1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 28 Protists

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: A World in a Drop of Water Even a low-power microscope – Can reveal an astonishing menagerie of organisms in a drop of pond water Figure 28.1 50  m

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings These amazing organisms – Belong to the diverse kingdoms of mostly single-celled eukaryotes informally known as protists Advances in eukaryotic systematics – Have caused the classification of protists to change significantly

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 28.1: Protists are an extremely diverse assortment of eukaryotes Protists are more diverse than all other eukaryotes – And are no longer classified in a single kingdom Most protists are unicellular – And some are colonial or multicellular

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Protists, the most nutritionally diverse of all eukaryotes, include – Photoautotrophs, which contain chloroplasts – Heterotrophs, which absorb organic molecules or ingest larger food particles – Mixotrophs, which combine photosynthesis and heterotrophic nutrition

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Protist habitats are also diverse in habitat And including freshwater and marine species Figure 28.2a–d 100  m 4 cm 500  m The freshwater ciliate Stentor, a unicellular protozoan (LM) Ceratium tripos, a unicellular marine dinoflagellate (LM) Delesseria sanguinea, a multicellular marine red alga Spirogyra, a filamentous freshwater green alga (inset LM) (a) (b) (c) (d)

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproduction and life cycles – Are also highly varied among protists, with both sexual and asexual species

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A sample of protist diversity Table 28.1

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endosymbiosis in Eukaryotic Evolution There is now considerable evidence – That much of protist diversity has its origins in endosymbiosis

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The plastid-bearing lineage of protists – Evolved into red algae and green algae On several occasions during eukaryotic evolution – Red algae and green algae underwent secondary endosymbiosis, in which they themselves were ingested

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 28.2: Diplomonads and parabasalids have modified mitochondria A tentative phylogeny of eukaryotes – Divides eukaryotes into many clades Figure 28.4 Diplomonads Parabasalids Kinetoplastids Euglenids Dinoflagellates Apicomplexans Ciliates Oomycetes Diatoms Golden algae Brown algae Chlorarachniophytes Foraminiferans Radiolarians Gymnamoebas Entamoebas Plasmodial slime molds Cellular slime molds Fungi Choanoflagellates Metazoans Red algae Chlorophytes Charophyceans Plants Ancestral eukaryote Chlorophyta Plantae Rhodophyta Animalia Fungi (Opisthokonta) (Viridiplantae) Diplomonadida Parabasala Euglenozoa AlveolataStramenopila Cercozoa Radiolaria Amoebozoa

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diplomonads and parabasalids – Are adapted to anaerobic environments – Lack plastids – Have mitochondria that lack DNA, an electron transport chain, or citric-acid cycle enzymes

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diplomonads – Have two nuclei and multiple flagella Figure 28.5a 5 µm (a) Giardia intestinalis, a diplomonad (colorized SEM)

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Parabasalids Parabasalids include trichomonads – Which move by means of flagella and an undulating part of the plasma membrane Figure 28.5b (b) Trichomonas vaginalis, a parabasalid (colorized SEM) Flagella Undulating membrane 5 µm

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 28.3: Euglenozoans have flagella with a unique internal structure Euglenozoa is a diverse clade that includes – Predatory heterotrophs, photosynthetic autotrophs, and pathogenic parasites

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The main feature that distinguishes protists in this clade – Is the presence of a spiral or crystalline rod of unknown function inside their flagella Flagella 0.2 µm Crystalline rod Ring of microtubules Figure 28.6

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Kinetoplastids – Have a single, large mitochondrion that contains an organized mass of DNA called a kinetoplast – Include free-living consumers of bacteria in freshwater, marine, and moist terrestrial ecosystems

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The parasitic kinetoplastid Trypanosoma – Causes sleeping sickness in humans Figure 28.7 9  m

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Euglenids – Have one or two flagella that emerge from a pocket at one end of the cell – Store the glucose polymer paramylon Figure 28.8 Long flagellum Short flagellum Nucleus Plasma membrane Paramylon granule Chloroplast Contractile vacuole Light detector: swelling near the base of the long flagellum; detects light that is not blocked by the eyespot; as a result, Euglena moves toward light of appropriate intensity, an important adaptation that enhances photosynthesis Eyespot: pigmented organelle that functions as a light shield, allowing light from only a certain direction to strike the light detector Pellicle: protein bands beneath the plasma membrane that provide strength and flexibility (Euglena lacks a cell wall) Euglena (LM) 5 µm

