Biology 2 Ch 26: Protists part 2

Green algae are divided into two main groups

, the charophytes and the chlorophytes. The charophytes are the algae most closely related to land plants, and we will discuss them along with plants in Chapter 29. The second group, the chlorophytes (from the Greek chloros, green), includes more than 7,000 species. Most live in fresh water, but there are also many marine and some terrestrial species.

Most chlorophytes have complex life cycles,

, with both sexual and asexual reproductive stages. Nearly all species of chlorophytes reproduce sexually by means of biflagellated gametes that have cup-shaped chloroplasts (Figure 28.23). Alternation of generations has evolved in some chlorophytes, including Ulva.

Amoebozoans

-Amoeba that have lobe-or tube-shaped pseudopodia: rather than threadlike. Include: slime molds, Tubulinids, and Entamoebas.

opisthokonts,:

-Choanoflagellates =Unicellular organisms =Common ancestor of Fungi and animal ==most like the common ancestor of Sponges -Propulsion by a single posterior flagellum

Tubulinids

-Diverse group of amoebozoans =Lobe- or tube-shaped pseudopodia -Common unicellular protists =Live in: Soil Freshwater Marine environments -Most Tubulinids are heterotrophic =Actively seek and consume bacteria and other protists

green algae

-Land plants arose from an ancestral green alga only once during evolution -Green alga -Paraphyletic clade =Does not include land plants consist of 2 monophyletic groups -Chlorophyta -Charophytes =Most closely related to land plants

chlorophytes,:

-Larger size and greater complexity evolved in chlorophytes by 1. The formation of colonies from individual cells 2.The formation of true multicellular bodies by cell division and differentiation (e.g., Ulva) 3. The repeated division of nuclei with no cytoplasmic division (e.g., Caulerpa)

Plasmodial slime molds:

-Many brightly colored -stream along as a plasmodium =non-walled, multinucleate mass of cytoplasm, form called feeding phase, ingests bacteria and other organic material -when food or moisture is scarce, forms sporangia, spores are produced

chlorophytes:

-Most live in fresh water Although many are marine -Other chlorophytes live in: -Damp soil ==As symbionts in lichens =Environments exposed to intense visible and ultraviolet radiation

Mycetozoans

-Once thought to be fungi =DNA sequence analyses indicate that the resemblance between slime molds and fungi is a result of convergent evolution

Opisthokonts:

-Opisthokonts include: Animals, Fungi, Several groups of protists, -Poorly organized clade

Entamoebas

-Parasites of vertebrates and some invertebrates -Entamoeba histolytica =Causes amebic dysentery the third-leading cause of human death due to eukaryotic parasites -Acanthomoeba =Enters the body through a wound =Crosses the blood-brain barrier into the brain

unicellular chlorophytes

-early green algae probably resembled Chlamydomonas reinhardtii, diverged from land plants over 1 BYA -several lines of evolutionary specialization derived from chlorophytes

Whereas most amoebozoans are free-living, those that belong to the genus Entamoeba are parasites.

They infect all classes of vertebrate animals as well as some invertebrates. Humans are host to at least six species of Entamoeba, but only one, E. histolytica, is known to be pathogenic. E. histolytica causes amebic dysentery and is spread via contaminated drinking water, food, or eating utensils. Responsible for up to 100,000 deaths worldwide every year, the disease is the third-leading cause of death due to eukaryotic parasites, after malaria (see Figure 28.16) and schistosomiasis (see Figure 33.11).

The simplest chlorophytes are unicellular organisms such as Chlamydomonas, which resemble gametes of more complex chlorophytes.

Various species of unicellular chlorophytes live independently in aquatic habitats as phytoplankton or inhabit damp soil. Some live symbiotically within other eukaryotes, contributing part of their photosynthetic output to the food supply of their hosts. Still other chlorophytes live in environments exposed to intense visible and ultraviolet radiation; these species are protected by radiation-blocking compounds in their cytoplasm, cell wall, or zygote coat.

