Review of the Classification of Living things One can view the chronology of the major episodes that shaped life as a phylogenetic tree. Fig Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Each branch is a clade which is a Greek word for branch. The concept is that each clade has one common ancestor.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Taxonomy employs a hierarchical system of classification The Linnean system, first formally proposed by Linneaus in Systema naturae in the 18th century, has two main characteristics. Each species has a two-part name. Species are organized hierarchically into broader and broader groups of organisms.

Genera are grouped into progressively broader categories: family, order, class, phylum, kingdom and domain. Fig Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

CHAPTER 28 THE ORIGINS OF EUKAYOTIC DIVERSITY Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section A: Introduction to the Protists 1.Systematists have split protists into many kingdoms 2.Protists are the most diverse of all eukaryotes

The Origin and Early Diversification of Eukaryotes 1.Endomembranes contributed to larger, more complex cells 2.Mitochondria and plastids evolved from endosymbiotic bacteria3. The eukaryotic cell is a chimera of prokaryote ancestors 4. Secondary endosymbiosis increased the diversity of algae 5. Research on the relationships between the three domains is changing ideas about the deepest branching in the tree of life 6. The origin of eukaryotes catalyzed a second great wave of diversification

Under one evolutionary scenario, the endomembrane system of eukaryotes (nuclear envelope, endoplasmic reticulum, Golgi apparatus, and related structures) may have evolved from infoldings of plasma membrane. Another process, called endosymbiosis, probably led to mitochondria, plastids, and perhaps other eukaryotic features. Fig Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Research on the relationships between the three domains is changing ideas about the deepest branching in the tree of life The chimeric origin of the eukaryotic cells contrasts with the classic Darwinian view of lineal descent through a “vertical” series of ancestors. The eukaryotic cell evolved by “horizontal” fusions of species from different phylogenetic lineages. The metaphor of an evolutionary tree starts to break down at the origin of eukaryotes and other early evolutionary episodes.

The conventional model of relationships among the three domains place the archaea as more closely related to eukaryotes than they are to prokaryotes. Similarities include proteins involved in transcription and translation. This model places the host cell in the endosymbiotic origin of eukaryotes as resembling an early archaean. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

All three domains seem to have genomes that are chimeric mixes of DNA that was transferred across the boundaries of the domains. This has lead some researchers to suggest replacing the classical tree with a web-like phylogeny Fig Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The origin of eukaryotes catalyzed a second great wave of diversification The first great adaptive radiation, the metabolic diversification of the prokaryotes, set the stage for the second. The second wave of diversification was catalyzed by the greater structural diversity of the eukaryotic cell. The third wave of diversification followed the origin of multicellular bodies in several eukaryotic lineages. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Protists are eukaryotes and thus are much more complex than the prokaryotes. The first eukaryotes were unicellular. Not only were they the predecessor to the great variety of modern protists, but also to all other eukaryotes - plants, fungi, and animals. The origin of the eukaryotic cell and the emergence of multicellularity unfolded during the evolution of protists. Introduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Eukaryotic fossils date back 2.1 billion years and “chemical signatures” of eukaryotes date back 2.7 billion years. For about 2 billion years, eukaryotes consisted of mostly microscopic organisms known by the informal name “protists.” Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In the five-kingdom system of classification, the eukaryotes were distributed among four kingdoms: Protista, Plantae, Fungi, and Animalia. The plant, fungus, and animal kingdoms are surviving the taxonomic remodeling so far, though their boundaries have been expanded to include certain groups formerly classified as protists. However, systematists have split protists into many kingdoms. Modern systematists has crumbled the former kingdom of protists beyond repair. 1. Systematists have split protists into many kingdoms Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Protista was defined partly by structural level (mostly unicellular eukaryotes) and partly by exclusion from the definitions of plants, fungi, or animals. However, this created a group ranging from single- celled microscopic members, simple multicellular forms, and complex giants like seaweeds. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 28.1

The kingdom Protista formed a paraphyletic group, with some members more closely related to animals, plants, or fungi than to other protists. Systematists have split the former kingdom Protista into as many as 20 separate kingdoms. Still,“protist” is used as an informal term for this great diversity of eukaryotic kingdoms. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 28.2

Protists are so diverse that few general characteristics can be cited without exceptions. Most of the 60,000 known protists are unicellular, but some are colonial and others multicellular. While unicellular protists would seem to be the simplest eukaryotic organisms, at the cellular level they are the most elaborate of all cells. A single cell must perform all the basic functions performed by the collective of specialized cells in plants and animals. 2. Protists are the most diverse of all eukaryotes Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Protists are the most nutritionally diverse of all eukaryotes, Most protists are aerobic, with mitochondria for cellular respiration. Some protists are photoautotrophs with chloroplasts. Still others are heterotrophs that absorb organic molecules or ingest larger food particles. A few are mixotrophs, combining photosynthesis and heterotrophic nutrition. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Euglena, a single celled mixotrophic protist, can use chloroplasts to undergo photosynthesis if light is available or live as a heterotroph by absorbing organic nutrients from the environment. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 28.3

These various modes of nutrition are scattered throughout the protists. The same group may include photosynthetic species, heterotrophic species, and mixotrophs. While nutrition is not a reliable taxonomic characteristic, it is useful in understanding the adaptations of protists and the roles that they play in biological communities. Protists can be divided into three ecological categories: protozoa - ingestive, animal-like protists absorptive, fungus-like protists algae - photosynthetic, plant-like protists. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Most protists move with flagella or cilia during some time in their life cycles. The eukaryotic flagella are not homologous to those of prokaryotes. The eukaryotic flagella are extensions of the cytoplasm with a support of the microtubule system. Cilia are shorter and more numerous than flagella. Cilia and flagella move the cell with rhythmic power strokes, analogous to the oars of a boat. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Reproduction and life cycles are highly varied among protists. Mitosis occurs in almost all protists, but there are many variations in the process. Some protists are exclusively asexual or at least employ meiosis and syngamy (the union of two gametes), thereby shuffling genes between two individuals. Others are primarily asexual but can also reproduce sexually at least occasionally. Protists show the three basic types of sexual life cycles, with some other variants, too. The haploid stage is the vegetative stage of most protists, with the zygote as the only diploid cell. Many protists form resistant cells (cysts) that can survive harsh conditions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Protists are found almost anywhere there is water. This includes oceans, ponds, and lakes, but also damp soil, leaf litter, and other moist terrestrial habitats. In aquatic habitats, protists may be bottom-dwellers attached to rocks and other anchorages or creeping through sand and silt. Protists are also important parts of the plankton, communities of organisms that drift passively or swim weakly in the water. Phytoplankton (including planktonic eukaryotic algae and prokaryotic cyanobacteria) are the bases of most marine and freshwater food chains. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Many protists are symbionts that inhabit the body fluids, tissues, or cells of hosts. These symbiotic relationships span the continuum from mutualism to parasitism. Some parasitic protists are important pathogens of animals, including those that cause potentially fatal diseases in humans. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings