How to become a tardigrade? Combining paleontology and developmental biology to understand tardigrade body plan evolution

Abstract

Tardigrades, also colloquially known as water bears or moss piglets, are a phylum of microscopic invertebrates that are characterized by having a segmented body plan and four pairs of lobe-like legs. Phylum Tardigrada is subdivided into the classes Heterotardigrada, which have high morphological disparity including various external cuticular specializations such as plates and spines, and Eutardigrada, which have a simpler plump-like appearance without external cuticular structures. Together with onychophorans and euarthropods, tardigrades form part of a larger clade known as Panarthropoda, with tardigrades having the distinction of being the only completely microscopic group in this context. Our current understanding on how tardigrades first evolved, and the processes leading to their morphological differences with the other panarthropods remains limited due to the scarcity on research that tackles these questions. In my dissertation, I performed an interdisciplinary approach that combined paleontological work and developmental genetics to shed light on the evolution of the tardigrade body plan. In Chapter 1, I described the youngest fossil tardigrade to date (in ~16 Ma amber) and showed that it had character combinations that are not present in extant relatives. This rejects the leading interpretation that tardigrades have remained morphologically static for millions of years. In Chapter 2, I redescribed the first two fossil tardigrades to be discovered, both preserved in a Cretaceous-age amber (~72 Mya) and resolved their phylogenetic relationships within tardigrades. This allowed me to effectively use one of them as a calibration point to calculate divergence time estimates (i.e., a proxy for when certain body plans existed) of the phylum and its major groups in deep time. In Chapter 3, I examined the gene expression patterns of distal limb patterning genes during embryonic leg development to infer which genes are involved in limb morphogenesis. Comparing the patterns to other panarthropod models suggest that tardigrade legs have a distal identity relative to the legs of onychophorans and euarthropods. In Chapter 4, I investigated the identity and fate of the eutardigrade embryonic tail – a feature that disappears during development. My results show that the tail gets internalized and potentially becomes the hindgut. Lastly, in Chapter 5, I redescribed the mid-Cambrian lobopodian Aysheaia pedunculata and provided support for its affinity as a stem-group tardigrade. This resolved relationship provided paleontological evidence for tardigrade miniaturization during their early evolutionary history, allowed the reconstruction of the ancestral tardigrade body plan, and revealed the morphological changes that have occurred in the lineage that led to the extant tardigrades. Overall, my dissertation utilized samples across the entire Phanerozoic to have different temporal perspectives – looking at the past and the present to ask how tardigrades have changed throughout their evolutionary history, and to uncover the macroevolutionary and genetic processes responsible for the stabilization of their distinctive body plan.