Larger size and greater complexity evolved in chlorophytes by The formation of colonies from individual cells The formation of true multicellular bodies by cell division and differentiation (e.g., Ulva) The repeated division of nuclei with no cytoplasmic division (e.g., Caulerpa

1. In Chlamydomonas, mature cells are haploid and contain a single cup-shaped chloroplast. 2. In response to a nutrient shortage, drying of the environment, or other stress, cells develop into gametes. 3. Gametes of different mating types (designated + and -) fuse (fertilization), forming a diploid zygote. 4. The zygote secretes a durable coat that protects the cell from harsh conditions. 5. After a dormant period, meiosis produces four haploid individuals (two of each mating type) that emerge and mature. 6. When a mature cell reproduces asexually, it resorbs its flagella and then undergoes two rounds of mitosis, forming four cells (more in some species). 7. These daughter cells develop flagella and cell walls and then emerge as swimming zoospores from the parent cell. The zoospores develop into mature haploid cells. Blue = haploid (n) Pink = diploid (2n)

The life cycle of Dictyostelium, a cellular slime mold (step 2)

1. In the feeding stage, solitary haploid amoebas engulf bacteria; these solitary cells periodically divide by mitosis (asexual reproduction). 2. During sexual reproduction, two haploid amoebas fuse and form a zygote. 3. The zygote becomes a giant cell by consuming haploid amoebas (not shown). After developing a resistant wall, the giant cell undergoes meiosis followed by several mitotic divisions. 4. The wall ruptures, releasing new haploid amoebas. 5. When food is depleted, hundreds of amoebas congregate in response to a chemical attractant and form a slug-like aggregate (see photo). 6. 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. 7. Other cells crawl up the stalk and develop into spores. 8. Spores are released. 9. In favorable conditions, amoebas emerge from the spore coats and feed. Blue = haploid (n) Pink = diploid (2n)

Amoebozoan clade includes:

1. Slime molds, or mycetozoans (from the Latin, meaning "fungus animals"), were once thought to be fungi because, like fungi, they produce fruiting bodies that aid in spore dispersal. However, DNA sequence analyses indicate that the resemblance between slime molds and fungi is a case of evolutionary convergence. DNA sequence analyses also show that slime molds descended from unicellular ancestors— an example of the independent origin of multicellularity in eukaryotes. Slime molds have diverged into two main branches, plasmodial slime molds and cellular slime molds. We'll compare their characteristics and life cycles.

Unikonta

is an extremely diverse supergroup of eukaryotes that includes animals, fungi, and some protists. There are two major clades of unikonts, the amoebozoans and the opisthokonts (animals, fungi, and closely related protist groups). Each of these two major clades is strongly supported by molecular systematics. The close relationship between amoebozoans and opisthokonts is more controversial. Support for this close relationship is provided by comparisons of myosin proteins and by some (but not all) studies based on multiple genes or whole genomes.

The Amoebozoan clade includes

many species of amoebas that have lobe- or tube-shaped pseudopodia, rather than the threadlike pseudopodia found in rhizarians. Amoebozoans include slime molds, Tubulinids, and Entamoebas.

Tubulinids constitute a large and varied group

of amoebozoans that have lobe- or tube-shaped pseudopodia. These unicellular protists are ubiquitous in soil as well as freshwater and marine environments. Most are heterotrophs that actively seek and consume bacteria and other protists; one such tubulinid species, Amoeba proteus, is shown in Figure 28.2. Some Tubulinids also feed on detritus (nonliving organic matter).

The grass-green chloroplasts

of green algae have a structure and pigment composition much like the chloroplasts of land plants. Molecular systematics and cellular morphology leave little doubt that green algae and land plants are closely related. In fact, some systematists now advocate including green algae in an expanded "plant" kingdom, Viridiplantae (from the Latin viridis, green). Phylogenetically, this change makes sense, since otherwise the green algae are a paraphyletic group.

