOLUTION

The development and evolution of the mesoderm in basal bilaterian acoel worms

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 Obiettivo del progetto (Objective)

'Understanding the evolution of the diversity of animal forms is one of the major tasks in biology. The use of molecular biology led to a better understanding of how evolutionary changes in the development can result in changes in the body plan. Modifications of the wiring between genes caused the adoption and co-option of gene regulatory networks that shape the morphology of the adult during the development. To get a better picture of how these changes led to evolutionary novelties one has to investigate animals that are representatives of key-nodes in the animal tree of life. A fundamental innovation of complex animals is the third germ layer – the mesoderm, that is characteristic for all bilateral symmetric animals that include flies, worms and humans. The mesoderm gives rise to important organ systems such as skeletal elements, musculature, heart and kidneys. Recent insights gained from studies of the development of cnidarians – the most closely related group of the bilaterians – suggest, that the mesoderm evolved from the internal germ layer the endoderm. Comparative studies between animals can reveal how the gene regulatory network that specifies the endoderm of cnidarians has been changed so that it gives rise to a novel germ layer, the mesoderm. The ideal organisms to investigate for answering this question are the simple acoel worms, which form the earliest evolutionary branch of all bilaterian animals. The proposed research project has the aim to investigate the gene regulatory network that specifies the endoderm and mesoderm of the acoel species Convolutriloba longifissura using modern molecular tools. By describing the detailed temporal and spatial expression of key regulatory genes and a following functional analysis important information will be gained that delivers the basis for a comparisons to identify the changes in the gene regulatory network that promoted the evolution of three germ layers and thus the evolution of more complex animals.'

Introduzione (Teaser)

A major turning point giving rise to development of complexity in animals was the advent of the mesoderm. EU researchers have analysed the gene networks that made this major evolutionary coup possible.

Descrizione progetto (Article)

More highly evolved animals have three germ layers and less complex creatures have only two. The difference is in the mesoderm that gives rise to important organs such as the skeletal, muscle and vascular systems. Study of the Cnidaria group comprising of jellyfish and sea anemones suggests that the mesoderm evolved from the inner germ layer, the endoderm.

Comparison of gene networks of different animals revealed what genes in the acoel worms gave rise to the new mesoderm layer. The EU-funded MESODERM EVOLUTION (The development and evolution of the mesoderm in basal bilaterian acoel worms) project has investigated mesoderm development in the acoel species, Isodiametra pulchra (I. pulchra). Techniques used included fate map analysis and gene expression studies. They also compared this genome and transcriptome with other acoel worms i.e. the Nemertoderma and Xenoturbella species.

The researchers have put together a more detailed fate map than from previous research using confocal imaging of living fluorescently labelled worms. Study of expression of mesoderm transcription factors revealed that only a small number of cell types are present. This suggests that the functional parts of the organs from the mesoderm must be secondarily derived.

Timing of expression of candidate genes is also a crucial factor in development. MESODERM EVOLUTION scientists have determined the rate of expression in more than 10 developmental stages. Tissue specific transcriptomes will also help to determine lineage specific genes responsible for mesoderm formation.

In the bid to identify specific functions of genes and thereby reconstruct the gene regulatory network, the team established RNA inference methods. By the end of project, the researchers had successfully knocked down the expression of some genes, which will help to identify their individual functions.

Project members used molecular biology to build a picture of gene regulatory networks. This has led to a better understanding of how evolutionary changes in development can result in changes in the body plan. This substantial knowledge base on the evolution of complexity in animals can also be used by other research teams.