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 28.4: Alveolates have sacs beneath the plasma membrane Members of the clade Alveolata – Have membrane-bounded sacs (alveoli) just under the plasma membrane Figure 28.9 Flagellum Alveoli 0.2 µm

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dinoflagellates – Are a diverse group of aquatic photoautotrophs and heterotrophs – Are abundant components of both marine and freshwater phytoplankton

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Each has a characteristic shape – That in many species is reinforced by internal plates of cellulose Two flagella – Make them spin as they move through the water Figure 28.10 3 µm Flagella

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rapid growth of some dinoflagellates – Is responsible for causing “red tides,” which can be toxic to humans

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Apicomplexans – Are parasites of animals and some cause serious human diseases – Are so named because one end, the apex, contains a complex of organelles specialized for penetrating host cells and tissues – Have a nonphotosynthetic plastid, the apicoplast

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 28.11 Inside mosquitoInside human Sporozoites (n) Oocyst MEIOSIS Liver Liver cell Merozoite (n) Red blood cells Gametocytes (n) FERTILIZATION Gametes Zygote (2n) Key Haploid (n) Diploid (2n) Merozoite Red blood cell Apex 0.5 µm Most apicomplexans have intricate life cycles – With both sexual and asexual stages that often require two or more different host species for completion An infected Anopheles mosquito bites a person, injecting Plasmodium sporozoites in its saliva. 1 The sporozoites enter the person’s liver cells. After several days, the sporozoites undergo multiple divisions and become merozoites, which use their apical complex to penetrate red blood cells (see TEM below). 2 The merozoites divide asexually inside the red blood cells. At intervals of 48 or 72 hours (depending on the species), large numbers of merozoites break out of the blood cells, causing periodic chills and fever. Some of the merozoites infect new red blood cells. 3 Some merozoites form gametocytes. 4 Another Anopheles mosquito bites the infected person and picks up Plasmodium gametocytes along with blood. 5 Gametes form from gametocytes. Fertilization occurs in the mosquito’s digestive tract, and a zygote forms. The zygote is the only diploid stage in the life cycle. 6 An oocyst develops from the zygote in the wall of the mosquito’s gut. The oocyst releases thousands of sporozoites, which migrate to the mosquito’s salivary gland. 7

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ciliates Ciliates, a large varied group of protists – Are named for their use of cilia to move and feed – Have large macronuclei and small micronuclei

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The micronuclei – Function during conjugation, a sexual process that produces genetic variation Conjugation is separate from reproduction – Which generally occurs by binary fission

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 28.12 50 µm Thousands of cilia cover the surface of Paramecium. The undigested contents of food vacuoles are released when the vacuoles fuse with a specialized region of the plasma membrane that functions as an anal pore. Paramecium, like other freshwater protists, constantly takes in water by osmosis from the hypotonic environment. Bladderlike contractile vacuoles accumulate excess water from radial canals and periodically expel it through the plasma membrane. Food vacuoles combine with lysosomes. As the food is digested, the vacuoles follow a looping path through the cell. Paramecium feeds mainly on bacteria. Rows of cilia along a funnel-shaped oral groove move food into the cell mouth, where the food is engulfed into food vacuoles by phagocytosis. Oral groove Cell mouth MicronucleusMacronucleus FEEDING, WASTE REMOVAL, AND WATER BALANCE Exploring structure and function in a ciliate Contractile Vacuole

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONJUGATION AND REPRODUCTION 8 7 2 MICRONUCLEAR FUSION Diploid micronucleus Haploid micronucleus MEIOSIS Compatible mates Key Conjugation Reproduction Macronucleus Two cells of compatible mating strains align side by side and partially fuse. 1 Meiosis of micronuclei produces four haploid micronuclei in each cell. 2 3 Three micronuclei in each cell disintegrate. The remaining micro- nucleus in each cell divides by mitosis. The cells swap one micronucleus. 4 The cells separate. 5 Micronuclei fuse, forming a diploid micronucleus. 6 Three rounds of mitosis without cytokinesis produce eight micronuclei. 7 The original macro- nucleus disintegrates. Four micronuclei become macronuclei, while the other four remain micronuclei. 8 Two rounds of cytokinesis partition one macronucleus and one micronucleus into each of four daughter cells. 9

