Eukaryotic Evolution
Eukaryotes evolved from prokaryotes. The earliest eukaryotic fossils (probably) date to about 2.1 billion years ago. However, there is geochemical evidence that eukaryotes may have existed long before, perhaps as early as 2.7 billion years ago, although the exact date is an open question.
Eukaryotic cells are generally larger and more complex than prokaryotic cells. Structures unique to eukaryotes include the nucleus, the endomembrane system, a complex cytoskeleton, and organelles specialized for energy transformations, e.g., mitochondria and plastids. Infoldings of the plasma membrane probably gave rise to the membrane-bound organelles of today’s eukaryotic cells.
The organelles of energy transformation have a different origin, however – prokaryotic cells. Sequencing of the mitochondrial genome indicates that mitochondria are derived from alphaproteobacteria. The presence of mitochondria is one of the many synapomorphies of eukaryotes, as all eukaryotic groups have (or had) mitochondria. Plastids, such as chloroplasts, originated from cyanobacteria, and are present in only some eukaryotic lineages.
Eukaryotes quickly radiated into a number of different groups. Eukaryotes were originally classified as either animals or plants, and all other eukaryotes, including fungi, amoebas, algae, etc. were sorted into one of these two groups. Fungi were eventually reclassified into their own kingdom, and organisms that did not fit neatly into one of these three kingdoms were called ‘protists.’ Some older textbooks, in fact, still refer to “Kingdom Protista”. We know now, from molecular analysis, that this classification scheme is not appropriate to describe the vast diversity of eukaryotes. “Kingdom Protista” is not monophyletic and therefore not a valid clade.
Currently, most eukaryotes are classified in one of five supergroups – Excavata, SAR, Archaeplastida, Amoebozoa, and Opisthokonta. There are also some taxa that do not seem to fit into any of these groups, or whose place in eukaryotic phylogeny is unclear. Eukaryote phylogeny is still very much a work in progress, and substantial revisions might still be made. The supergroups used in this lab manual are:
- Excavata – includes (mostly) unicellular, flagellated, heterotrophs (= zooflagellates). This supergroup includes two lineages, united primarily by molecular traits. Each of these includes two main clades:
- Metamonads
- Diplomonads
- Parabasalids
- Oxymonads
- Euglenozoans
- Kinetoplastids
- Euglenids
- Metamonads
- Amoebozoa – includes organisms that have a stage in their life cycle that is unicellular, lacks a cell wall, and moves or feeds by lobose pseudopodia. There is molecular evidence that this group is a sister group to the Opisthokonta.
- Amoebae
- Plasmodial Slime Molds
- Cellular Slime Molds
- Opisthokonta – diverse group of heterotrophic organisms that includes the fungi and animals.
- Archaeplastida – includes the evolutionary lineage that acquired plastids through primary endosymbiosis with a cyanobacterium. The plant kingom “nests” within this clade. Non-plant members of the Archaeplastida are commonly referred to as algae (as are some members of supergroup SAR). Major clades include:
- Rhodophyta – red algae
- Viridiplantae
- Chlorophyta – green algae
- Streptophyta
- Charophyta – charophytes
- Plantae – land plants (embryophytes)
- SAR (Stramenopiles, Alveolates, and Rhizarians). This grouping includes a diverse array of phyla that have been united on the basis of molecular data. (Older textbooks combine the stramenopiles and alveolates in the supergroup Chromalveolata and maintains the Rhizaria as a separate supergroup). Photosynthetic members of this supergroup are commonly referred to as algae, and unicellular heterotrophs in this group as protozoans.
- Stramenopiles
- Oomycetes – water molds
- Bacillariophyta – diatoms
- Chrysophyta – golden algae
- Phaeophyta – brown algae
- Alveolates
- Ciliophora – ciliates
- Dinoflagellata – dinoflagellates
- Apicomplexa – apicomplexans
- Rhizarians
- Foraminifera – forams, foraminiferans
- Radiolaria – radiolarians
- Cercozoa – cercozoans
- Stramenopiles
This figure presents the most recent picture of eukaryote evolution, but keep in mind that it is a hypothesis.
Supergroup Excavata
Many of the protist species classified into the supergroup Excavata are asymmetrical, single-celled organisms with a feeding groove “excavated” from one side. This supergroup includes heterotrophic predators, photosynthetic species, and parasites. Among its subgroups are the metamonads, including diplomonads, parabasalids, and oxymonads, and the euglenozoans, including euglenids and kinetoplastids. In this lab, we will look at representatives from each of these groups.
Diplomonads
Diplomonads are named for their unusual trait of having two nuclei. Until recently, these heterotrophic protists were believed to lack mitochondria. Mitochondrial remnant organelles, called mitosomes, have since been identified in diplomonads, but these mitosomes are essentially nonfunctional. Diplomonads exist in anaerobic environments and use alternative pathways, such as glycolysis, to generate energy. They generally use several flagella for locomotion.
