The origin of blinking in both mudskippers and tetrapods is linked to life on land.

Blinking, the transient occlusion of the eye by one or more membranes, serves several functions including wetting, protecting, and cleaning the eye. This behavior is seen in nearly all living tetrapods and absent in other extant sarcopterygian lineages suggesting that it might have arisen during the water-to-land transition. Unfortunately, our understanding of the origin of blinking has been limited by a lack of known anatomical correlates of the behavior in the fossil record and a paucity of comparative functional studies. To understand how and why blinking originates, we leverage mudskippers (Oxudercinae), a clade of amphibious fishes that have convergently evolved blinking. Using microcomputed tomography and histology, we analyzed two mudskipper species, Periophthalmus barbarus and Periophthalmodon septemradiatus, and compared them to the fully aquatic round goby, Neogobius melanostomus. Study of gross anatomy and epithelial microstructure shows that mudskippers have not evolved novel musculature or glands to blink. Behavioral analyses show the blinks of mudskippers are functionally convergent with those of tetrapods: P. barbarus blinks more often under high-evaporation conditions to wet the eye, a blink reflex protects the eye from physical insult, and a single blink can fully clean the cornea of particulates. Thus, eye retraction in concert with a passive occlusal membrane can achieve functions associated with life on land. Osteological correlates of eye retraction are present in the earliest limbed vertebrates, suggesting blinking capability. In both mudskippers and tetrapods, therefore, the origin of this multifunctional innovation is likely explained by selection for increasingly terrestrial lifestyles.

Lipid Membrane-Based Antigen Presentation to B Cells Using a Fully Synthetic Ex Vivo Germinal Center Model.

High-affinity antigen-specific B cells are generated within specialized structures, germinal centers (GCs), inside lymphoid organs. In GCs, follicular dendritic cells (FDCs) present antigens on their membrane surface to cognate B cells, inducing rapid proliferation and differentiation of the B cells toward antibody-secreting cells. The FDC's fluid membrane surface allows B cells to "pull" the antigens into clusters and internalize them, a process that frequently involves tearing off and internalizing FDC membrane fragments. To study this process ex vivo, liposomal membranes are used as the antigen-presenting FDC-like fluid lipid surface to activate B cells. In a fully synthetic in vitro GC model (sGC), which uses the microbead-based presentation of the CD40 Ligand and a cytokine cocktail to mimic T follicular helper cell signals to B cells, liposomes presenting a model antigen mimic effectively engage B cell receptors (BCRs) and induce greater BCR clustering compared to soluble antigens, resulting in rapid antigen internalization and proliferation of the B cells. B cells showed GC-like reactions and undergo efficient IgG1 class-switching. Taken together, the results suggest that fluid membrane-bound antigen induces a strong GC response and provides a novel synthetic in vitro system for studying GC biology in health and diseases, and for expanding therapeutic B cells ex vivo.