The Origin and Diversification of Eukaryotes 25 The Origin and Diversification of Eukaryotes

Overview: Shape Changers Protist is the informal name of the diverse group of mostly unicellular eukaryotes Some protists, like the ciliate Didinium, are able to perform dramatic shape changes due to the structural complexity of their cells © 2014 Pearson Education, Inc. 2

Figure 25.1 Figure 25.1 What enables the cell on the left to engulf its prey? 3

Concept 25. 1: Eukaryotes arose by endosymbiosis more than 1 Concept 25.1: Eukaryotes arose by endosymbiosis more than 1.8 billion years ago Early eukaryotes were unicellular Eukaryotic cells have organelles and are structurally more complex than prokaryotic cells A well-developed cytoskeleton enables eukaryotic cells to have asymmetrical forms and to change shape © 2014 Pearson Education, Inc. 4

The Fossil Record of Early Eukaryotes Chemical evidence for the presence of eukaryotes dates back to 2.7 billion years ago The earliest fossils of eukaryotic cells are 1.8 billion years old © 2014 Pearson Education, Inc. 5

Initial diversification of eukaryotes occurred 1. 8 to 1 Initial diversification of eukaryotes occurred 1.8 to 1.3 billion years ago Novel features of eukaryotes, including complex multicellularity, sexual life cycles, and photosynthesis, arose 1.3 billion to 635 million years ago The first large, multicellular eukaryotes represented by the Ediacaran biota evolved 635 to 535 million years ago © 2014 Pearson Education, Inc. 6

Figure 25.2 Exploring the early evolution of eukaryotes 4.5 bya 3.5 2.5 1.5 500 mya 550 mya 535 mya 1.8 bya 1.5 bya 1.3 bya 1.2 bya 750 mya 635 mya 600 mya 0.5 cm 1 cm 20 m 20 m 20 m (b) Tappania, a 1.5-billion-year-old fossil that may represent an early alga or fungus (a) A 1.8-billion- year-old fossil eukaryote (e) Bonneia, a vase-shaped eukaryote (f) An early member of the Ediacaran biota (g) Spriggina floundersi, an early animal with many body segments Figure 25.2 Exploring the early evolution of eukaryotes 25 m 25 m (c) Bangiomorpha, an ancient red alga (d) Proterocladus, classified as a green alga 7

Eukaryotic cells exhibited greater structural complexity than prokaryotic cells as early as 1.5 billion years ago The earliest multicellular eukaryotic fossils are of red algae, which date back to 1.2 billion years ago, but large, multicellular eukaryotes did not arise until 635 million years ago Severe ice ages from 750 to 580 million years ago may have hindered the rise of large eukaryotes © 2014 Pearson Education, Inc. 8

Endosymbiosis in Eukaryotic Evolution DNA sequence data indicate that eukaryotes are “combination” organisms Eukaryotic genes and characteristics show evidence of both archaeal and bacterial origins Evidence of mixed origins may be a consequence of endosymbiosis, a symbiotic relationship in which one organism lives inside the body or cell of another organism © 2014 Pearson Education, Inc. 9

Table 25.1 Table 25.1 Inferred origins of key eukaryotic features 10

Origin of Mitochondria and Plastids Endosymbiont theory proposes that mitochondria and plastids were formerly small prokaryotes that began living within larger cells An endosymbiont is a cell that lives within a host cell Prokaryote ancestors to mitochondria and plastids probably entered the host cell as undigested prey or internal parasites © 2014 Pearson Education, Inc. 11

The relationship between endosymbiont and host cells was mutually beneficial Anaerobic host cells benefited from endosymbionts’ ability to take advantage of an increasingly aerobic world Heterotrophic host cells benefited from the nutrients produced by photosynthetic endosymbionts In the process of becoming more interdependent, the host and endosymbionts would have become a single organism © 2014 Pearson Education, Inc. 12

Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events © 2014 Pearson Education, Inc. 13

Cytoplasm DNA Ancestral prokaryote Engulfing of aerobic bacterium Figure 25.3 Cytoplasm DNA Ancestral prokaryote Engulfing of aerobic bacterium Plasma membrane Nucleus Engulfing of photo- synthetic bacterium Endoplasmic reticulum Nuclear envelope Mitochondrion Mito- chondrion Figure 25.3 A hypothesis for the origin of eukaryotes through endosymbiosis Ancestral heterotrophic eukaryote Plastid Ancestral photosynthetic eukaryote 14