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 28.5: Stramenopiles have “hairy” and smooth flagella The clade Stramenopila – Includes several groups of heterotrophs as well as certain groups of algae

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Most stramenopiles – Have a “hairy” flagellum paired with a “smooth” flagellum Smooth flagellum Hairy flagellum 5 µm Figure 28.13

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oomycetes (Water Molds and Their Relatives) Oomycetes – Include water molds, white rusts, and downy mildews – Were once considered fungi based on morphological studies

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Most oomycetes – Are decomposers or parasites – Have filaments (hyphae) that facilitate nutrient uptake

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The life cycle of a water mold Figure 28.14 Cyst Zoospore (2n) ASEXUAL REPRODUCTION Zoosporangium (2n) Germ tube Zygote germination FERTILIZATION SEXUAL REPRODUCTION Zygotes (oospores) (2n) MEIOSIS Oogonium Egg nucleus (n) Antheridial hypha with sperm nuclei (n) Key Haploid (n) Diploid (2n) Encysted zoospores land on a substrate and germinate, growing into a tufted body of hyphae. 1 Several days later, the hyphae begin to form sexual structures. 2 Meiosis produces eggs within oogonia (singular, oogonium). 3 On separate branches of the same or different individuals, meiosis produces several haploid sperm nuclei contained within antheridial hyphae. 4 Antheridial hyphae grow like hooks around the oogonium and deposit their nuclei through fertilization tubes that lead to the eggs. Following fertilization, the zygotes (oospores) may develop resistant walls but are also protected within the wall of the oogonium. 5 A dormant period follows, during which the oogonium wall usually disintegrates. 6 The zygotes germinate and form hyphae, and the cycle is completed. 7 The ends of hyphae form tubular zoosporangia. 8 Each zoospor- angium produces about 30 biflagellated zoospores asexually. 9

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The ecological impact of oomycetes can be significant – Phytophthora infestans causes late blight of potatoes

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diatoms Diatoms are unicellular algae – With a unique two-part, glass-like wall of hydrated silica Figure 28.15 3 µm

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diatoms are a major component of phytoplankton – And are highly diverse Figure 28.16 50 µm

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Accumulations of fossilized diatom walls – Compose much of the sediments known as diatomaceous earth

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Golden Algae Golden algae, or chrysophytes – Are named for their color, which results from their yellow and brown carotenoids The cells of golden algae – Are typically biflagellated, with both flagella attached near one end of the cell

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Most golden algae are unicellular – But some are colonial Figure 28.17 25 µm

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Brown Algae Brown algae, or phaeophytes – Are the largest and most complex algae – Are all multicellular, and most are marine

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Brown algae – Include many of the species commonly called seaweeds Seaweeds – Have the most complex multicellular anatomy of all algae Figure 28.18 Blade Stipe Holdfast

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Kelps, or giant seaweeds – Live in deep parts of the ocean Figure 28.19

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Human Uses of Seaweeds Many seaweeds – Are important commodities for humans – Are harvested for food Figure 28.20a–c (a) The seaweed is grown on nets in shallow coastal waters. (b) A worker spreads the harvested sea- weed on bamboo screens to dry. (c) Paper-thin, glossy sheets of nori make a mineral-rich wrap for rice, seafood, and vegetables in sushi.