In trying to determine the root of the eukaryotic tree

researchers have based their phylogenies on different sets of genes, some of which have produced conflicting results. Researchers have also tried a different approach, based on tracing the occurrence of a rare evolutionary event (Figure 28.24). Results from this "rare event" approach suggest that the unikonts were the first eukaryotes to diverge from other eukaryotes. If this hypothesis is correct, animals and fungi belong to an early-diverging group of eukaryotes, while protists that lack typical mitochondria (such as the diplomonads and parabasalids) diverged later in the history of life. This idea remains controversial and will require more supporting evidence to be widely accepted.

unikonts include protists that are closely related to fungi and animals:,

root of eukaryotic tree remains controversial: unclear whether unikonts separated relatively early or late.

unikonts include protists that are closely related to fungi and animals

supergroup Unikonta includes two clades: amoebozoans and opisthokonts

Most red algae are multicellular. Although none are as big as the giant brown kelps,

the largest multicellular red algae are included in the informal designation "seaweeds." You may have eaten one of these multicellular red algae, Porphyra (Japanese "nori"), as crispy sheets or as a wrap for sushi (see Figure 28.21). Red algae reproduce sexually and have diverse life cycles in which alternation of generations is common. However, unlike other algae, red algae do not have flagellated gametes, so they depend on water currents to bring gametes together for fertilization.

slime molds, or Mycetozoans (from the Latin, meaning "fungus animals")

were once thought to be fungi because, like fungi, they produce fruiting bodies that aid in spore dispersal. However, DNA sequence analyses indicate that the resemblance between slime molds and fungi is a case of evolutionary convergence. DNA sequence analyses also show that slime molds descended from unicellular ancestors— an example of the independent origin of multicellularity in eukaryotes. Slime molds have diverged into two main branches, plasmodial slime molds and cellular slime molds. We'll compare their characteristics and life cycles.

Cellular Slime Molds The life cycle of the protists called cellular slime molds can prompt us to question

what it means to be an individual organism. The feeding stage of these organisms consists of solitary cells that function individually, but when food is depleted, the cells form a sluglike aggregate that functions as a unit (Figure 28.26). Unlike the feeding stage (plasmodium) of a plasmodial slime mold, these aggregated cells remain separated by their individual plasma membranes. Ultimately, the aggregated cells form an asexual fruiting body.

most chlorophytes have complex life cycle,

with both sexual and asexual reproductive stages. Nearly all species of chlorophytes reproduce sexually by means of biflagellated gametes that have cup-shaped chloroplasts (Figure 28.23). Alternation of generations has evolved in some chlorophytes, including Ulva.

Cellular Slime Molds

Form multicellular aggregates Cells are separated by their membranes Individual organisms behave as separate amoebas Move through soil ingesting bacteria Cells feed individually

Recent discoveries suggest an answer to this question.

Cheating cells lack a specific surface protein and noncheating cells can recognize this difference. Noncheaters preferentially aggregate with other noncheaters, thus depriving cheaters of the chance to exploit them. Such a recognition system may have been important in the evolution of other multicellular eukaryotes, such as animals and plants.

red algae

Many of the 6,000 known species of red algae (rhodophytes, from the Greek rhodos, red) are reddish, owing to a photosynthetic pigment called phycoerythrin, which masks the green of chlorophyll (Figure 28.21). However, other species (those adapted to more shallow water) have less phycoerythrin. As a result, red algal species may be greenish red in very shallow water, bright red at moderate depths, and almost black in deep water. Some species lack pigmentation altogether and function heterotrophically as parasites on other red algae.

most chlorophytes

Most chlorophytes have complex life cycles with both sexual and asexual reproductive stages

Slime Molds:

Mycetozoans and Slime molds include two lineages

Rhodophyta (Red Algae)

Reddish in color Accessory pigment called phycoerythrin Color varies Greenish-red in shallow water Dark red or almost black in deep water Lack flagella and centrioles Usually multicellular range from microscopic to very large Largest are seaweeds Most abundant large algae in coastal waters of the tropics

Larger size and greater complexity evolved in chlorophytes by three different mechanisms;

The formation of colonies of individual cells, as seen in Volvox (see Figure 28.2) and in filamentous forms that contribute to the stringy masses known as pond scum The formation of true multicellular bodies by cell division and differentiation, as in Ulva (Figure 28.22a) The repeated division of nuclei with no cytoplasmic division, as in Caulerpa (Figure 28.22b)

Red algae Bonnemaisonia hamifera. This red alga has a filamentous form, Nori. The red alga Porphyra is the source of a traditional Japanese food. Dulse (Palmaria palmata). This edible species has a "leafy" form.