Key evidence supporting an endosymbiotic origin of mitochondria and plastids: Inner membranes are similar to plasma membranes of prokaryotes Division is similar in these organelles and some prokaryotes DNA structure is similar to that of prokaryotes These organelles transcribe and translate their own DNA Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes © 2014 Pearson Education, Inc. 15

Eukaryotic mitochondria descended from a single common ancestor DNA sequence analysis indicates that mitochondria arose from an alpha proteobacterium Eukaryotic mitochondria descended from a single common ancestor Plastids arose from an engulfed cyanobacterium Some photosynthetic protists may have been engulfed to become endosymbionts themselves © 2014 Pearson Education, Inc. 16

The ancestral host cell may have been an archaean or a “protoeukaryote,” from a lineage related to, but diverged from, archaeal ancestors © 2014 Pearson Education, Inc. 17

Plastid Evolution: A Closer Look There is now considerable evidence that much protist diversity has its origins in endosymbiosis Mitochondria arose first through descent from a bacterium that was engulfed by a cell from an archaeal lineage The plastid lineage evolved later from a photosynthetic cyanobacterium that was engulfed by a heterotrophic eukaryote © 2014 Pearson Education, Inc. 18

Secondary endo- Dinoflagellates symbiosis Membranes are represented Figure 25.4 Secondary endo- symbiosis Dinoflagellates Membranes are represented as dark lines in the cell. Cyano- bacterium Red alga Plastid 1 2 Primary endo- symbiosis 3 Stramenopiles Secondary endo- symbiosis Nucleus Plastid Heterotrophic eukaryote One of these membranes was lost in red and green algal descendants. Figure 25.4 Diversity of plastids produced by endosymbiosis Euglenids Secondary endo- symbiosis Green alga Chlorarachniophytes 19

The plastid-bearing lineage of protists evolved into red and green algae Like cyanobacteria, plastids of red algae and green algae have two membranes Transport proteins in the membranes of red and green algae are homologous to those found in cyanobacteria On several occasions during eukaryotic evolution, red and green algae underwent secondary endosymbiosis, in which they were ingested by a heterotrophic eukaryote © 2014 Pearson Education, Inc. 20

Concept 25.2: Multicellularity has originated several times in eukaryotes The evolution of eukaryotic cells allowed for a greater range of unicellular forms A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals © 2014 Pearson Education, Inc. 21

Multicellular Colonies The first multicellular forms were colonies, collections of cells that are connected to one another but show little or no cellular differentiation Multicellular colonies consist of simple filaments, balls, or cell sheets Colonial cells may be attached by shared cell walls or, in cells that lack rigid walls, held together by proteins that physically connect adjacent cells © 2014 Pearson Education, Inc. 22

Figure 25.5 Figure 25.5 Pediastrum Pediastrum 23

Independent Origins of Complex Multicellularity Multicellular organisms with differentiated cells originated multiple times over the course of eukaryotic evolution Genetic and morphological evidence indicates that lineages of red, green, and brown algae, plants, fungi, and animals arose independently from different single-celled ancestors © 2014 Pearson Education, Inc. 24

Volvox is a multicellular green algae that forms a monophyletic group with a single-celled alga (Chlamydomonas) and several colonial species Volvox and all colonial members of this group have proteins homologous to those found in the cell wall of Chlamydomonas Multicellularity in Volvox may have originated through evolution of increasingly complex colonial forms descended from a single-celled common ancestor © 2014 Pearson Education, Inc. 25

Extracellular matrix (ECM) Figure 25.6 Flagellum Cytoplasm Chlamydomonas Outer cell wall Inner cell wall Gonium Pandorina Figure 25.6 Morphological change in the Volvox lineage Outer cell wall Cytoplasm Volvox Extracellular matrix (ECM) 26

Only a few novel genes account for morphological differences between Volvox and Chlamydomonas The transition to multicellularity may result from changes in how existing genes are used rather than the origin of large numbers of novel genes © 2014 Pearson Education, Inc. 27

Steps in the Origin of Multicellular Animals Choanoflagellates are the closest living relatives of animals The common ancestor of choanoflagellates and living animals may have resembled present-day choanoflagellates © 2014 Pearson Education, Inc. 28

Individual choanoflagellate Choano- flagellates OTHER EUKARY- OTES Figure 25.7 Individual choanoflagellate Choano- flagellates OTHER EUKARY- OTES Sponges Figure 25.7 Three lines of evidence that choanoflagellates are closely related to animals Animals Collar cell (choanocyte) Other animals 29

