Figure 1: Body size plasticity of adult Schmidtea mediterranea.
Figure 2: Different-sized planarians can be generated from different-sized fragments through whole-body regeneration.
Figure 3: Biodiversity of planarian body size. A. Simplified planarian phylogeny, adapted from Alvarez-Presas et al., 2008. B. Representative examples of marine (Camerata sp.), freshwater (Schmidtea mediterranea), and land (Bipalium kewense) planarians. Bipalium kewense image credit: Pestnet.org.
+++ At EMBL from December 2021 +++
Through comparative studies of planarian flatworms, the Vu Group aims to understand the control principles that define animal body size.
Previous and current research
Body size is indisputably one of the most defining features of a species, with important implications for physiology, ecology, and evolution. However, it remains poorly understood what determines the size of a species and why it differs so much across the animal kingdom. Over the past few decades, converging studies of growth, patterning, endocrinology, and evolution (particularly in insect model systems, such as Drosophila and Manduca) have provided important insights into the genetic and environmental regulation of body size. However, the mechanism by which the animal ‘senses’ that it has reached its final size and therefore stops growing is still largely unknown. Furthermore, the predominant existing paradigm for studying size control – embryonic development – encompasses a small fraction of an animal’s life history and can explain, at most, a three- to fourfold variation in body size. Understanding orders of magnitude variations in body size across the animal kingdom is further complicated by the significant physiological differences among different species. A comprehensive mechanistic understanding of body size control will therefore require new concepts and approaches.
Our group has chosen the planarian flatworm to tackle these questions because they harbour unique biological properties that make them an ideal model system to identify mechanisms of body size control and evolution. First, planarians exhibit extreme adult body size plasticity. For instance, the model species Schmidtea mediterranea can reversibly adjust its length over a 40-fold range and its mass over a 20,000-fold range, according to food availability (Figure 1). Second, the unparalleled ability of planarians to generate a miniature but perfectly proportioned animal from any cut (Figure 2) provides a unique paradigm for understanding how size is sensed and proportionality re-established. Third, there is a variation of three orders of magnitude in body size across planarian species, from the millimetre-long aquatic planarian Procerodes sp. to the metre-long land planarian Bipalium nobile. This tremendous biodiversity and the powerful comparative approaches it enables offer an unparalleled mechanistic handle on the evolutionary origin of body size diversity.
Future projects and goals
Regulatory logic of body size determination
We take an interdisciplinary approach, spanning biochemistry, cell biology, functional genomics, and mathematical modelling to investigate mechanisms of body size determination in the planarian model species Schmidtea mediterranea. Specifically, we ask:
- How is body size ‘sensed’ and converted into gene expression that tunes growth, patterning, and physiology to suit body size?
- What are the genetic and environmental factors that control growth and ultimately tell the body when to stop growing?
- How are tissues and organs proportionally maintained in the face of constant body size fluctuations, and re-established in whole-body regeneration?
Evolution of body size diversity
While a detailed investigation of how a single species determines its body size has the potential to reveal underlying mechanisms, understanding how the tremendous diversity of animal body sizes evolved necessitates a comparative approach spanning multiple species. Taking advantage of the three-orders-of-magnitude body size range across planarian phylogeny, and in particular the independent evolution of gigantism (when a group of animals is much larger on average than closely related groups) in land planarians (Figure 3), we aim to identify the mechanisms by which body size diversity evolved. Specifically, we ask:
- Which elements of the body size control mechanism were altered during the water-to-land transition?
- Which features of the terrestrial environment ‘lift the brakes’ on growth? And how is growth constrained in aquatic environments?
- How is species-specific size encoded in the genome?
Beyond planarian body size
While many of the projects in the lab are related to planarian body size, we’re always interested in exploring new questions, approaches, and even research organisms – so please don’t hesitate to reach out!