Predatory Protists the Two Words Used in the Main Title Grell, K.G

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Chalker, D.L., and Yao, M.C. (2011). DNA Modes of feeding in predatory elimination in ciliates: transposon Primer domestication and genome surveillance. Annu. protists Rev. Genet. 45, 227–246. Predatory protists The two words used in the main title Grell, K.G. (1979). Cytogenetic systems and have defi nitions of convenience. Use evolution in foraminifera. J. Foram. Res. 9, 1–13. of the term ‘protist’ here refers to Hamilton, E.P., Kapusta, A., Huvos, P.E., Brian S. Leander all (mostly single-celled) eukaryotes Bidwell, S.L., Zafar, N., Tang, H., Hadjithomas, M., Krishnakumar, V., excluding the following multicellular Badger, J.H., Caler, E.V., et al. (2016). Among the most impactful events in lineages: green algae/land plants, Structure of the germline genome of the history of life was the evolutionary animals, fungi, brown algae and red Tetrahymena thermophila and relationship to the massively rearranged somatic genome. origin of phagotrophy over a billion algae. Use of the term ‘predator’ eLife 5, e19090. years ago, which triggered the ability refers to eukaryotes capable of Iwamoto, M., Hiraoka, Y., and Haraguchi, T. (2016). of a cell to ingest a particle of organic hunting and ingesting relatively Uniquely designed nuclear structures of lower eukaryotes. Curr. Opin. Cell Biol. 40, 66–73. material, whether dead or alive, as large prey cells. Different kinds of Iwamoto, M., Koujin, T., Osakada, H., Mori, C., food. Without it, there would be no predators represent vastly distantly Kojidani, T., Matsuda, A., Asakawa, H., Hiraoka, Y., and Haraguchi, T. (2015). Biased animals and no plants, let alone the related lineages across the tree of assembly of the nuclear pore complex is vast number of single-celled lineages of eukaryotes, and this general lifestyle required for somatic and germline nuclear eukaryotes that either photosynthesize can blend into the defi nitions of other differentiation in Tetrahymena. J. Cell Sci. 128, 1812–1823. and/or consume other organisms to modes of nutrition, such as stalked Keeling, P.J., and Burki, F. (2019). Progress sustain themselves long enough to suspension feeding and parasitism. towards the tree of eukaryotes. Curr. Biol. 29, reproduce. In fact, the most recent Although parasites are smaller than R808–R817. Klobutcher, L.A., and Herrick, G. (1997). common ancestor of all eukaryotes and usually do not kill their hosts, Developmental genome reorganization in was almost certainly capable of some predators and parasites use the ciliated protozoa: the transposon link. Prog. Nucleic Acid Res. Mol. Biol. 56, 1–62. phagotrophy, a trait that fundamentally same feeding mechanism to extract Maurer-Alcala, X.X., Yan, Y., Pilling, O.A., distinguishes eukaryotes from all other nutrients from prey cells and host Knight, R., and Katz, L.A. (2018). Twisted tales: forms of life, namely archaea and cells, respectively, which can blur the insights into genome diversity of ciliates using single-cell ‘omics. Genome Biol. Evol. 10, bacteria. distinction between the two lifestyles. 1927–1939. The evolution of phagotrophy was For instance, the ability to feed like Maurer-Alcala, X.X., and Nowacki, M. (2019). Evolutionary origins and impacts of genome predicated by a dynamic proteinaceous a vampire by piercing the surface of architecture in ciliates. Ann. NY Acad. Sci. cytoskeleton comprising microtubules, a cell and sucking out its contents 1447, 110–118. actin fi laments and associated as food, known as ‘myzocytosis’, Prescott, D.M. (1994). The DNA of ciliated protozoa. Microbiol. Rev. 58, 233–267. molecular motors, which together is found in several different kinds of Raikov, I.B. (1985). Primitive never-dividing preceded the origin of other distinctive predators, such as didinid ciliates, macronuclei of some lower ciliates. Int. Rev. traits of eukaryotes, such as the colpodellids, colponemids, noctilucoid Cytol. 