Multiple evolutionary routes to cytoskeletal arborization revealed by the rhizarian amoeba Filoreta ramosa

Abstract

The eukaryotic cytoskeleton generates remarkable diversity in cellular architecture despite being built from deeply conserved actin and tubulin polymers. Diversification of cytoskeletal regulators, motors, and filament-organizing proteins produces highly varied cellular morphologies across eukaryotes, yet certain higher-order architectures repeatedly emerge in distantly related lineages. One notable example is cytoskeletal arborization, which occurs not only in metazoan neurons but also in amoeboid lineages distributed throughout the eukaryotic tree. Whether these similar branched architectures arise through conserved cytoskeletal organization, independent reuse of shared molecular systems, or convergence driven by common physical constraints remains unresolved. Here, we investigate the rhizarian amoeba Filoreta ramosa, which forms a multinucleate reticulated network through branching and anastomosis. Using live imaging, immunofluorescence, morphometric analyses, and cytoskeletal drugs, we define how actin and microtubule systems organize branch formation, intracellular transport, and large-scale network architecture. Actin-rich protrusions initiate exploratory branchlets that become selectively stabilized through microtubule incorporation. Longitudinal microtubule arrays reinforce mature branches and support rapid bidirectional organelle transport, while branch nodes function as distributed sites of microtubule nucleation. These cytoskeletal features parallel key mechanisms underlying neuronal arborization, including actin-driven exploration, microtubule-dependent branch stabilization, and transport systems that scale with increasingly extended cytoskeletal networks. However, unlike neurons, Filoreta develops a decentralized reticulated network through repeated anastomosis and distributed microtubule organization, demonstrating that similar arborized morphologies can emerge through distinct architectural strategies. Our findings indicate that arborization can arise through multiple evolutionary adaptations to common cellular constraints. Shared cytoskeletal mechanisms repeatedly support branching architectures, but distinct topologies and modes of cellular organization demonstrate that evolution can reach arborization through different routes. Similar cytoskeletal networks may repeatedly emerge in diverse lineages when cells face the challenges of exploration, stabilization, and transport across increasingly larger scales. Filoreta therefore provides an experimentally tractable model for investigating how conserved cytoskeletal systems generate diverse arborized cellular architectures across eukaryotic evolution.