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alternation of Generations A variety of life cycles – Have evolved among the multicellular algae The most complex life cycles include an alternation of generations – The alternation of multicellular haploid and diploid forms

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The life cycle of the brown alga Laminaria Figure 28.21 Sporophyte (2n) Zoospores Female Gametophytes (n) MEIOSIS FERTILIZATION Developing sporophyte Zygote (2n) Mature female gametophyte (n) Egg Sperm Male Sporangia Key Haploid (n) Diploid (2n) The sporophytes of this seaweed are usually found in water just below the line of the lowest tides, attached to rocks by branching holdfasts. 1 In early spring, at the end of the main growing season, cells on the surface of the blade develop into sporangia. 2 Sporangia produce zoospores by meiosis. 3 The zoospores are all structurally alike, but about half of them develop into male gametophytes and half into female gametophytes. The gametophytes look nothing like the sporo- phytes, being short, branched filaments that grow on the surface of subtidal rocks. 4 Male gametophytes release sperm, and female gametophytes produce eggs, which remain attached to the female gameto- phyte. Eggs secrete a chemical signal that attracts sperm of the same species, thereby increasing the probability of fertilization in the ocean. 5 Sperm fertilize the eggs. 6 The zygotes grow into new sporophytes, starting life attached to the remains of the female gametophyte. 7

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 28.6: Cercozoans and radiolarians have threadlike pseudopodia A newly recognized clade, Cercozoa – Contains a diversity of species that are among the organisms referred to as amoebas Amoebas were formerly defined as protists – That move and feed by means of pseudopodia Cercozoans are distinguished from most other amoebas – By their threadlike pseudopodia

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Foraminiferans (Forams) Foraminiferans, or forams – Are named for their porous, generally multichambered shells, called tests Figure 28.22 20 µm

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pseudopodia extend through the pores in the test Foram tests in marine sediments – Form an extensive fossil record

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Radiolarians Radiolarians are marine protists – Whose tests are fused into one delicate piece, which is generally made of silica – That phagocytose microorganisms with their pseudopodia

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The pseudopodia of radiolarians, known as axopodia – Radiate from the central body Figure 28.23 200 µm Axopodia

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 28.7: Amoebozoans have lobe- shaped pseudopodia Amoebozoans – Are amoeba that have lobe-shaped, rather than threadlike, pseudopodia – Include gymnamoebas, entamoebas, and slime molds

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gymnamoebas – Are common unicellular amoebozoans in soil as well as freshwater and marine environments

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Most gymnamoebas are heterotrophic – And actively seek and consume bacteria and other protists Figure 28.24 Pseudopodia 40 µm

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Entamoebas – Are parasites of vertebrates and some invertebrates Entamoeba histolytica – Causes amebic dysentery in humans

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Slime Molds Slime molds, or mycetozoans – Were once thought to be fungi Molecular systematics – Places slime molds in the clade Amoebozoa

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plasmodial Slime Molds Many species of plasmodial slime molds – Are brightly pigmented, usually yellow or orange Figure 28.25 4 cm

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings At one point in the life cycle – They form a mass called a plasmodium Figure 28.26 Feeding plasmodium Mature plasmodium (preparing to fruit) Young sporangium Mature sporangium Spores (n) Germinating spore Amoeboid cells (n) Zygote (2n) 1 mm Key Haploid (n) Diploid (2n) MEIOSIS SYNGAMY Stalk Flagellated cells (n) The feeding stage is a multinucleate plasmodium that lives on organic refuse. 1 The plasmodium takes a weblike form. 2 The plasmodium erects stalked fruiting bodies (sporangia) when conditions become harsh. 3 Within the bulbous tips of the sporangia, meiosis produces haploid spores. 4 These cells are either amoeboid or flagellated; the two forms readily convert from one to the other. 6 The cells unite in pairs (flagellated with flagellated and amoeboid with amoeboid), forming diploid zygotes. 7 The resistant spores disperse through the air to new locations and germinate, becoming active haploid cells when conditions are favorable. 5

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The plasmodium – Is undivided by membranes and contains many diploid nuclei – Extends pseudopodia through decomposing material, engulfing food by phagocytosis

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cellular Slime Molds Cellular slime molds form multicellular aggregates – In which the cells remain separated by their membranes

