The photophobic response in the apusomonad Podomonas kaiyoae is mediated by coordination of cilia and actin filaments.

Light responsiveness is a fundamental characteristic of eukaryotic organisms, both unicellular and multicellular. However, no photoresponsive behavior has previously been reported in Apusomonadida-a group of small, free-living biflagellates phylogenetically positioned as the sister group to Opisthokonta (comprising animals, fungi, and their unicellular relatives). As such, apusomonads are critical for understanding the evolutionary origins of opisthokonts. Here, we report an avoidance response to blue light in the apusomonad Podomonas kaiyoae. This response is characterized by an increase in gliding velocity, transient changes in flagellar waveforms, and alterations in cell shape. Dynamic cell contraction is triggered by an increase in intracellular free Ca²⁺ and is inhibited by either a dynein inhibitor or an actin-disrupting agent. These findings suggest that the photophobic behavior of Podomonas kaiyoae depends on cytoskeletal responses mediated by both the dynein/tubulin and myosin/actin systems, which likely originated early in eukaryotic evolution. The functional dominance of the posterior flagellum in cilia-driven directional changes further supports the phylogenetic placement of apusomonads as the sister group to opisthokonts, rather than to other eukaryotic lineages.