The Protists
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CHAPTER 35 Protists are a diverse group of eukaryotes 000 Where did eukaryotic cells come from? 000 The protists Origin of the nucleus 000 The endomembrane system: extension of the nuclear envelope 000 Mitochondria and plastids arose by endosymbiosis 000 he protists include a weird Cilia and flagella: extensions of the cytoskeleton 000 Are simple protists ancient eukaryotes? 000 Tand wonderful potpourri of Sponge-like protists 000 ‘Collar’ flagellates: choanoflagellates 000 eukaryotic organisms that few people Slime moulds 000 Cellular slime moulds 000 ever see. Most protists are single-celled organisms (unicellular) and Acellular slime moulds: myxomycetes 000 live in aquatic habitats. There are at least 100 000 species and new Parasitic flagellates that contaminate water supplies: diplomonads 000 ones are being discovered continually. Photosynthetic protists are Symbionts and parasites: parabasalids 000 Amoebae 000 major primary producers in lakes, rivers and oceans, and during Rhizopods are amoebae that can alter their shape 000 photosynthesis they release into the atmosphere at least 30% of the Actinopods are radially symmetrical unicells 000 Protists with plastids 000 planet’s oxygen. Herbivorous protists are the link in food chains Protists with primary plastids: the ‘green lineage’ 000 between algal primary producers and larger animal consumers, Missing links in endosymbiosis: glaucophytes 000 Red algae: rhodophytes 000 such as fishes and invertebrates. Parasitic protists are responsible for Green algae: chlorophytes 000 Applications Green algae and biotechnology 000 serious human diseases, such as malaria, sleeping sickness and certain Protistan pirates with second-hand plastids 000 types of dysentery. Protists also parasitise other animals and plants, Chromist protists: the ‘brown lineage’ 000 Flagellates with second-hand plastids: causing agricultural losses. cryptomonads 000 Golden flagellates: chrysophytes 000 The classification of protists is undergoing major changes as Chalk comes from dead algae: haptophytes 000 their relationships are still being discovered. Some groups that were Algae in glass houses: diatoms 000 Brown algae: phaeophytes 000 traditionally classified as ‘orders’ are now treated at a higher level—as Water moulds and downy mildews: oomycetes 000 Applications Dieback disease 000 new ‘phyla’. The protists are polyphyletic, including a number Alveolates: dinoflagellates, ciliates and parasites 000 of major lines of evolution; various types that were once classified Dinoflagellates: whirling algae 000 Small but deadly: apicocomplexans 000 together (such as the ‘algae’) are now known to be only distantly Research Malaria 000 Ciliates: eukaryotes with two different nuclei 000 related. Thus, in this chapter we will not use formal taxonomic Euglenoids and kinetoplasts 000 Euglenoid flagellates 000 names for the different groups until protistologists agree on a new Flagellate parasites: kinetoplasts 000 system of classification. Cercozoa and forams 000 Amoebae with second-hand chloroplasts: chlorarachniophytes 000 More chalky protists: forams 000 KKNOXNOX CCH35.inddH35.indd 883535 22/4/09/4/09 11:14:57:14:57 PPMM Protists are a diverse group of into kingdom Protista. However, it is patently obvious that there is no such kingdom, and many of its members are more eukaryotes closely related to other kingdoms than to each other. Green Protists are diverse. Comparing two protistan phyla is like algae, for example, are the closest relatives of land plants comparing elephants with mushrooms or eels with tomatoes. (Chapter 36), and choanoflagellates are an early offshoot on In the past, protists were grouped together based on their form the way to animals (Chapter 38). So why do we still put most of nutrition—whether they were autotrophic (able to pro- protists together in one chapter as though they were one evo- duce food by photosynthesis) or heterotrophic (consumers of lutionary lineage? The answer is partly historical and partly organic substances or other organisms). Photosynthetic protists practical. There are still groups of unicellular eukaryotes of were known as algae, protists that ate smaller organisms were unknown evolutionary relationships, some not even named. known as protozoa (simple animals), and some protists that For convenience, these organisms are temporarily grouped absorbed small food molecules from the environment were together under the banner of protists. The study of protists is considered to be fungi. at a very exciting stage; new insights are being made daily and It is now obvious that this system was far too simplistic. revolutionary changes are sweeping through the discipline of Numerous photosynthetic protists, for example, swim about