While the morphological drivers of ecological radiations have been a frequent and productive subject of study, our understanding of the developmental mechanisms that underlie morphological change during rapid diversification remains much more limited. In last decade, more and more individual case studies provided insights into the specific developmental processes driving the evolution of specific novel and dramatic structures. However, the general rules that govern the evolution of developmental processes remain poorly understood. My primary research goals are to: 1) Identify developmental processes that have evolved in tandem with environmental factors to generate diversity in organismal form, 2) Characterize and model any general rules governing how developmental mechanisms can evolve while still maintaining core developmental functions.
Over its 37 million-year history, this group of 207 species has radiated into almost every mammalian ecological dietary niche (Fig1) and evolved a striking variation in the size and shape of various organs, including the teeth, nose, and ears, as well as an incredible range of sensory adaptations. As a result, this group is ideal to study the developmental foundation of adaptive radiations.
Teeth are modular structure at two levels: the tooth level (the organ itself) and at the cusps level
Adaptive radiations shape morphological and ecological diversity. While the morphological drivers of ecological radiations have been a frequent and productive subject of study (e.g., key innovations), our understanding of the developmental mechanisms that underlie morphological change during rapid diversification remains much more limited. As “evolution is the filtering of development by ecology” this is a fundamental gap in knowledge for the fields of evolutionary, ecological, and developmental biology. Among the developmental mechanisms proposed to underlie adaptive radiation, the modular structure of developmental networks shows particular promise. A developmental module is defined as a component of a developing organism that is semi-autonomous relative to pattern formation and differentiation, or to a signaling cascade. In theory, modular networks allow the rapid evolution of structures while preserving the core network needed for their basic formation and function. However, there is little direct, experimentally-based knowledge of how networks and their modules are modified in response to environmental selection. As a result, the evo-devo field lacks experimental evidence to assess (1) how the relative evolvability of network modules impacts morphological change in response to ecological pressures, and (2) the role of development as an enabling or constraining force during morphological innovation or convergence and lineage diversification. My primary goal is to model the evolution of developmental modules and associated morphologies using the dramatic adaptive radiation of noctilionoid bats and their molars as a model system. My primary hypothesis is that a robust, core developmental module controls the development of molar morphological traits that are conserved across noctilionoids, with evolutionary modification of more plastic developmental submodules leading to morphological variation across species. This project, that I developped in Sears's lab in collaboration with Sharlene Santana, at UW, will serve as a fondation for my future lab as a PI for the next five years.
In our paper, we show that bats possess either L or L and S cones and that color vision evolve in a mosaic fashion.
Pteronotus parnellii cochlea, a bat that possess constant frequency echolocation.
The ecological theory of adaptive radiation remains the single theoretical framework to explain the taxonomic and functional diversity of all organisms in the biosphere. The driving force at the center of this theory is ecological opportunity, a set of resources or adaptive zone that becomes available to a taxonomic group through the evolution of a key innovation, colonization of new environment, or the extinction of competitors. Key innovations, novel phenotypic traits that promote diversification, often involve sensory adaptations that enable access to hitherto unavailable ecological resources. Where they emerge, these sensory adaptations confer such dramatic advantages that they raise an enduring question: why don’t species evolve multiple sensory adaptations simultaneously?
To elucidate the genomic and developmental mechanisms underlying sensory adaptations we focused on noctilionoid bats —an exceptionally diverse and ecologically important clade of mammals. In a collaboration involving four labs, we propose will uncover the genomic and functional morphological basis of variation in four sensory systems: hearing by targeting the cochlea, vision via photoreceptor cells and relative eye size, olfaction by studying the olfactory epithelium and olfactory bulb, and chemosensation by analyzing the vomeronasal organ and accessory olfactory bulb.
In this project, I specifically studied the evolution of vision (see preprint Sadier et al. 2018 ) in Noctilionoid bats, showing that vision plays un important role in bat ecology and that the evolution of color vision is bats if highly mosaic and involve multiple processes at different levels of regulation. Our future research regarding that project is to investigate the developmental mechanisms at the origin of this rapid evolution of color vision by combining classical developmental approaches, RNAseq and modeling. In parallel, I am leading a study on the evolution of hearing and echolocation in the same group of bats to investigate the evolution of echolocation in relation with ecological parameters in this group. Ultimately, these projects will highlight the evolution of sensory systems as a whole, and lead us to conclude about the existence of possible trade-off between sensory systems in mammals.
Aside from my mains projects, I also collaborated on various project inside my team. These projects include some experiments about the identification of limb enhancers, the generation of the first transgenic oppossum or fieldwork.
I am also implicated in collaborations regarding tooth development and evolution with external labs.
During my PhD, I worked on the evo-devo of the Eda Pathway, a signaling pathway which is implicated in the development of ectodermal appendages (such as hair, tooth, nails, scales, feather etc) and which has been shown to be involved in adaptive evolution in vertebrates (see our review in my CV, Sadier et al. TIG). My project was divided in two parts that focused on different aspects of the evolution of this pathway.
In the first project, I studied the role and the evolution of the two isoforms of the intracellular adaptor EDARADD in mammals by combining developmental, genomic, molecular and phylogenetic studies. I demonstrated a role for the two isoforms of Edaradd in signal transduction and proposed a putative role for gain/loss of isoforms in the evolution of developmental pathways. This work was published in 2015 in BMC Evolutionary Biology: Sadier et al. 2015
In my second project, I focused on the receptor EDAR which is implicated in dental development and evolution. I studied its role in the establishment of the molar tooth row and the molar shape in the mouse during development using classical developmental techniques, organ culture and mouse mutants. We used our results to propose a new conceptual framework for the development of the molar row and the first molar. This work is actually submitted to Plos Biology.
I also worked with other colleagues in collaboration on tooth development in fish and on the role of the Eda pathway in cancer (my 4th year of PhD was funded by this project). These collaborations led to two publication (see CV).