95, 267–325. Singh, D.P., Saudemont, B., Guglielmi, G., nucleus, endomembrane system dinofl agellates and vampyrellid Arnaiz, O., Goût, J.F., Prajer, M., Potekhin, A., and mitochondria. Phagotrophy also cercozoans, and some marine intestinal Przybòs, E., Aubusson-Fleury, A., Bhullar, S., et al. (2014). Genome-defence small facilitated major evolutionary events parasites, such as archigregarine RNAs exapted for epigenetic mating type that transformed the diversity of life and apicomplexans. inheritance. Nature 509, 447–452. the planet as a whole, such as multiple A modifi cation of myzocytosis Vogt, A., Goldman, A.D., Mochizuki, K., and Landweber, L.F. (2013). Transposon origins of photosynthesis and multiple involves a free-living predator, such domestication versus mutualism in ciliate independent origins of parasitism as some colpodellids, perforating and genome rearrangements. PLoS Genet. 9, across the tree of eukaryotes. entering a prey cell and eating it from e1003659. Warren, A., Patterson, D.J., Dunthorn, M., However, despite these major the inside out, leaving only an empty Clamp, J.C., Achilles-Day, U.E.M., Aescht, E., events, many different lineages of shell of what once was. This particular Al-Farraj, S.A., Al-Quraishy, S., Al-Rasheid, K., Carr, M., et al. (2017). Beyond the “code”: a eukaryotes have maintained lifestyles feeding strategy is also found in the guide to the description and documentation most consistent with their deepest zoospores of some parasites, such of biodiversity in ciliated protists (Alveolata, ancestors in the form of free-living as perkinsozoan alveolates; after Ciliophora). J. Eukaryot. Microbiol. 64, 539–554. predators capable of hunting, killing and entering the host cell, the zoospores Xiong, J., Yang, W., Chen, K., Jiang, C., Ma, Y., consuming other prey organisms. This feed on the cell contents and grow Chai, X., Yan, G., Wang, G., Yuan, D., Liu, Y., et al. (2019). Hidden genomic evolution in general mode of nutrition in single-celled a large multicellular sporangium that a morphospecies-The landscape of rapidly eukaryotes has resulted in dynamic completely fi lls the inside of the now evolving genes in Tetrahymena. PLoS Biol. 17, predator–prey relationships and a exterminated host cell. New zoospores e3000294. diverse array of traits associated with are then released from the mature their feeding apparatus, motility systems sporangium and hunt for a new host 1Department of Molecular Genetics and Cell and hunting mechanisms. A brief cell to perpetuate the parasitic lifecycle. Biology, University of Chicago, Chicago, survey of these traits across the tree of Evidence of myzocytosis in the form IL 60637, USA. 2Department of Molecular, eukaryotes is the focus of this primer in of perforations in the protective shells Cellular and Developmental Biology, order to introduce the reader to some of other protists shows up in the fossil University of California Santa Barbara, outstanding examples of convergent record about 750 million years ago. Santa Barbara, CA 93106, USA. 3Institute of Molecular Biology, Academia Sinica, Taipei, evolution, structural complexity and Many different lineages of predatory Taiwan. behavioral sophistication within the protists acquire food using whole *E-mail: [email protected] microbial world. prey cell phagocytosis, which is

Figure 1. Montage of some predatory protists of bacteria and their traits. (A) Light micrograph of the bicosoecid Cantina showing two fl agella (double arrowheads) and the ventral groove (arrow) used to feed on bacteria (image courtesy of Yubuki and Leander). (B) Scanning electron micrograph of the excavate Aduncisulcus in ventral view showing the ventral groove and two fl agella (double arrowhead) (image courtesy of Yubuki and Leander). (C,D) Scanning electron micrographs of the excavate Kipferlia showing a rod-shaped bacterium (arrowhead) trapped in the ventral groove (C) and drawn into the mouth-like cell opening (D) (image courtesy of Yubuki and Leander). Scale bars: A = 8 µm, B = 2 µm, C,D = 1 µm. accomplished in a diverse number of As predatory protists evolved more In addition to armor, many prey cells ways. Predators capable of eating large sophisticated feeding strategies, their defend themselves with subcellular prey cells, such as other eukaryotes, preferred eukaryotic prey evolved more weapons, called ‘extrusive organelles’, tend to have specialized structures sophisticated forms of evasion and that come in many different forms, to facilitate this feat. For instance, protection. Multicellular aggregations, such as the coiled ejectisomes of crawling amoebae ooze dynamic for instance, in the form of fi laments, cryptomonads and the telescopic pseudopods around prey cells in prostrate sheets and arborescent trichocysts of