Chapter 26

1. Loss of the stiff prokaryotic cell wall allowed infolding of the cell membrane, greatly increasing the surface area of the cell without increasing its volume. These infoldings also allowed for greater structure within the cell and eventually led to the evolution of the endoplasmic reticulum.

2. When atmospheric oxygen concentrations were low early in Earth’s history, organisms could use only anaerobic respiration. Increased availability of atmospheric oxygen allowed the evolution of aerobic respiration, which proceeds more rapidly and harvests energy more efficiently than anaerobic respiration does. But as cells increased in size, they would decrease in surface area-to-volume ratio unless the cell membrane had infoldings. Increased levels of atmospheric oxygen also led to the endosymbiosis of an aerobic bacterium (the ancestor of the eukaryotic mitochondrion), which facilitated the evolution of aerobic eukaryotes.

3. The three rRNA genes of corn are not one another’s closest relatives because the nuclear, mitochondrial, and chloroplast genomes have different origins, and the relationships shown in the tree reconstruct the endosymbiotic events that gave rise to mitochondria and chloroplasts.

4. The mitochondrial genome of corn is more closely related to the genome of E. coli than it is to the nuclear genome of corn because the mitochondria were derived from an endosymbiosis with a proteobacterium. Likewise, the chloroplast genome of corn is more closely related to the genome of Chlorobium than it is to the nuclear genome of corn because the chloroplasts were derived from an endosymbiosis with a cyanobacterium.

5. The term “protists” is used, for convenience, to refer to a diverse group of distantly related, polyphyletic microbial eukaryotes. Some protists are more closely related to each of the major groups of multicellular eukaryotes (including plants, fungi, and animals) than they are to one another.

1. 1. Foraminiferans have external shells of calcium carbonate, whereas radiolarians have long, stiff pseudopods and radial symmetry. The external shells of foraminiferans and the internal skeletons of radiolarians are both important components of ocean sediments and sedimentary rocks.

   2. Ciliates are covered with numerous hairlike cilia, whereas dinoflagellates generally have two flagella (one in an equatorial groove and the other in a longitudinal groove). Both ciliates and dinoflagellates have sacs, called alveoli, just beneath their cell membranes, which identify them as alveolates.

   3. Diatoms are unicellular and are typically composed of two nested plates (like a petri dish). In contrast, brown algae are large, multicellular organisms composed of branched elements or leaflike growths. Both diatoms and brown algae are photosynthetic.

   4. The vegetative unit of a plasmodial slime mold is a plasmodium: a wall-less mass of cytoplasm containing numerous diploid nuclei. In contrast, the vegetative unit of cellular slime molds consists of separate, single amoeboid cells. In both groups, when environmental conditions become unfavorable, the vegetative units form fruiting structures.

2. Most eukaryotes were unicellular until the beginning of the Cambrian. The rapid diversification and increased size of multicellular eukaryotes (especially animals) near the beginning of the Cambrian led to greatly increased chances of fossilization.

3. Examples; other answers are possible.

   1. Alveolates: Dinoflagellates can be toxic (e.g., they can poison mollusks during red tides); the apicomplexan Plasmodium is the causative agent of malaria.

   2. Stramenopiles: Some brown algae are harvested and used by humans as an emulsifier in ice cream, cosmetics, and other products.

   3. Excavates: The parasitic Giardia lamblia, a diplomonad, causes the intestinal disease giardiasis (an infection common among hikers and campers who drink spring or stream water in recreational areas). Kinetoplastids include several medically important species of pathogenic trypanosomes that cause diseases such as sleeping sickness, Chagas disease, and leishmaniasis.

1. Two paramecia exchange genetic material during conjugation, but there is no increase in the number of individuals. As a result, there is sex (genetic recombination) but no reproduction.

2. The sex that occurs in conjugation allows the removal of deleterious mutations through genetic recombination. Without conjugation, paramecia can only reproduce clonally, which is not viable over long periods of time.

3. Alternation of generations involves alternation of multicellular, independent haploid and diploid stages. Sperm and eggs are unicellular and are not independent organisms.

1. The female mosquitoes transmit the Plasmodium parasites to vertebrate hosts. The mosquitoes take up Plasmodium along with blood from an infected host and transmit the parasite to new, previously uninfected hosts when they feed on the blood of the new hosts.

2. Red tides are massive blooms of free-living protists (often dinoflagellates) that occur in oceans (and sometimes freshwater bodies) around the world. Many dinoflagellates involved in red tides produce toxins that kill many other species, especially fish and other vertebrates. Dinoflagellates are important symbionts of corals; when environmental conditions change, the dinoflagellates die or are ejected from the host corals, resulting in bleaching. The corals may take up new dinoflagellates, or they may become weakened and perish.

3. Diatoms are important as primary producers in many ecosystems. Of more direct importance to humans, fossilized oils from diatoms are the primary source of petroleum and natural gas. In addition, the remains of diatom skeletons produce diatomaceous earth, which is used for insulation, filtration, polishing, and as an insecticide.

1. The new strain (B211 in this experiment) was taken up by 92 percent of the coral colonies after bleaching (and by 100% of the surviving colonies), even though at least 58 percent of the experimental colonies retained some of their original symbionts after bleaching. This shows that bleached coral colonies are likely to take up strains of Symbiodinium that are available in the environment, even if some of their original symbionts survive the bleaching.

2. Between week 3 and week 6 after bleaching, an additional 17 percent of the experimental colonies died despite having acquired new strains of Symbiodinium.

3. This observation suggests that the detection assays for Symbiodinium are not sensitive enough to detect very small levels of the symbionts. Presumably, the non-B211 strains were present in some of the experimental colonies in week 3, but at such low levels that they were not detected. These symbionts then recovered in 8 percent of the colonies and were at detectable levels by week 6. Another possibility is that contamination of the experimental samples with non-B211 symbionts took place between weeks 3 and 6 of the experiment. Neither possibility challenges the conclusion that corals can acquire new strains of Symbiodinium from their environment after bleaching events.

1. All of the apicomplexans shown here have a parasitic lifestyle. Therefore this lifestyle was likely present in the common ancestor of the apicomplexans. The hosts of the three apicomplexans differ, but they are all mammals. Thus it is reasonable to infer that the common ancestor of these species likely had a mammalian host.

2. No, all of the dinoflagellates are equally distantly related to the apicomplexans, so there is no reason to favor one genus over another in choosing an outgroup.

3. The genus Alexandrium is more closely related to Pfiesteria than to any of the other genera, so this genus is a good choice for comparison, as the two genera share the most recent common ancestor. Therefore comparing Alexandrium to Pfiesteria would help the investigator determine which attributes evolved in Pfiesteria, and which were already present in the common ancestor of Alexandrium and Pfiesteria.

4. Answers will vary. One example is the morphological similarity of loboseans and heteroloboseans, despite these groups not being closely related.