A fundamental issue in evolutionary biology is the nature of transitions from unicellular to multicellular organisms. From the point of view of natural selection, increased fitness of even the simplest multicellular organisms relative to their unicellular ancestors is subtle, for there are both costs and benefits. The costs include metabolic investment in the mechanical structures and adhesive proteins that give strength to tissues, and greater gene regulation to allow for distinct cell types, while the benefits of increased size may include faster motility, decreased predation, and greater rates of nutrient uptake.
Over the past 15 years, there has been considerable progress in understanding - from a physical perspective - various aspects of these trade-offs. This has been achieved primarily using green algae and uni- and multicellular choanoflagellates, a sister group of animals that are their closest living relatives, and that has been conjectured historically to be closely related to choanocytes, the flagellated filtering cells in sponges (the basalmost of animals).
Researching how the simplest multicellular organisms function, for example, how a multicellular alga like Volvox carteri (Figure 1) steers toward the light without a central nervous system, has not only shed light on these important biological issues, but has had a significant impact on physical sciences. It has led to discovery of “hydrodynamic bound states” mediated by surface interactions, measurements of the flows around freely-swimming microorganisms, an understanding of flagellar synchronization, progress in the mechanics of cytoplasmic streaming, discovery of spontaneous coherent flows of confined active matter, and tests of interactions between fluid flow and elastic biopolymers.
Progress thus far on the physical understanding of transitions to multicellularity has exploited the elegant geometries of the adult model organisms, whose high degree of symmetry simplifies experimental and theoretical study. Yet, we do not understand how these organisms create their wondrous geometric forms, and what those processes can tell us about the structure and function of the earliest animals. Phrased another way, a profound question is: How do cells build structures external to themselves in a robust and accurate manner?
This question is central to understanding the directionality, evolution, and form of embryonic development, and our research program involves an interdisciplinary effort to address these issues through the study of model organisms representing the simplest examples of multicellularity. It is in this context that the distinction between germ cells (responsible for reproduction) and somatic cells first appears, leading to well-defined embryos distinct from a parent. We are studying embryonic development in two classes of simple multicellular organisms, green algae and aquatic sponges, to elucidate goal-directed processes and develop quantitative mathematical approaches to directionality and form.
In the service of this new science, we emphasize the importance of a holistic approach that is grounded in quantitative experimental measurements of carefully chosen model systems, using a range of suitable species, strains, and mutants, and which seamlessly connects to mathematical and physical theories. Our work will produce an experimental platform of this type that can serve as a model for others to do research in this area, particularly in the context of the cohort activities that will bring together researchers of many different types to tackle these questions.
Volvocine green algae, spanning from unicellular Chlamydomonas to multicellular germ-soma differentiated Volvox, have long been recognized as models for evolutionary transitions to multicellularity. The spherical daughter colonies of Volvox grow in embryonic vesicles through cell division, then undergo a fascinating process known as “embryonic inversion” in which they literally turn themselves inside out. Inversion shares the key feature of tissue folding that is found in developmental processes such as gastrulation, neurulation, and optic cup formation in higher organisms. Subsequent to inversion, cells secrete an extracellular matrix (ECM), a complex three-dimensional structure of glycoproteins and enzymes, and the colony grows as an expanding sphere with cells arranged in a lattice with a high degree of orientational order, essential for motility. While inversion has now been quantified and modelled with generalized elasticity theory and has been shown to involve both considerable spatio-temporal coordination and noise, the regulation of subsequent ECM generation that gives rise to the adult form is unknown; its elucidation will be a major thrust of this work.
Marine and freshwater sponges are arguably the basalmost of animals and display a level of tissue complexity beyond that found in multicellular algae. Aspects of their oogenesis and larval development are similar to inversion in green algae. Most inhabit a regime of inertia-dominated flows external to the organism, yet are comprised of a complex network of minute interconnected chambers lined with choanocytes, within which viscosity dominates.
This microfluidic network, termed the “sponge pump”, interfaces with the outside world through pores in the sponge wall and can achieve substantial volumetric throughput. Thus, in contrast to green algae, which have external feeding currents, sponges thus have internal flows, the evolutionary start of a gastric system. The fluid mechanics of the sponge pump has been of interest for decades, yet it remains poorly understood.
This is a multiscale problem, linking flagellar dynamics at the micron scale to inertial flows spanning tens of centimetres. These structures can be viewed as the simplest type of vascular systems and must develop from the embryo through a sequence of topological rearrangements; the irreversibility of these topological transitions likely lies at the heart of the temporal ratchet of this developmental process, yet to date there have been no quantifications of this process in vivo nor any proposal of the biophysical mechanisms involved. Achieving this understanding is a second major goal of the work.
Choanocytes are closely related to choanoflagellates, uni- and multicellular organisms that are the closest living relatives of animals and are models for the study of multicellularity. They are known to exhibit many of the key adhesion proteins that exist in animals and have recently been discovered to exhibit geometrical transformations of cell sheets analogous to inversion. By linking the formation of multicellular choanoflagellates through ECM generation with body plan establishment in sponges, we aim to reveal evolutionary aspects that underlie sponge embryogenesis.
My group uses light sheet and confocal microscopy, microfluidics and state-of-the-art image analysis to obtain four dimensional (space+time) data on development, and mutant strains and biochemical/biological interventions as appropriate to test mechanisms. Additionally, we develop physical/mathematical approaches to the fluid-structure problem of network development and topological changes driven by cell growth and elasticity. With such theories, we will address broadly applicable questions about how to compare to each other growing structures which have developmental fluctuations (“noise”), answering the question: What does it mean to say two developing organisms are similar? Our preliminary results suggest that there are new mechanisms of regulation at work, best described as geometrical feedback loops.