alveolates. The rapid their path. Some dinofl agellates unzip arrays create larger body sizes that discharge of extrusive organelles specifi c regions of their otherwise rigid limit the abilities of protistan predators from prey cells serves to both repel cells to internalize prey cells. Some to feed on them; as such, protistan a predatory attack like a shield dinofl agellates (e.g., Protoperidinium, predators were likely a major selective and forcefully propel the prey cell Gyrodinium) use a large hood-like cell driver for the independent origins in unpredictable directions and extension, called a ‘pallium’, to envelop of multicellularity across the tree of away from the pursuing predator. fi lamentous prey and enzymatically fold eukaryotes. Larger prey sizes then set The effectiveness of this defensive it in half several times before ingesting the stage for a switch from whole-cell mechanism has even led to peculiar it. Ciliates use a highly expandable phagocytosis to myzocytosis in some episymbiotic relationships between oral pocket to ingest prey cells. Many predatory protists. Different lineages (verrucomicrobial) bacteria capable euglenids use a robust system of of prey cells also secrete hard parts as of rapidly discharging a tightly coiled longitudinal rods and pinwheel-like armor, such as thick cell walls, cellulosic thread when disturbed and a large membranous vanes to grab and pull thecal plates in dinofl agellates, calcium protistan host (e.g., ciliates and in prey cells like a Chinese fi nger trap. carbonate coccoliths, chrysophyte euglenozoans); presumably, the Most predatory protists, however, scales, siliceous diatom frustules, network of discharged threads from are less than 10 microns long and euglenophyte loricas, ebriid skeletons, the episymbiotic bacteria serve to eat bacteria using a distinctive foraminiferan tests, agglutinated loricas protect the underlying host and the ventral groove with an opening of tintinnid ciliates, and the shells of remaining (undischarged) episymbionts called a ‘cytopharynx’ (e.g., jakobids, euglyphid and arcellinid amoebae. from predatory attacks. Of course, carpediomonads, percolomonads, Regardless, some larger predatory predators have evolved weapons of malawimonads, bicosoecids, protists (e.g., heteronemid euglenids, their own to counteract the defenses of cercomonads and colponemids) ciliates and dinofl agellates) can their preferred prey, some of which are (Figure 1); this overall feeding strategy consume armored prey cells whole, outlined below. and associated cell morphology spans move the hard parts through their the tree of eukaryotes and almost cells like a pseudo-digestive track, Subcellular projectiles certainly represents the traits in the and release the inorganic waste via Like the defensive extrusive organelles most recent predatory ancestor of the exocytosis through a specialized anus- of prey cells, predatory protists use entire group. like pore, called a ‘cytoproct’. similar subcellular weapons to hunt.

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Figure 2. Montage of some predatory protists of other eukaryotes and their traits. (A) Light micrograph showing the ciliate Didinium in the process of catching and consuming the ciliate Paramecium; both ciliates use extrusive organelles, namely toxicysts and trichocysts, respectively, to battle each other (image courtesy of Gerald Helbig). (B) Light micrograph of the warnowiid dinofl agellate Erythropsidinium showing the eye-like ocelloid (arrowhead); this cell is also equipped with nematocysts used to capture prey cells (image courtesy of Gavelis and Leander). (C,D) Three-dimensional reconstructions of subcellular nematocysts in polykrikoid dinofl agellates (C) and warnowiid dinofl agellates (D); the former are organized as a thread within a pressurized capsule, and the latter contain up to 15 projectiles arranged in a ring (image courtesy of Gavelis and Leander). (E) Scanning electron micrograph showing a predatory ameba using a pseudopod to envelop and consume bacteria (image purchased from Science Photo Library). (F,G) Light micrograph and scanning electron micrograph of the mixotrophic euglenid Rapaza (below) in the process of consuming an entire green algal cell (above), namely Tetraselmis (image courtesy of Yamaguchi, Yubuki and Leander). Scale bars: A = 5 µm, B = 20 µm, C,D = 2 µm; E = 2 µm, F,G = 5 µm.

