FORAGING MECHANISMS IN EXCAVATE FLAGELLATES SHED NEW LIGHT ON THE FUNCTIONAL ECOLOGY OF EARLY EUKARYOTES

Abstract

The phagotrophic flagellates described as typical excavates have been hypothesized to be morphologically similar to the Last Eukaryotic Common Ancestor and understanding the functional ecology of excavates may therefore help shed light on the ecology of these early eukaryotes. Typical excavates are characterized by a posterior flagellum equipped with a vane that beats in a ventral groove. Here, we combined flow visualization and observations of prey capture in representatives of the three clades of excavates with computational fluid dynamic modelling, to understand the functional significance of this cell architecture. We record substantial differences amongst species in the orientation of the vane and the beat plane of the posterior flagellum. Clearance rate magnitudes estimated from flow visualization and modelling are like that of other similarly sized phagotrophic flagellates. The interaction between a vaned flagellum beating in a confinement is modelled to produce a very efficient feeding current at low energy costs, irrespective of the beat plane and vane orientation and of all other morphological variations. Given this predicted uniformity of function, we suggest that the foraging systems of typical excavates studied here may be good proxies to understand those potentially used by our distant ancestors more than 1 billion years ago. SignificanceHuman sperm reminds us of our ancestry: flagellates, unicellular organisms equipped with a flagellum. The last common eukaryotic ancestor (LECA) was a flagellate. Phylogenetic analyses suggest that Excavates, an assemblage of flagellates, are the living organisms most similar to LECA. They have distinct characteristics in common: a ventral groove within which a vaned flagellum is beating. We show how the shared morphology and foraging behavior among 3 excavate clades is fluid dynamically efficient. A similar flagellar arrangement, potentially homologous to that found in the excavates, is found among flagellates from other deep branches of the eukaryotic tree, suggesting that the typical excavate foraging system studied here may have been used by our distant ancestors more than 1 billion years ago.