The origin of multicellularity in animals required mechanisms for cells to adhere to each other and to communicate with each other (cell signaling) Molecular similarities in domains of proteins functioning in cell adherence (cadherins) and cell signaling have been found between modern choanoflagellates and representative animals © 2014 Pearson Education, Inc. 30

Choano- flagellate Hydra Fruit fly “CCD” domain Mouse Figure 25.8 Figure 25.8 Cadherin proteins in choanoflagellates and animals Mouse 31

The transition to multicellularity in animals involved new ways of using proteins encoded by genes found in choanoflagellates rather than the evolution of many novel genes © 2014 Pearson Education, Inc. 32

Concept 25.3: Four “supergroups” of eukaryotes have been proposed based on morphological and molecular data Eukaryote diversity is influenced by their hybrid origins and the independent origins of complex multicellularity One hypothesis divides all eukaryotes into four supergroups © 2014 Pearson Education, Inc. 33

Four Supergroups of Eukaryotes It is no longer thought that amitochondriates (lacking mitochondria) are the oldest lineage of eukaryotes Many have been shown to have mitochondria and have been reclassified Our understanding of the relationships among protist groups continues to change rapidly One hypothesis divides all eukaryotes (including protists) into four supergroups © 2014 Pearson Education, Inc. 34

Figure 25.9 Exploring eukaryotic diversity Excavata 5 m Archaeplastida 20 m 50 m Diplomonads Parabasalids Excavata Euglenozoans Stramen- opiles Diatoms Brown algae Dinoflagellates “SAR” Clade 50 m Unikonta “SAR” clade lates Alveo- Apicomplexans Ciliates rians Rhiza- Forams Cercozoans Red algae algae Green Chlorophytes Charophytes Archaeplastida 100 m Land plants Figure 25.9 Exploring eukaryotic diversity Amoebo- zoans Gymnamoebas Slime molds 100 m Nucleariids Unikonta Fungi Opisthokonts Choanoflagellates Animals 35

Excavates The supergroup Excavata is characterized by its cytoskeleton Some members have a feeding groove This monophyletic group includes the diplomonads, parabasalids, and euglenozoans © 2014 Pearson Education, Inc. 36

Diplomonads Excavata Parabasalids Euglenozoans SAR clade Figure 25.UN02 Diplomonads Parabasalids Excavata Euglenozoans SAR clade Archaeplastida Figure 25.UN02 In-text figure, Excavata, p. 492 Unikonta 37

Diplomonads and Parabasalids These two groups lack plastids and have modified mitochondria, and most live in anaerobic environments Diplomonads Have reduced mitochondria called mitosomes Derive energy from anaerobic biochemical pathways Are often parasites, for example, Giardia intestinalis Move using multiple flagella © 2014 Pearson Education, Inc. 38

Parabasalids Have reduced mitochondria called hydrogenosomes that generate some energy anaerobically Include Trichomonas vaginalis, a sexually transmitted parasite © 2014 Pearson Education, Inc. 39

The parabasalid parasite, Trichomonas vaginalis (colorized SEM) Figure 25.10 Flagella 5 m Figure 25.10 The parabasalid parasite, Trichomonas vaginalis (colorized SEM) Undulating membrane The parabasalid parasite, Trichomonas vaginalis (colorized SEM) 40

Euglenozoans Euglenozoa is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, and parasites The main feature distinguishing them as a clade is a spiral or crystalline rod inside their flagella This clade includes the euglenids and kinetoplastids © 2014 Pearson Education, Inc. 41

Euglenozoan flagellum Figure 25.11 Flagella 0.2 m 8 m Crystalline rod (cross section) Figure 25.11 Euglenozoan flagellum Ring of microtubules (cross section) Euglenozoan flagellum 42

Some species can be both autotrophic and heterotrophic Euglenids have one or two flagella that emerge from a pocket at one end of the cell Some species can be both autotrophic and heterotrophic © 2014 Pearson Education, Inc. 43

Kinetoplastids have a single mitochondrion with an organized mass of DNA called a kinetoplast They include free-living consumers of prokaryotes in aquatic ecosystems and parasites of animals, plants, and other protists This group includes Trypanosoma, which causes sleeping sickness in humans © 2014 Pearson Education, Inc. 44

Trypanosoma, the kinetoplastid that causes sleeping sickness Figure 25.12 Figure 25.12 Trypanosoma, the kinetoplastid that causes sleeping sickness 9 m Trypanosoma, the kinetoplastid that causes sleeping sickness 45