The seaweed is grown on nets in shallow coastal waters. Paper-thin, glossy sheets of dried nori make a mineral-rich wrap for rice, seafood, and vegetables in sushi.

Opisthokonts are an extremely diverse group of eukaryotes that includes animals, fungi, and several groups of protists.

We will discuss the evolutionary history of fungi and animals in Chapters 31-34. Of the opisthokont protists, we will discuss the nucleariids in Chapter 31 because they are more closely related to fungi than they are to other protists. Similarly, we will discuss choanoflagellates in Chapter 32, since they are more closely related to animals than they are to other protists. The nucleariids and choanoflagellates illustrate why scientists have abandoned the former kingdom Protista: A monophyletic group that includes these single-celled eukaryotes would also have to include the multicellular animals and fungi that are closely related to them.

cellular slime molds: aggregate to migrate and form a fruiting body

When food is scarce, organisms aggregate to form a slug Slug differentiates into a sorocarp

multicellular chlorophytes (seaweeds)

a. Ulva, or sea lettuce. This multicellular, edible chlorophyte has differentiated structures, such as its leaflike blades and a rootlike holdfast that anchors the alga. b. Caulerpa, an intertidal chlorophyte. The branched filaments lack crosswalls and thus are multinucleate. In effect, the body of this alga is one huge "supercell.

Red algae are the most

abundant large algae in the warm coastal waters of tropical oceans. Some of their photosynthetic pigments, including phycoerythrin, allow them to absorb blue and green light, which penetrate relatively far into the water. A species of red alga has been discovered near the Bahamas at a depth of more than 260 m. There are also a small number of freshwater and terrestrial species.

Many plasmodial slime molds are

brightly colored, often yellow or orange (Figure 28.25). As they grow, they form a mass called a plasmodium, which can be many centimeters in diameter. (Don't confuse a slime mold's plasmodium with the genus Plasmodium, which includes the parasitic apicomplexan that causes malaria.) Despite its size, the plasmodium is not multicellular; it is a single mass of cytoplasm that is undivided by plasma membranes and that contains many nuclei. This "supercell" is the product of mitotic nuclear divisions that are not followed by cytokinesis. The plasmodium extends pseudopodia through moist soil, leaf mulch, or rotting logs, engulfing food particles by phagocytosis as it grows. If the habitat begins to dry up or there is no food left, the plasmodium stops growing and differentiates into fruiting bodies which function in sexual reproduction.

Cellular Slime Molds The life cycle of the protists

called cellular slime molds can prompt us to question what it means to be an individual organism. The feeding stage of these organisms consists of solitary cells that function individually, but when food is depleted, the cells form a sluglike aggregate that functions as a unit (Figure 28.26). Unlike the feeding stage (plasmodium) of a plasmodial slime mold, these aggregated cells remain separated by their individual plasma membranes. Ultimately, the aggregated cells form an asexual fruiting body.

Dictyostelium discoideum, a cellular slime mold

commonly found on forest floors, has become a model organism for studying the evolution of multicellularity. One line of research has focused on the slime mold's fruiting body stage. During this stage, the cells that form the stalk die as they dry out, while the spore cells at the top survive and have the potential to reproduce (see Figure 28.26). Scientists have found that mutations in a single gene can turn individual Dictyostelium cells into "cheaters" that never become part of the stalk. Because these mutants gain a strong reproductive advantage over noncheaters, why don't all Dictyostelium cells cheat?

Another controversy involving the unikonts

concerns the root of the eukaryotic tree. Recall that the root of a phylogenetic tree anchors the tree in time: Branch points close to the root are the oldest. At present, the root of the eukaryotic tree is uncertain; hence, we do not know which group of eukaryotes was the first to diverge from other eukaryotes. Some hypotheses, such as the amitochondriate hypothesis described earlier, have been abandoned, but researchers have yet to agree on an alternative. If the root of the eukaryotic tree were known, scientists could infer characteristics of the common ancestor of all eukaryotes.