Domain Eubacteria_ Domain Archaebacteria includes bacteria that live in exotic environments like hot springs and thermal vents in the deep ocean and some other marine bacteria. Domain Eubacteria “new bacteria include the common bacteria” we encounter in our daily lives. The third domain, Domain Eukarya (the eukaryotes), includes other organisms. It may seem surprising that all eukaryotes are put into a single domain, but the uniting feature is the similar structure of their cells. Three defining features of all Eukaryotes include: (1) DNA organized into rod-like chromosomes, (2) mitosis (bacteria divide very differently, a process called fission), and (3) eukaryotic cells have organelles. Therefore, protozoans, algae, plants, fungi, and animals are all considered subgroups under the domain Eukarya. Before the first living cells evolved, a number of features must have already existed that would add together to create the first eukaryotic cells. Some of those primordial features would have included cell membranes, RNA, and DNA. Both Eubacteria and Archaebacteria (called Archaea) have these three apomorphies, and their DNA is arranged into a circular chromosome (a fourth synapomorphy). Eubacteria differ from Archaea in that they have a substance called peptidoglycan in their cell wall. Archaea are placed on a branch (clade) closer to Eukaryotes because one of their RNA types is more similar to RNA in Eukaryotes than it is to RNA of Eubacteria. Also, Archaea have some proteins involved in their chromosome structure that Eukaryotes have, but Eubacteria don’t. A special feature of Archaea is that many of them can survive at temperatures greater than 100°C. Remember the Rules of Cladograms: Each “bar” in the cladogram represents a defining apomorphy. The placement of any “bar” on the horizontal line of the cladogram means that everyone on the right side of it (including vertical branches) has that apomorphy or some derived modification of it. Using a similar logic, any “bar” on a vertical branch defines everyone above that apomorphy. This means that an apomorphy placed on a vertical clade (branch) is not part of the evolution of any group to the right side of that clade. Label each of the apomorphies (“bars”) in the cladogram on the next page. Review the definitions of autapomorphy and synapomorphy before starting the cladogram.
The Correct Answer and Explanation is:
The cladogram in question describes the evolutionary relationships between the three domains of life: Eubacteria, Archaebacteria (Archaea), and Eukarya. To label the apomorphies in the cladogram, we need to consider the key features that define each group and their evolutionary significance.
Apomorphies and Their Placement
- DNA organized into rod-like chromosomes:
- This is a defining feature of Eukarya. All eukaryotes have DNA in the form of rod-shaped chromosomes, which is an advanced trait (synapomorphy) that evolved after the divergence of prokaryotes (Eubacteria and Archaebacteria).
- This feature would be placed on the vertical clade that represents Eukarya. The placement of this apomorphy separates Eukarya from Eubacteria and Archaebacteria, as these prokaryotic domains typically have circular DNA.
- Mitosis:
- Mitosis, the process of cell division in eukaryotes, is a key feature distinguishing Eukarya from prokaryotes, which undergo fission (a different process of cell division). This feature would also be placed on the vertical clade representing Eukarya.
- It marks the divergence of eukaryotes from prokaryotes and should be positioned as a synapomorphy for Eukarya.
- Organelles:
- The presence of organelles (such as the nucleus, mitochondria, and chloroplasts) is a defining feature of eukaryotic cells. This feature should be placed in Eukarya as well and acts as a synapomorphy for this domain.
- The appearance of organelles is a later development in evolutionary history, distinguishing Eukarya from both Eubacteria and Archaebacteria.
- Peptidoglycan in cell walls:
- Eubacteria are defined by the presence of peptidoglycan in their cell walls. This is an apomorphy (derived feature) that is unique to Eubacteria and distinguishes them from Archaebacteria and Eukarya.
- The placement of this apomorphy on the cladogram would be before the divergence of Eubacteria from Archaebacteria.
- Survival in extreme temperatures:
- Archaebacteria are known for their ability to survive in extreme environments, such as high-temperature habitats (e.g., hot springs, thermal vents). This is a specialized feature of Archaebacteria and would be placed as an apomorphy that defines this group.
- The ability to survive extreme heat would be placed on the branch leading to Archaebacteria, indicating that it is a defining feature of this domain.
- RNA similarity to Eukarya:
- Archaebacteria share more similarities with Eukarya in terms of RNA, particularly in certain types of RNA sequences. This would be an apomorphy shared by Archaebacteria and Eukarya, but not Eubacteria.
- This RNA similarity is an evolutionary feature that places Archaebacteria closer to Eukarya than to Eubacteria.
Explanation
The cladogram represents evolutionary relationships based on shared derived characteristics (synapomorphies) that define each group. The placement of apomorphies along the branches shows which characteristics are shared by organisms within each group.
- Eubacteria are separated by their distinctive feature of peptidoglycan in their cell walls.
- Archaebacteria are defined by the ability to live in extreme conditions, as well as their RNA similarities to eukaryotes.
- Eukarya is characterized by more complex features, such as rod-shaped chromosomes, mitosis, and organelles.
The concept of autapomorphy refers to a unique characteristic that is specific to a single group, whereas synapomorphy refers to a shared derived characteristic found in multiple groups. All of the defining features mentioned above are synapomorphies for their respective groups, helping to illustrate the evolutionary tree and the divergence of these three domains.