A classic example of a predator–prey to immobilize Paramecium prior to are either poorly understood or entirely battle involving the simultaneous attempting to slurp it up whole; in unknown. discharge of extrusive organelles by response, Paramecium discharges a The most complex extrusive both combatants can be observed on dense network of trichocysts toward organelles described so far have been a microscope slide when the ciliate Didinium as it struggles to escape characterized in a speciﬁ c group Didinium attacks the ciliate Paramecium the attack. This kind of predator–prey of marine predatory dinoﬂ agellates (Figure 2A). Once a prey cell has been interaction is thought to occur between that live amongst plankton, called detected, Didinium launches a battery countless numbers of species in marine warnowiids and polykrikoids of paralyzing extrusive organelles, and freshwater ecosystems around (Figure 2B). Some of these so-called called ‘toxicysts’, from its apical end the world, the vast majority of which ‘nematocysts’ are strikingly similar

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in form and function to the harpoon- from starch-like material, and one protistan predator actually represents like stinging cells of jellyfi sh and their or more iris-like rings (Figure 2B). several distinct groups that diverged closest relatives. These consist of We know that warnowiids consume from one another over one billion years a pressurized subcellular capsule other dinofl agellates as food, so it is ago, such as euglenids (Excavata), containing a tightly wound thread tempting to interpret the complexity of dinofl agellates (Alveolata), cryomonads capable of rapid discharge when the eye-like ocelloid as a light-sensing (Rhizaria) and katablepharids disturbed (Figure 2C). A paralytic toxin mechanism for detecting prey cells (Hacrobia). Euglenids, dinofl agellates is delivered when the thread penetrates and aiming the complex revolver-like and cryomonads each contain a a prey cell, allowing the predator to nematocysts at them. Light-sensing multitude of lineages with extremely feed without the risk of harm by the (visual) systems in predatory animals diverse morphologies and lifestyles, prey fi ghting to escape. Although are common (e.g., ocelli and eyes), so the streamlined predatory species similar in overall form, the nematocysts but they are not common in predatory that look like fl attened ovals under a of dinofl agellates and jellyfi sh are protists; warnowiid dinofl agellates and microscope are not representative of fundamentally different in molecular possibly some predatory euglenids the groups as a whole. Despite their composition, development, detailed (e.g., Urceolus) are among the only vast phylogenetic distance, these structure and operation. In jellyfi sh, known exceptions. Even though some predators look remarkably similar to the thread is ejected from the capsule, predatory protists show extraordinary one another not only in gross cell shape which usually remains within the cell it subcellular complexity in their but also at many different subcellular formed in; in dinofl agellates, both the weapons and possibly prey detection, levels. All of these groups have capsule and the thread are launched many predatory protists are highly independently acquired a sophisticated toward a prey cell and remain attached streamlined and have evolved traits feeding apparatus, modes of gliding to the dinofl agellate cell by a different that are nearly indistinguishable in very locomotion, extrusive organelles, ‘tow line’ of unclear origin. These distantly related lineages. robust cell surfaces and chromosomes differences refl ect convergent evolution that remain permanently condensed over a vast phylogenetic distance; Convergent evolution of predatory throughout the cell cycle (Figure 3G–I). dinofl agellates and jellyfi sh diverged protists Despite these similarities, traits from each other over one billion years The spaces between grains of marine refl ecting their ‘supergroup’ affi nities ago. sand are teeming with microbial life, remain. For instance, the dinofl agellate In addition to nematocysts an Earth-enveloping habitat where predators have intracellular cellulosic consisting of a coiled thread within countless protistan predators are in armor, fl agellar-based gliding, tubular a capsule, some dinofl agellates, constant pursuit of prey. This collection mitochondrial cristae and lattice-like namely warnowiids, have even more of ‘meiofaunal’ organisms contains trichocysts; the euglenid predators complex extrusive organelles capable very distantly related lineages that have proteinaceous (articulin) pellicle of launching up to 15 projectiles lined represent nearly every major group in strips, tubular extrusive organelles, up in a circular arrangement similar to the tree of eukaryotes, including tiny fl agellar-based gliding and discoidal a revolver (Figure 2D). These extrusive multicellular animals that compete with mitochondrial cristae; the cryomonad organelles contain several intricately and are about the same size as the predators have an extracellular cell organized subcomponents with names single cells of large predatory protists. wall, tubular mitochondrial cristae, reminiscent of a machine, such as When examining the overall diversity of pseudopod-based gliding and tubular nozzles, gaskets, shafts, rosettes predatory protists in this habitat, some trichocysts (Figure 3G–I) and stylets; however, their specifi c species look very similar to one another, These groups of predatory protists functions are essentially unknown. mainly because of their streamlined represent an example of convergent These complex weapons are found cell shapes when viewed under a light evolution over vast phylogenetic in predatory cells with an even more microscope. These particular predators distances; however, the selective complex subcellular apparatus are adapted for moving within the tight forces responsible for their subcellular reminiscent of the camera eyes in spaces between grains of sand by similarities are poorly understood cephalopods and vertebrates. This eliminating unnecessary complexity and currently left only to speculation. so-called ‘ocelloid’ consists of a retinal that would get in the way of movement; Improved knowledge of predators body built from a highly modifi ed the cells are essentially fl attened like these will provide evidence chloroplast, a cornea-like layer built ovals capable of gliding along the necessary for understanding cellular from highly modifi ed mitochondria, surfaces of sand grains (Figure 3A–F). adaptation, subcellular streamlining, a crystalline lens presumably built Upon closer examination, this type of subcellular complexity and the overall

Figure 3. Convergent evolution of streamlined predatory protists living in the spaces between grains of marine sand. Images compare similarities in representatives of three fundamentally different groups of predatory eukaryotes that diverged from one another over one billion years ago: (A,D,G,J) euglenids (Excavata); (B,E,H,K) cryomonad cercozoans (Rhizaria); (C,F,I,L) dinofl agelates (Alveolata). (A–C) Light micrographs showing the fl attened oval cells of a benthic predatory euglenid, cryomonad and mixotrophic dinofl agellate, respectively. (D–F) Scan- ning electron micrographs showing the smooth, robust cell surfaces of a benthic predatory euglenid, cryomonad and dinofl agellate, respectively. Unlike euglenids and dinofl agellates, cryomonads hunt prey cells with pseudopodia (arrowheads) that emerge from a ventral groove. (G–I) Transmis- sion electron micrographs showing the permanently condensed chromosomes in the nucleus of a benthic euglenid, cryomonad and dinofl agellate, respectively. (J–L) Transmission electron micrographs showing the extrusive organelles (pink) in a benthic predatory euglenid, cryomonad and dinofl agellate, respectively. Images from the Leander lab. Scale bars: Images A–F are at the same scale = 5 µm, G–I = 2 µm, J = 1 µm, K,L = 0.5 µm.

