Role of neutral drift, developmental constraints and natural selection in the long term evolutionary stability of embryonic tunicate morphologies. – TED (Tunicate Evo devo)

Evolutionary biology studies have mostly focused on the mechanisms driving innovation. By contrast, evolutionary stability over long periods of time, in spite of progressive genome sequence drift, is a very widespread phenomenon that remains poorly understood. It probably involves a careful balance between external stabilizing selection, internal developmental constraints and their consequences on organismal robustness to neutral drift.

Within tunicates, the vertebrate sister group, ascidian embryos show exceptional morphological stability since the Cambrian, over 500 million years ago, and despite extreme genome sequence divergence. By contrast, the embryos of thaliaceans, which are considered paraphyletic to ascidians, have undergone a radical morphological change associated to an ecological transition. We propose to use ascidians to unravel some of the molecular mechanisms underlying morphological stability and to study how this stability has been lost in thaliaceans. The project will address three complementary problems, involving different levels of biological complexity:

• Task 1: Elucidating the tunicate phylogeny and the evolution of the repertoire of developmental regulatory proteins in ascidians and thaliaceans.

• Task 2: Unraveling the evolution of the transcriptional program of ascidian embryogenesis: temporal and spatial structure of patterns of evolutionary divergence of gene expression, relative contributions of cis-and trans-regulatory divergence to these patterns.

• Task 3: Revealing the causes of the extreme evolutionary stability of ascidians at the systems level: relative impact of developmental constraints, selection and genetic drift.

This project combines genomics, systems biology and embryology and bridges micro- and macro-evolutionary studies. It associates two neighboring Montpellier groups with complementary expertise and belonging to distinct CNRS institutes: P. Lemaire (CRBM, CNRS-INSB; developmental systems biology, genomics and evolution) and E. Douzery (ISEM, CNRS-INEE; molecular phylogeny and systematics, molecular clock).