Chapter 27 Diversification of Eukaryotes

27.1 Why Do Biologists Study Protists?

• Protists are all eukaryotes except plants, fungi, and animals.

• They are very common in water, both in oceans and freshwater, and play a key role in food chains and carbon fixation, impacting the global carbon cycle.

• Some protists are parasites that cause diseases like malaria.

• Protists that produce toxins can cause harmful algal blooms when they grow in large numbers.

27.2 How Do Biologists Study Protists?

• Microscopic investigation: Many protists are single-celled, but unlike bacteria, they have mitochondria, a nucleus, a membrane system, and a cytoskeleton.

• Direct sequencing: This has helped discover new groups of protists.

• Molecular data: Phylogenetic studies show that protists form seven major groups and are paraphyletic (not all descendants are included in the group).

27.3 What Themes Occur in the Diversification of Protists?

• The first eukaryote likely formed when a prokaryotic cell (probably an archaeal cell) engulfed an alpha-proteobacterium, which became today's mitochondria.

• Chloroplasts have features like a circular chromosome, ribosomes, and a double membrane, supporting the idea that they evolved from an ancient cyanobacterium.

• After primary endosymbiosis, chloroplasts were passed to other protist groups through secondary endosymbiosis.

• The evolution of the nuclear envelope allowed transcription and translation to happen in separate areas of the cell.

• Multicellularity evolved independently in different protist groups.

• Protists vary in how they obtain food, including predatory, parasitic, and photosynthetic lifestyles, which evolved independently in different groups.

• Ingestive feeding was key because it allowed eukaryotes to eat bacteria and other cells, and it made endosymbiosis (and the evolution of mitochondria and chloroplasts) possible.

• Protists reproduce in different ways: most reproduce asexually via mitosis, but many also reproduce sexually through meiosis at some point in their life cycle.

• Alternation of generations is common in multicellular protists, where they have separate haploid (gametophyte) and diploid (sporophyte) forms.

27.4 Key Lineages of Eukaryotes

• Protists are a diverse group, organized into seven major lineages.

• These lineages are identified based on DNA sequences and features like the presence of chloroplasts or unique flagella.

• Traits like parasitism, autotrophy, and multicellularity evolved independently in different protist groups.

Why are protists considered paraphyletic?

a. They include many extinct forms, including lineages that no longer have any living representatives.

b. They include some but not all descendants of their most recent common ancestor.

c. They represent all of the descendants of a single common ancestor.

d. Not all protists have all of the synapomorphies that define the Eukarya, such as a nucleus.

b. They include some but not all descendants of their most recent common ancestor.

The most important primary producers in marine ecosystems are ______.

phytoplankton

T/F Amoeboid motion is common in species with cell walls.

False

T/F Some protists use flagella to swim.

True

T/F Some protists use cilia to swim, which are shorter and more numerous than flagella.

True

T/F Amoeboid motion requires ATP and interactions between the proteins actin and myosin.

True

How can dinoflagellates be harmful to humans?

a. They are transmitted by mosquitoes and cause malaria.

b. They produce toxins that can be absorbed by clams and other shellfish which, when eaten by people, can lead to paralytic shellfish poisoning.

c. They cause amoebic dysentery which leads to severe diarrhea and dehydration.

d. They are transmitted by tsetse flies and cause “sleeping sickness.”

b. They produce toxins that can be absorbed by clams and other shellfish which, when eaten by people, can lead to paralytic shellfish poisoning.

Which of the following best describes movement in Physarum?

a. Cilia propel the slime mold.

b. Flagella propel the slime mold.

c. The slime mold moves by amoeboid motion.

d. The slime mold moves by gliding motility.

c. The slime mold moves by amoeboid motion.

Explain the logic behind the claim that the nuclear envelope is a synapomorphy that defines eukaryotes as a monophyletic group.

The nuclear envelope is a synapomorphy because it is a trait that all eukaryotes have, but no prokaryotes do. This means it defines eukaryotes as a group that shares a common ancestor with this feature, making eukaryotes a monophyletic group.

Consider the endosymbiosis theory for the origin of the mitochondrion. How did each endosymbiotic partner benefit from the relationship?

The mitochondrion is believed to have originated from an ancient relationship between a host cell and an engulfed aerobic bacterium.

• The host cell benefited because the engulfed bacterium could efficiently produce energy (ATP) through aerobic respiration, providing the host with more energy.

• The engulfed bacterium benefited by living inside a protected environment, gaining access to resources from the host cell.

Over time, the bacterium became an integral part of the host cell, evolving into the mitochondrion.