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evolutionary history within marine cells that were either cyanobacteria and archaea as food. This ancestor interstitial habitats. This research will (primary), photosynthetic eukaryotes already had a complex cytoskeleton also bring into the forefront the cellular with cyanobacterial endosymbionts that supported a ventral feeding context needed for interpreting the (secondary), or photosynthetic groove and two fl agella used for huge amount of genomic data being eukaryotes with photosynthetic gliding along substrates, an overall generated around the world, especially eukaryotic endosymbionts (tertiary). cell morphology and lifestyle similar to environmental sequencing surveys Ultimately, this process transforms many extant species that span the tree of microbial diversity. Nonetheless, predators into photosynthesizers. of eukaryotes. As eukaryotes began to continuous interactions between Different stages in the process diversify and multiply, some lineages predators like these and their prey of endosymbiosis have been expanded their prey preferences create circumstances that have led to described in a wide range of living and acquired the ability to consume major events in the evolution of life, predatory protists. One of the other eukaryotes by either whole-cell namely transformative shifts in lifestyles hallmarks of an intermediate stage phagocytosis or myzocytosis. These and modes of nutrition. in the transformation of a predator predator–prey interactions resulted in to a photosynthesizer is a mode of selective forces that led to arms races Counterintuitive outcomes nutrition called ‘mixotrophy’, where involving subcellular projectiles, like of continuous predator–prey a protist continues to consume prey trichocysts and nematocysts, and interactions cells even though it has already complex suites of armor, such as shells, Predatory protists can either established a photosynthetic symbiont scales, cell walls, frustules, loricas have specifi c prey preferences or through endosymbiosis. The euglenid and tests. Predatory protists were also indiscriminately eat a wide-range Rapaza viridis, for instance, is both selective drivers for the independent of encountered prey cells. Some a predator and a photosynthesizer origins of multicellularity across the tree predatory protists are limited to eating (Figure 2F,G); however, the robust of eukaryotes, because prey organisms bacteria because of their small cell feeding apparatus found in euglenid with larger body sizes are more sizes under 10 microns and their predators has become signifi cantly likely to avoid and survive predatory restricted feeding apparatus, such reduced in Rapaza, indicating a stage attacks. Different lineages of predatory as the bacteria-sized ventral feeding in the gradual loss of the predatory protists living in similar environments groove in excavates (Figure 1). lifestyle. Looking deeper into the evolved similar traits, resulting in Some predatory protists that hunt diversity of photosynthetic euglenids, examples of convergent evolution over and consume other eukaryotes can which acquired chloroplasts from vast phylogenetic distances, such be so fi nicky that they will eat only green algal prey cells, demonstrates as the nematocysts in polykrikoid a particular strain of a recognized a highly reduced, vestigial feeding dinofl agellates and jellyfi sh and the species, such as the euglenid Rapaza apparatus and a suite of other fl attened oval cell shapes in marine viridis that, so far as we know, eats subcellular modifi cations that refl ect sandy habitats. Ultimately, continuous only a specifi c strain of the green alga the transformation from a predatory predator–prey interactions set up Tetraselmis sp. found in the same lifestyle to a photosynthetic lifestyle. conditions for multiple endosymbiotic tide pool (Figure 2F,G). Predatory For instance, the proteinacious cell events involving photosynthetic prey prey interactions like these have surface, or pellicle, in photosynthetic cells, which transformed predators gone on continuously for thousands euglenids tends to be protectively into photosynthesizers and created if not millions of years. As such, the thick and rigid when compared to completely new ecosystems across population of predatory protists is the thin and pliable cell surface of the planet. Because the vast majority constantly exposed to the properties, predatory euglenids, which must be of predatory protists are unknown including the genetic material, of the able to accommodate the ingestion and have only been minimally population of prey cells they prefer of large prey cells. This is only one described with line drawings using to