Solitary choanoflagellate dynamics and microconfined directed transport

Abstract

In evolutionary biology, choanoflagellates are broadly investigated as the closest animal ancestors. Under suitable environmental cues, choanoflagellate Salpingoeca rosetta can differentiate into two types of solitary motile cells. Each group is recognized by its own strategy to swim and its morphology. Moreover, under nutrient limited conditions, S. rosetta experience a haploid-to-diploid transition evidenced by the presence of gametes. It is challenging to determine if there is a connection between the two types of swimming strategies and the male and female gametes. Therefore a current interest is to isolate and concentrate the fast swimming cells, for instance, using a microfluidic device. Following this aim we measured their body sizes and characterized their motilities. We determined that fast cells swim remarkably different from slow cells and proposed a phenomenological model to reproduce the observed dynamics. We solved the Langevin dynamical equations of motion using experimental parameters for choanoflagellates swimming in a confined flat microdevice divided by a wall of asymmetric obstacles. A systematic study of the directed transport efficiency was performed in order to optimize the geometry of the obstacles wall. Numerical results showed that fast choanoflagellates can be directed efficiently for a wide range of geometric parameters of the obstacles wall while slow cells are hardly directed independently of its geometry. The clear differences found in the rectification of fast and slow choanoflagellates suggest that an efficient micro-sorter device could be designed for further applications in evolutionary biology.