61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The life cycle of Dictyostelium, a cellular slime mold Spores (n) Emerging amoeba Solitary amoebas (feeding stage) ASEXUAL REPRODUCTION Fruiting bodies Aggregated amoebas Migrating aggregate SYNGAMY MEIOSIS SEXUAL REPRODUCTION Zygote (2n) Amoebas 600 µm 200 µm Key Haploid (n) Diploid (2n) Figure 28.27 In the feeding stage of the life cycle, solitary haploid amoebas engulf bacteria. 1 During sexual repro- duction, two haploid amoebas fuse and form a zygote. 2 The zygote becomes a giant cell (not shown) by consuming haploid amoebas. After developing a resistant wall, the giant cell undergoes meiosis followed by several mitotic divisions. 3 The resistant wall ruptures, releasing new haploid amoebas. 4 When food is depleted, hundreds of amoebas congregate in response to a chemical attractant and form a sluglike aggregate (photo below left). Aggregate formation is the beginning of asexual reproduction. 5 The aggregate migrates for a while and then stops. Some of the cells dry up after forming a stalk that supports an asexual fruiting body. 6 Other cells crawl up the stalk and develop into spores. 7 Spores are released. 8 In a favorable environment, amoebas emerge from the spore coats and begin feeding. 9

62. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dictyostelium discoideum – Has become an experimental model for studying the evolution of multicellularity

63. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 28.8: Red algae and green algae are the closest relatives of land plants Over a billion years ago, a heterotrophic protist acquired a cyanobacterial endosymbiont – And the photosynthetic descendants of this ancient protist evolved into red algae and green algae

64. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Red Algae Red algae are reddish in color – Due to an accessory pigment call phycoerythrin, which masks the green of chlorophyll

65. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Red algae – Are usually multicellular; the largest are seaweeds – Are the most abundant large algae in coastal waters of the tropics Figure 28.28a–c (a) Bonnemaisonia hamifera. This red alga has a filamentous form. Dulse (Palmaria palmata). This edible species has a “leafy” form. (b) A coralline alga. The cell walls of coralline algae are hardened by calcium carbonate. Some coralline algae are members of the biological communities around coral reefs. (c)

66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Green Algae Green algae – Are named for their grass-green chloroplasts – Are divided into two main groups: chlorophytes and charophyceans – Are closely related to land plants

67. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Most chlorophytes – Live in fresh water, although many are marine Other chlorophytes – Live in damp soil, as symbionts in lichens, or in snow Figure 28.29

68. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chlorophytes include – Unicellular, colonial, and multicellular forms Volvox, a colonial freshwater chlorophyte. The colony is a hollow ball whose wall is composed of hundreds or thousands of biflagellated cells (see inset LM) embedded in a gelatinous matrix. The cells are usually connected by strands of cytoplasm; if isolated, these cells cannot reproduce. The large colonies seen here will eventually release the small “daughter” colonies within them (LM). (a) Caulerpa, an inter- tidal chlorophyte. The branched fila- ments lack cross-walls and thus are multi- nucleate. In effect, the thallus is one huge “supercell.” (b) Ulva, or sea lettuce. This edible seaweed has a multicellular thallus differentiated into leaflike blades and a rootlike holdfast that anchors the alga against turbulent waves and tides. (c) 20 µm 50 µm Figure 28.30a–c

69. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 28.31 Flagella Cell wall Nucleus Regions of single chloroplast Zoospores ASEXUAL REPRODUCTION Mature cell (n) SYNGAMY SEXUAL REPRODUCTION Zygote (2n) MEIOSIS 1 µm Key Haploid (n) Diploid (2n)  + +   + + Most chlorophytes have complex life cycles – With both sexual and asexual reproductive stages In Chlamydomonas, mature cells are haploid and contain a single cup-shaped chloroplast (see TEM at left). 1 In response to a shortage of nutrients, drying of the pond, or some other stress, cells develop into gametes. 2 Gametes of opposite mating types (designated + and –) pair off and cling together. Fusion of the gametes (syngamy) forms a diploid zygote. 3 The zygote secretes a durable coat that protects the cell against harsh conditions. 4 After a dormant period, meiosis produces four haploid individuals (two of each mating type) that emerge from the coat and develop into mature cells. 5 When a mature cell repro- duces asexually, it resorbs its flagella and then undergoes two rounds of mitosis, forming four cells (more in some species). 6 These daughter cells develop flagella and cell walls and then emerge as swimming zoospores from the wall of the parent cell that had enclosed them. The zoospores grow into mature haploid cells, completing the asexual life cycle. 7