consume. Over long periods of of many examples of how the cell light microscopy, there is enormous time, components of the digested structure of photosynthetic lineages potential for discoveries using single- prey cells, such as fragments of DNA differs signifi cantly from their predatory cell high-resolution microscopy containing complete genes, can ancestors. Although counterintuitive, all and comparative genomics/ become permanently incorporated photosynthetic eukaryotes, including transcriptomics that have implications into the predatory cells through land plants, green algae, red algae and for our understanding of cell biology, a process called horizontal gene giant kelps, are, from an evolutionary developmental biology, paleontology transfer. This process can ultimately perspective, highly modifi ed predatory and evolutionary history. set up the conditions for acquiring protists that were transformed organelles, such as chloroplasts, from following an endosymbiotic event FURTHER READING photosynthetic prey cells leading to a involving photosynthetic prey cells. merger of distantly related lineages, Archibald, J.M. (2015). Endosymbiosis and eukaryotic with one living inside the other, called Concluding remarks cell evolution. Curr. Biol. 25, R911–R921. Breglia, S.A., Yubuki, N., Hoppenrath, M., and ‘endosymbiosis’. Endosymbiosis has The most recent common ancestor Leander, B.S. (2010). Ultrastructure and occurred several times independently of all eukaryotes was almost certainly molecular phylogenetic position of a novel euglenozoan with extrusive episymbiotic across the tree of eukaryotes and a tiny predator that used whole-cell bacteria: Bihospites bacati n. gen. et sp. has involved incorporated prey phagocytosis to consume bacteria (Symbiontida). BMC Microbiol. 10, 145

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Gavelis, G.S., Hayakawa, S., White III, R.A., Fungi are closely related to animals Gojobori, T., Suttle, C.A., Keeling, P.J., and Primer Leander, B.S. (2015). Eye-like ocelloids are through a common opisthokont built from different endosymbiotically acquired Chytrid fungi ancestor that lived in an aquatic components. Nature 523, 204–207. environment over one billion years ago Gavelis, G., Wakeman, K., Tillman, U., Ripken, C., Mitarai, S., Herranz, M., Ozebek, S., Holstein, T., (Figure 1). Chytrids and other early- Keeling P., and Leander, B.S. (2017). Edgar M. Medina1 diverging fungi have persisted in this Microbial arms race: ballistic “nematocysts” and Nicolas E. Buchler2,* in dinofl agellates represent a new extreme in ancestral habitat and have retained organelle complexity. Sci. Adv. 3, e1602552. traits that make them well adapted to Hess, S. (2017). Hunting for agile prey: trophic Fungi have distinguishing traits, foraging for resources in water. For specialisation in leptophryid amoebae (Vampyrellida, Rhizaria) revealed by two novel such as hyphae and cell walls, that example, chytrids produce spores predators of planktonic algae. FEMS Microbiol. evolved in a fungal ancestor over one (known as zoospores) that lack a cell Ecol. 93, fi x104. Leander, B.S. (2004). Did trypanosomatid parasites billion years ago. Chytrid fungi are wall and swim via a motile cilium and/ have photosynthetic ancestors? Trends some of the earliest diverging fungal or crawl on surfaces via amoeboid Microbiol. 12, 251–258. lineages that retained features of motion (Figure 1). The presence of a Leander, B.S. (2008). A hierarchical view of convergent evolution in microbial eukaryotes. J. the opisthokont ancestor of animals centriole and a motile cilium is unique Eukaryot. Microbiol. 55, 59–68. and fungi (Figure 1). For example, to chytrids and other zoosporic fungi Leander, B.S. (2008). Marine gregarines - evolutionary prelude to the apicomplexan radiation? Trends chytrids make reproductive cells within the fungal kingdom. The cilium Parasitol. 24, 60–67. known as zoospores that swim is attached to a basal body that Leander, B.S., Esson, H.J. and Breglia, S.A. (2007). with a motile cilium or crawl like an contains a classic centriole with nine Macroevolution of complex cytoskeletal systems in euglenids. BioEssays 29, 987–1000. amoeba. The aim of this primer is to circularly arranged triplet microtubules Leander, B.S. and Hoppenrath, M. (2008). introduce the reader to the life cycle, that nucleate the axoneme. Similar to Ultrastructure of a novel tube-forming, intracellular parasite of dinofl agellates: biology, and ecology of chytrids and many animal cells, chytrids resorb the Parvilucifera prorocentri sp. nov. (Alveolata, other zoosporic fungi. We highlight cilium and the centriole is repurposed Myzozoa). Europ. J. Protistol. 