The “SAR” Clade The “SAR” clade is a diverse monophyletic supergroup named for the first letters of its three major clades, stramenopiles, alveolates, and rhizarians © 2014 Pearson Education, Inc. 46

Excavata Diatoms Stramenopiles Brown algae SAR clade Dinoflagellates Figure 25.UN03 Excavata Diatoms Stramenopiles Brown algae Dinoflagellates SAR clade Apicomplexans Alveolates Ciliates Forams Rhizarians Figure 25.UN03 In-text figure, the SAR clade, p. 493 Cercozoans Archaeplastida Unikonta 47

Stramenopiles The subgroup stramenopiles includes the most important photosynthetic organisms on Earth Diatoms and brown algae are members of the stramenopiles clade Diatoms are highly diverse, unicellular algae with a unique two-part, glass-like wall of silicon dioxide Video: Diatoms Moving Video: Various Diatoms © 2014 Pearson Education, Inc. 48

The diatom Triceratium morlandii (colorized SEM) Figure 25.13 Figure 25.13 The diatom Triceratium morlandii (colorized SEM) 40 m The diatom Triceratium morlandii (colorized SEM) 49

Brown algae are the largest and most complex algae All are multicellular, and most are marine Brown algae include many species commonly called “seaweeds” © 2014 Pearson Education, Inc. 50

Brown algal seaweeds have plantlike structures: the rootlike holdfast, which anchors the alga, and a stemlike stipe, which supports the leaflike blades Similarities between algae and plants are examples of analogous structures © 2014 Pearson Education, Inc. 51

Blade Stipe Holdfast Figure 25.14 Figure 25.14 Seaweeds: adapted to life at the ocean’s margins Holdfast 52

Alveolates The alveolates are a subgroup of the SAR clade that have membrane-bounded sacs (alveoli) just under the plasma membrane Dinoflagellates and ciliates are members of the alveolata clade © 2014 Pearson Education, Inc. 53

Flagellum Alveoli Alveolate 0.2 m Figure 25.15 Figure 25.15 Alveoli 54

Toxic “red tides” are caused by dinoflagellate blooms Dinoflagellates have two flagella, and each cell is reinforced by cellulose plates They are abundant components of both marine and freshwater phytoplankton They are a diverse group of aquatic phototrophs, mixotrophs, and heterotrophs Toxic “red tides” are caused by dinoflagellate blooms © 2014 Pearson Education, Inc. 55

Most ciliates are predators of bacteria or small protists Ciliates, a large varied group of protists, are named for their use of cilia to move and feed Most ciliates are predators of bacteria or small protists © 2014 Pearson Education, Inc. 56

Contractile vacuole Oral groove Cell mouth Cilia Micronucleus Food Figure 25.17 Contractile vacuole Oral groove Cell mouth Cilia Micronucleus Food vacuoles Macronucleus 50 m Figure 25.17 Structure and function in the ciliate Paramecium caudatum 57

Rhizarians Many species of rhizarians are amoebas Amoebas move and feed by pseudopodia; some but not all belong to the clade Rhizaria Forams and cercozoans are members of the rhizarian clade © 2014 Pearson Education, Inc. 58

Foraminiferans, or forams, are named for porous shells, called tests Pseudopodia extend through the pores in the test Many forams have endosymbiotic algae Forams include both marine and freshwater species © 2014 Pearson Education, Inc. 59

They are common in marine, freshwater, and soil ecosystems Cercozoans include amoeboid and flagellated protists with threadlike pseudopodia They are common in marine, freshwater, and soil ecosystems Most are heterotrophs, including parasites and predators © 2014 Pearson Education, Inc. 60

Paulinella chromatophora is an autotroph with a unique photosynthetic structure called a chromatophore This structure evolved from a different cyanobacterium than the plastids of other photosynthetic eukaryotes © 2014 Pearson Education, Inc. 61

Chromatophore 5 m Figure 25.18 Figure 25.18 A second case of primary endosymbiosis? 5 m 62

Archaeplastids Plastids arose when a heterotrophic protist acquired a cyanobacterial endosymbiont The photosynthetic descendants of this ancient protist evolved into red algae and green algae Land plants are descended from the green algae Archaeplastida is the supergroup that includes red algae, green algae, and land plants © 2014 Pearson Education, Inc. 63