Parabasalids
Parabasalids are named for their parabasal body, which is a large modified Golgi apparatus. Like diplomonads, these heterotrophs lack functional mitochondria. Instead, they have hydrogenosomes, highly modified mitochondria that perform anaerobic respiration. Many parabasalid species are symbionts. Some live in the guts of insects, such as termites or cockroaches, and help their host break down cellulose. Others are parasites, such as Trichomonas vaginalis, which is a common cause of vaginitis.
Oxymonads
Oxymonads are flagellated heterotrophs that live in the guts of termites and other wood-eating insects. It is presumed that they also play a role in cellulose digestion, although this has not been directly demonstrated. They are often covered in spirochete bacteria.
Jessica Maurer, CC BY-SA 4.0
Kinetoplastids
The kinetoplastid subgroup is named after the kinetoplast, a DNA mass carried within the single, oversized mitochondrion possessed by each of these cells. This subgroup includes several parasites, collectively called trypanosomes, which cause devastating human diseases and infect an insect species during a portion of their life cycle. Diseases caused by trypanosomes include sleeping sickness, leishmaniasis, and Chagas disease.
Euglenids
Euglenids move through their aquatic habitats using two long flagella that guide them toward light sources sensed by a primitive ocular organ called an eyespot. The genus Euglena encompasses some mixotrophic species that display a photosynthetic capability only when light is present. In the dark, the chloroplasts of Euglena shrink up and temporarily cease functioning, and the cells instead take up organic nutrients from their environment.
Supergroup Amoebozoa
Amoebozoa is a group consisting of about 2,400 described species. It includes many of the amoeboid organisms, but not all. The word amoeba, or amoeboid organism, is a nonspecific term used to describe a cell that can change its shape by moving, extending, and retracting pseudopodia (or pseduopods). Pseudopods are projections of the cytoplasm that move by polymerization and depolymerization of actin. They allow the cell to move and engulf food particles. Amoebozoan pseudopods are lobe- or tube-shaped, as opposed to the slender, hair-like pseudopodia of Rhizarians and other groups. Some amoebozoans also have flagella.
Most amoebozoan species are unicellular, amoeboid organisms, although some are multicellular or have a multicellular phase of their life cycle. Many slime molds, for example, are classified in this supergroup. Amoebozoans are heterotrophs and acquire food via phagocytosis. They lack cell walls. Some amoebae have a shell, or test, for protection. Others, the “naked” amoebae, lack a test.
Many amoebozoan species are free-living in water or soil; others are symbionts. These include the Entamoebas, which are animal parasites or commensals. Entamoeba histolytica is the most common cause of amoebic dysentery, which can be fatal. Other amoebozoans, such as Entamoeba coli and Endolimax nana, are human commensals and do not cause disease.
Recent molecular analyses place Supergroup Amoebozoa as the sister clade of Opisthokonta, which includes animals and fungi. Some scientists have proposed a higher-order clade, called Amorphea, which includes Amoebozoans and Opisthokonts.
Amoebae
The image that probably comes immediately to mind when you hear the word ‘amoeba’ is that of a lobose amoebozoan, with broad pseudopods and no test. We will examine two naked amoebae (Gymnamoebae) and two testate amoebae.
Amoeba proteus is a naked amoeba that is widespread in freshwater environments. It feeds on ciliates, other amoebas, and small animals such as rotifers.
Chaos carolinensis is a naked amoeba, also known as the giant amoeba. It can be large enough (1-5mm) to be seen with the naked eye. Chaos can have multiple nuclei.
Arcella vulgaris is a testate amoeba, with a shell composed of chitin. Its test is approximately hemispherical in shape and has an opening through which the pseudopodia extend. It has two nuclei.
Sage Wintle, CC BY-SA 4.0
Difflugia lobostoma is a testate amoeba that constructs its shell from debris scavenged from the environment, generally grains of sand, which the amoeba cements together. The test has an opening through which the pseudopodia extend.
Sage Wintle
Plasmodial Slime Molds
Plasmodial slime molds are essentially huge, multinucleate cells enclosed by a plasma membrane. They can be over a meter in diameter and are easily seen with the naked eye in their plasmodial form. They do not have plastids. They are heterotrophs, and may feed on bacteria, yeast, fungi, and decaying organic material.
Plasmodial slime molds form fruiting bodies in which spores are produced by meiosis. These are carried by wind and germinate to form gametes, which are either amoeboid or flagellated. Fusion of an amoeboid gamete with a flagellated gamete results in a diploid cell, which divides its nucleus mitotically many times. It does not complete cytokinesis, however, resulting in a large, multinucleate cell called a plasmodium.
Plasmodial slime molds can sense the environment through protein receptors. They have been investigated for their navigation abilities. Slime molds can “solve” mazes by finding the shortest path between food sources.
Cellular Slime Molds
Cellular slime molds are heterotrophic organisms that exist as individual amoeboid cells when food is plentiful. When food becomes scare, they begin to secrete a signaling molecule into the environment. This signals other amoebae in the vicinity to aggregate into a slug-like mass that can move. It eventually reorganizes into a fruiting body that produces spores.
Dictyostelium discoideum is an important model organism. It has been used for decades to studying cell signaling, taxis, programmed cell death (apoptosis), and the origin of multicellularity.
BIOL 1625 