44, 55–70. how chytrids are well positioned to as a centrosome to organize the Lukes, J., Leander, B.S. and Keeling, P.J. (2009). Cascades of convergent evolution: elucidate both the cell biology of mitotic spindle for nuclear division the corresponding evolutionary histories of the animal–fungal ancestor and the cycles. euglenozoans and dinofl agellates. Proc. Natl. Acad. Sci. USA. 106, 9963–9970. evolution of derived fungal features. The fungal ancestor evolved new Porter, S.M. (2016). Tiny vampires in ancient seas: traits (‘derived traits’) that are shared evidence for predation via perforation in fossils Chytrids exhibit fungal and by all fungi including chytrids. For from the 780–740 Ma Chuar Group, Grand Canyon, USA. Proc. R. Soc. B 283, 20160221. ancestral features example, the chytrid life cycle includes Rosati, G., Petroni, G., Quochi, S., Modeo, L., Life fi rst evolved in the ocean and a vegetative body (‘thallus’) with a cell and Verni, F. (1999). Epixenosomes: peculiar epibionts of the hypotrich ciliate Euplotidium itoi the last eukaryotic common ancestor wall and hyphal-like feeding structure defend their host against predators. J. Eukaryot. (LECA) likely swam and engulfed known as a rhizoid (Figure 1). Fungal Microbiol. 46, 278–282. organic matter via phagocytosis. Based hyphae are branching, fi lamentous Rundell, R.J., and Leander, B.S. (2010). Masters of miniaturization: convergent evolution among on the shared features found across tubes that penetrate organic matter interstitial eukaryotes. BioEssays 32, 430–437. eukaryotes, LECA had a nucleus, and secrete digestive enzymes to Simpson, A.G.B., Inagaki, Y., and Roger, A.J. (2006). Comprehensive multi-gene phylogenies mitochondria, an endo-membrane extract nutrients for cell growth. of excavate protists reveal the evolutionary system, actin and tubulin cytoskeleton, Hyphae grow into substrates by positions of ‘primitive’ eukaryotes. Mol. Biol. and a centriole for building a mitotic depositing cell wall materials and Evol. 23, 615–625. Tikhonenkov, D.V., Strasser, J.F.H., Janouskovec, J., spindle and cilium. LECA gave rise to remodeling enzymes at the hyphal Mylnikov, P., Aleoshin, V.V., Burki, F., and diverse eukaryotes, some of which tip via directed vesicle traffi cking Keeling, P.J. (2020). Predatory colponemids are the sister group to all other alveolates. BioRxiv. remained in aquatic environments on a cytoskeletal network. The cell https://doi.org/10.1101/2020.02.06.936658 and others which colonized land over wall is critical because it holds large, Yamaguchi, A., Yubuki, N., and Leander, B.S. (2012). 500 million years ago. Fungi (e.g. hydrostatic pressures caused by Morphostasis in a novel eukaryote illuminates the evolutionary transition from phagotrophy chytrids, rusts, molds, mushrooms, and internal osmolytes, which generate the to phototrophy: Description of Rapaza viridis n. yeast) are a large eukaryotic kingdom biomechanical forces that drive cell gen. et sp. (Euglenozoa, Euglenida). BMC Evol. Biol. 12, 29. Yubuki, N., Huang, S.Z.S., and found in many environments and wall expansion at the hyphal tip. As Leander, B.S. (2016). Comparative ultrastructure ecological niches. These eukaryotes are in other fungi, the hyphal-like rhizoid of fornicate excavates, including a novel free- decomposers that live on organic matter is important for colonizing substrates living relative of diplomonads: Aduncisulcus palustris gen. et sp. nov. Protist 167, 584–596. or as parasites of plants and animals. and extracting nutrients to fuel chytrid Yubuki, N., and Leander, B.S. (2013). Evolution of Fungi are also important symbionts: cell growth. microtubule organizing centers across the tree of eukaryotes. Plant J. 75, 230–244. they are partners of algae and Yubuki, N., Simpson, A.G.B., and Leander, B.S. cyanobacteria in lichens or they form Chytrid ecology and the evolution of (2013). Comprehensive ultrastructure of Kipferlia mycorrhizae that colonize plant roots zoosporic fungi bialata provides evidence for character evolution within the Fornicata (Excavata). Protist 164, and extract water and nutrients from soil We use the term zoosporic fungi 423–439. in exchange for sugars. The successful to describe chytrids and other expansion and colonization of terrestrial early diverging fungi that have a Departments of Botany and Zoology, environments by the plant and fungal zoospore stage during their life Biodiversity Research Centre, 6270 University Boulevard, University of British Columbia, kingdoms is likely the consequence of cycle (Figure 2A). Meta-genomic Vancouver, BC, V6T 1Z4, Canada. a symbiotic relationship between early sequencing has shown that E-mail: [email protected] fungi and photosynthetic algae. zoosporic fungi comprise much