CHAPTER 28 THE ORIGINS OF EUKAYOTIC DIVERSITY Section A: Introduction to the Protists 1. Systematists have split protists into many kingdoms 2. Protists are the most diverse of all eukaryotes Protists are eukaryotes and thus much more complex than prokaryotes. The first eukaryotes were unicellular. Protists are biology’s answer to rock’n roll! 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. Ranging from single- celled microscopic members, simple multicellular forms, and to complex giants Modern systematists has crumbled the former kingdom of protists beyond repair! 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. Fig. 28.2 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Protists are the most diverse of all eukaryotes 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. Nutritionally diverse; (photo)autotrophs, heterotrophs mixotrophs

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

Protists can be divided into three ecological categories: protozoa - ingestive, animal-like protists absorptive, fungus-like protists algae - photosynthetic, plant-like protists. 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 9 + 2 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.

Symbionts and parasites 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; e.g. malaria Trypanosoma Giardia

The early origin of eukaryotes: One trend was the evolution of multicellular prokaryotes, where cells specialized for different functions (ex. some cyanobacteria). A second trend was the evolution of complex communities of prokaryotes, with species benefiting from the metabolic specialties of others. A third trend was the compartmentalization of different functions within single cells, an evolutionary solution that contributed to the origins of eukaryotes.

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 (chloroplasts), and perhaps other eukaryotic features. Plants Animals

This evolved into a mutually beneficial symbiosis. The organelle ancestors probably entered the host cells as undigested prey or internal parasites. This process would be facilitated by the presence of an endomembrane system and cytoskeleton, allowing the larger host cell to engulf the smaller prokaryote and to package them within vesicles. This evolved into a mutually beneficial symbiosis. Chloroplasts: A heterotrophic host could derive nourishment from photosynthetic endosymbionts. Mitochondria: In an increasingly aerobic world, an anaerobic host cell would benefit from aerobic endosymbionts that could exploit oxygen. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The eukaryotic cell is a chimera of prokaryotic ancestors The chimera of Greek mythology was part goat, part lion, and part serpent. Similarly, the eukaryotic cell is a chimera of prokaryotic parts: mitochondria from one bacteria plastids from another nuclear genome from the host cell Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

While mitochondria and plastids contain DNA and can build proteins, they are not genetically self-sufficient. The endosymbionts transferred some of their DNA to the host genome during the evolutionary transition from symbiosis to integrated eukaryotic organism. Key event: gene transfer form the symbiomt to the host nucleus

Secondary endosymbiosis increased the diversity of algae The best current explanation for the diversity of plastids is that plastids were acquired independently several times during the early evolution of eukaryotes. Those algal groups with more than two membranes were acquired by secondary endosymbiosis. It was by primary endosymbiosis that certain eukaryotes first acquired the ancestors of plastids by engulfing cyanobacteria. Secondary endosymbiosis occurred when a heterotrophic protist engulfed an algae containing plastids.

Secondary endosymbiosis Each endosymbiotic event adds a membrane derived from the vacuole membrane of the host cell that engulfed the endosymbiont. Fig. 28.5 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In some algae, there are remnants of the secondary endosymbionts. In most cases of secondary endosymbiosis, the endosymbiont lost most of its parts, except its plastid. In some algae, there are remnants of the secondary endosymbionts. For example, the plastids of cryptomonad algae contain vestiges of the endosymbiotic nucleus, cytoplasm, and even ribosomes. Thus, a cryptomonad is a complex chimera, like a box containing a box containing a box. The cryptomonad situation Secondary plastid Brown algae, diatoms, Cryptomonads, Euglena etc

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. Fig. 28.6 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 ? ? In this new model, the three domains arose from an ancestral community of primitive cells that swapped DNA promiscuously This explains the chimeric genomes of the three domains. Gene transfer across species lines is still common among prokaryotes. Fig. 28.7

Fig. 28.8

Protists lacking mitochondria The diplomonads have multiple flagella, two separate nuclei, a simply cytoskeleton, and no mitochondria or plastids. The parabasalids include trichomonads. The best known species, Trichomonas vaginalis, inhabits the vagina of human females According to the “archaezoa hypothesis,” these protists are derived from ancient eukaryotic lineages before the acquisition of endosymbiotic bacteria that evolved into mitochondria. DNA data: these protists lost their mitochondria during their evolution. (They have mitochondrial genes in their nuclei) Trichomonas vaginalis Giardia lamblia

Euglenozoa: euglenoids and kinetoplastids Several protistan groups, including the euglenoids and kinetoplastids, use flagella for locomotion. The euglenoids (Euglenozoa) are characterized by an anterior pocket from which one or two flagella emerge. While Euglena is chiefly autotrophic, other euglenoids are mixotrophic or heterotrophic. The kinetoplastids (Kinoplastida) have a single large mitochondrion associated with a unique organelle, the kinetoplast. Trypanosoma – sleeping sickness

Alveolata: The Alveolata combines flagellated protists (dinoflagellates), parasites (apicomplexans), and ciliated protists (the ciliates). This clade has been supported by molecular systematics. Members of this clade have alveoli, small membrane-bound cavities, under the cell surface. Dinoflagellate blooms, red tides in costal waters. Harmful algal blooms – toxins For instance in blue mussels

Other species of dinoflagellates are heterotrophic. Dinoflagellates and other phytoplankton form the foundation of most marine and many freshwater food chains. Other species of dinoflagellates are heterotrophic. Most dinoflagellates are unicellular, but some are colonial. Two flagella Plates of cellulose: characteristic pattern of each species Fig. 28.12