Excavata SAR clade Red algae Archaeplastida Chlorophytes Green algae Figure 25.UN04 Excavata SAR clade Red algae Chlorophytes Green algae Archaeplastida Charophytes Land plants Figure 25.UN04 In-text figure, Archaeplastida, p. 495 Unikonta 64

Red Algae Red algae are reddish in color due to an accessory pigment called phycoerythrin, which masks the green of chlorophyll The color varies from greenish-red in shallow water to dark red or almost black in deep water Red algae are usually multicellular; the largest are seaweeds Red algae are the most abundant large algae in coastal waters of the tropics Red algae reproduce sexually © 2014 Pearson Education, Inc. 65

Bonnemaisonia hamifera Nori 8 mm Red algae Figure 25.19 Figure 25.19 Red algae 8 mm Red algae 66

Green Algae Green algae are named for their grass-green chloroplasts Plants are descended from the green algae Green algae are a paraphyletic group The two main groups are charophytes and chlorophytes Charophytes are most closely related to land plants © 2014 Pearson Education, Inc. 67

Nearly all species of chlorophytes reproduce sexually Most chlorophytes live in fresh water, although many are marine and some are terrestrial Nearly all species of chlorophytes reproduce sexually Some unicellular chlorophytes are free-living while others live symbiotically within other eukaryotes Larger size and greater complexity are found in multicellular species including Volvox and Ulva © 2014 Pearson Education, Inc. 68

Multicellular chlorophytes Figure 25.20 (a) Ulva, or sea lettuce 2 cm Figure 25.20 Multicellular chlorophytes (b) Caulerpa, an intertidal chlorophyte Multicellular chlorophytes 69

Unikonts The supergroup Unikonta includes animals, fungi, and some protists This group includes two clades: the amoebozoans and the opisthokonts (animals, fungi, and related protists) The root of the eukaryotic tree remains controversial It is unclear whether unikonts separated from other eukaryotes relatively early or late © 2014 Pearson Education, Inc. 70

Excavata SAR clade Archaeplastida Gymnamoebas Slime molds Nucleariids Figure 25.UN05 Excavata SAR clade Archaeplastida Gymnamoebas Slime molds Nucleariids Unikonta Fungi Figure 25.UN05 In-text figure, Unikonta, p. 496 Choanoflagellates Animals 71

Results Choanoflagellates Animals Unikonta Fungi Common ancestor Figure 25.21 Results Choanoflagellates Animals Unikonta Fungi Common ancestor of all eukaryotes Amoebozoans Diplomonads Excavata Euglenozoans Stramenopiles Alveolates SAR clade Figure 25.21 Inquiry: Where is the root of the eukaryotic tree? DHFR-TS gene fusion Rhizarians Red algae Green algae Archaeplastida Plants 72

Amoebozoans Amoebozoans are amoebas that have lobe- or tube-shaped, rather than threadlike, pseudopodia Most amoebozoans are free-living, but those that belong to the genus Entamoeba are parasites © 2014 Pearson Education, Inc. 73

Slime molds are free-living amoebozoans that were once thought to be fungi DNA sequence analyses indicate that slime molds belong in the clade Amoebozoa © 2014 Pearson Education, Inc. 74

Dictyostelium is an example of a cellular slime mold Cellular slime molds consist of solitary cells that feed individually but can aggregate to form a fruiting body © 2014 Pearson Education, Inc. 75

Spores (n) FERTILIZATION Emerging amoeba (n) Zygote (2n) Fruiting Figure 25.22 Spores (n) FERTILIZATION Emerging amoeba (n) Zygote (2n) Fruiting bodies (n) SEXUAL REPRODUCTION Solitary amoebas (feeding stage) (n) MEIOSIS ASEXUAL REPRODUCTION Amoebas (n) Aggregated amoebas 600 m Migrating aggregate Figure 25.22 The life cycle of Dictyostelium, a cellular slime mold 200 m Key Haploid (n) Diploid (2n) 76

Opisthokonts Opisthokonts include animals, fungi, and several groups of protists © 2014 Pearson Education, Inc. 77

Concept 25.4: Single-celled eukaryotes play key roles in ecological communities and affect human health The majority of the eukaryotic lineages are composed of protists Protists exhibit a wide range of structural and functional diversity © 2014 Pearson Education, Inc. 78

Structural and Functional Diversity in Protists Single-celled protists can be very complex, as all biological functions are carried out by organelles in each individual cell © 2014 Pearson Education, Inc. 79

Protists are found in diverse aquatic environments Some protists reproduce asexually, while others reproduce sexually, or by the sexual processes of meiosis and fertilization © 2014 Pearson Education, Inc. 80

Protists show a wide range of nutritional diversity, including Photoautotrophs, which contain chloroplasts Heterotrophs, which absorb organic molecules or ingest larger food particles Mixotrophs, which combine photosynthesis and heterotrophic nutrition © 2014 Pearson Education, Inc. 81

Photosynthetic Protists Many protists are important producers that obtain energy from the sun In aquatic environments, photosynthetic protists and prokaryotes are the main producers In aquatic environments, photosynthetic protists are limited by nutrients These populations can explode when limiting nutrients are added © 2014 Pearson Education, Inc. 82

Other consumers Herbivorous plankton Carnivorous plankton Prokaryotic Figure 25.23 Other consumers Herbivorous plankton Carnivorous plankton Figure 25.23 Protists: key producers in aquatic communities Prokaryotic producers Protistan producers 83

Diatoms are a major component of the phytoplankton After a diatom population has bloomed, many dead individuals fall to the ocean floor undecomposed This removes carbon dioxide from the atmosphere and “pumps” it to the ocean floor © 2014 Pearson Education, Inc. 84

The biomass and growth of photosynthetic protists and prokaryotes have declined as sea surface temperature has increased Warmer surface temperatures may prevent nutrient upwelling, a process critical to the growth of marine producers © 2014 Pearson Education, Inc. 85

If sea surface temperature continues to warm due to global warming, this could have large effects on Marine ecosystems Fishery yields The global carbon cycle © 2014 Pearson Education, Inc. 86

Figure 25.24 NP EP A Arctic (A) NA North Atlantic (NA) NI EA Equatorial Atlantic (EA) South Atlantic (SA) North Indian (NI) South Indian (SI) North Pacific (NP) Equatorial Pacific (EP) South Pacific (SP) SI SA Southern (S) S SP −0.02 −0.01 0.00 0.01 0.02 Figure 25.24 Effects of climate change on marine producers −2.50 −1.25 0.00 1.25 2.50 Chlorophyll change (mg/m2 • yr) Sea-surface temperature (SST) change (C) (a) (b) 87

Symbiotic Protists Some protist symbionts benefit their hosts Dinoflagellates nourish coral polyps that build reefs Wood-digesting protists digest cellulose in the gut of termites © 2014 Pearson Education, Inc. 88

Figure 25.25 Figure 25.25 A symbiotic protist 10 m 89

Some protists are parasitic Plasmodium causes malaria Pfiesteria shumwayae is a dinoflagellate that causes fish kills Phytophthora ramorum causes sudden oak death Phytophthora infestans causes potato late blight © 2014 Pearson Education, Inc. 90

Effects on Human Health Protists that cause infectious disease can pose major challenges to human immune systems and public health Trypanosoma is an excavate that causes sleeping sickness in humans © 2014 Pearson Education, Inc. 91

Trypanosomes evade immune responses by switching surface proteins A cell produces millions of copies of a single protein The new generation produces millions of copies of a different protein These frequent changes prevent the host from developing immunity © 2014 Pearson Education, Inc. 92

Entamoebas are amoebozoan parasites of vertebrates and some invertebrates Entamoeba histolytica causes amebic dysentery, the third-leading cause of human death due to eukaryotic parasites © 2014 Pearson Education, Inc. 93

Apicomplexans are alveolate parasites of animals, and some cause serious human diseases Most have sexual and asexual stages that require two or more different host species for completion © 2014 Pearson Education, Inc. 94

The apicomplexan Plasmodium is the parasite that causes malaria Plasmodium requires both mosquitoes and humans to complete its life cycle Approximately 900,000 people die each year from malaria Efforts are ongoing to develop vaccines that target this pathogen © 2014 Pearson Education, Inc. 95

Inside mosquito Inside human Merozoite Sporozoites (n) Liver Liver Figure 25.26 Inside mosquito Inside human Merozoite Sporozoites (n) Liver Liver cell Oocyst Apex MEIOSIS Merozoite (n) Red blood cell 0.5 m Red blood cells Zygote (2n) Figure 25.26 The two-host life cycle of Plasmodium, the apicomplexan that causes malaria FERTILIZATION Gametes Gametocytes (n) Key Haploid (n) Diploid (2n) 96

Figure 25.UN01 Figure 25.UN01 Skills exercise: interpreting comparisons of genetic sequences 97

Figure 25.UN06 Figure 25.UN06 Summary of key concepts: supergroups 98