Current challenges in evolutionary biology

V.I. MARCONI

VIÑA del Mar

Congreso; Physics of Active Matter. Statphys 2019 Satellite Meeting; 2019

Choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals, their sis- ter group. The changes in gene content that preceded the origin of animals can be reconstructed by comparison with them. Choanoflagellates Salpingoeca rosetta, under suitable environmental cues can di↵erentiate into two types of solitary cells. Each group is recognised by its own swimming strategy and its body size: fast and small or slow and large. Under nutrient limited conditions, S. rosetta experience a haploid-to-diploid transition, evidenced by the presence of gametes. It is challeng- ing to determine if there is a connection between the two types of cells and the male and female gametes. Here is where active matter research and microfluidics application cause under laboratory conditions always they are mixed populations. Motivated with this current interest, we have measured their sizes and characterised their motilities, developing a new soft for automatic track- ing analysis of big amount of data [S ́anchez et. al., 2016, ReyesThesis2017, Min ̃o et al Preprint 2019]. We find that fast cells swim remarkably different from slow cells, being their trajectories quasi-straight and interrupted by changes of direction while the latter are strongly tortuous. We propose a phenomenological model to reproduce the observed choanoflagellate dynamics. Further, cells are confined into a flat device divided by a wall of asymmetric obstacles separated by micro-gaps. A systematic study of the directed transport is performed in order to optimize the device geometry. We solve the Langevin dynamical equations of motion ussing the experimental parameters. Simulations show that fast choanoflagellates are remarkably easier to be directed eciently for a wide range of wall geometries, while slow cells are hardly directed independently of the geometry due to their tor- tuous swimming behavior. As a consequence, a rigorous characterization of each micro-swimmer motility is crucial for a proper micro-device optimization, using the largest amount of data available, and automatically. Even more, due to the clear di↵erences between fast and slow choanoflagellates rectification results, an ecient micro-sorter device was fabricated, obtaining a good agreement between theory and experiments [Miño et al, preprint].