protist research. like animals and even capture smaller cells and eat them. These organisms are both animal-like and plant-like and can- • Protists may be photosynthetic, parasitic, predatory or absorb not be classified on the basis of nutrition. A more natural clas- small food molecules from the environment. sification based on morphological, biochemical and molecular • Relationships among them are still unclear but they are a diverse features, particularly gene sequences, is now emerging. Most range of eukaryotic cell types, and the kingdom Protista is of the newly recognised natural groups include organisms with polyphyletic. various modes of nutrition. Alveolates (p. 000), for example, have photosynthetic, parasitic and predatory members, but all are close relatives based on comparison of the fine structural Where did eukaryotic cells come details of their cells and their DNA sequences. from? From the phylogenetic tree in Figure 35.1, you can see that protists are not a monophyletic group (Chapter 31). The oldest fossils of eukaryotic organisms do not appear until For a long time all protists have been collectively grouped about 1.4 billion years ago. Since fossils of prokaryotes are older The ‘primary lineage’ Fig. 35.1 Current view of the phylogeny of eukaryotes (super kingdom Eukarya). Everything other FUNGI red algae red amoebae ANIMALS than the three kingdoms fungi, parabasilids green algae green diplomonads glaucophytes animals and plants are protists. cryptomonads LAND PLANTS Several protist lineages are nearest choanoflagellates relatives to these more familiar actinopods, rhizopods fossils, eukaryotic kingdoms. Other protist 450 mya lineages form large groups, such as mycetozoa, slime moulds the chromists and the alveolates. fossils, 590 mya How these protist groups relate to fossils, one another is not yet clear so the 600 mya tree has a comb-like appearance to reflect this lack of understanding Precambrian fossils, Precambrian fossils, of the branching orders. Some 700 mya 900 mya groups are recorded in the fossil record and their ages are shown. The lineages that have plastids (e.g. photosynthetic chloroplasts or remnant non-photosynthetic organelles) are indicated on the Precambrian tree. Eukarya 1.5 billion years 836 Part 5 Evolution and biodiversity For further reading and revision KKNOXNOX CCH35.inddH35.indd 883636 22/4/09/4/09 11:14:57:14:57 PPMM (3.5 billion years ago), it is generally thought that eukaryotes The endomembrane system: extension of evolved from prokaryotic organisms. As we have seen in earlier chapters, prokaryotic and the nuclear envelope eukaryotic cells share many cellular processes but the internal The endomembrane system of eukaryotes forms a conduit layout of their cells is different. Prokaryotic cells are essentially from the nuclear envelope to various subcellular compartments one single compartment, whereas eukaryotic cells contain and also to the exterior of the cell via the plasma membrane. several membrane-bound subcompartments. How did these subcompartments originate? The answer turns out to be quite nucleoid (DNA) a surprise. Origin of the nucleus prokaryote The eukaryotic nucleus differs from the prokaryotic nucleoid in numerous respects. Two major distinctions are the nuclear envelope and the multiple linear chromosomes of eukaryotes (Chapter 8). Prokaryotes lack a nuclear envelope and usually have a single circular chromosome. Transformation from a circular chromosome to linear chromosomes might have arisen from a break in the circle and duplication of the linear chromo- endoplasmic reticulum some to give multiple copies. nuclear envelope The origin of the nuclear envelope can be explained by No extant the accumulation of vesicles resulting from the infolding descendants (invagination) of the cell membrane around the prokaryotic known nucleoid. If the vesicles flatten around the nucleoid, as shown in Figure 35.2, they form a rudimentary double envelope nucleated cell complete with gaps or nuclear pores. Such accumulations of membrane vesicles around the nucleoid are known to occur in certain cyanobacteria (Chapter 34). aerobic bacterial Chromists—the ‘brown lineage’ Alveolates endosymbiont mitochondrion Animals Fungi ciliates forams diatoms photosynthetic oomycetes euglenoids cercozoans bacterial kinetoplasts haptophytes chrysophytes phaeophytes endosymbiont cryptomonads dinoflagellates apicomplexans C H A P T E R 3 5 fossils, Cambrian 540 mya fossils, 540 mya chloroplast Precambrian fossils, Green algae 700 mya chromalveolates Plants Key primary plastid Fig. 35.2 The evolution of eukaryotic cells. The origin of secondary plastid the nucleus and endomembrane system. Mitochondria originated from an aerobic bacterial endosymbiont. Chloroplasts originated mya = million years ago from a photosynthetic