What are the general characteristics of a prokaryote shown below? Lack of a nucleus? Cell walls made of specialized sugars called peptidoglycans? Lack of membrane-bound organelles? All of the above?
What about none of the above? What if we found a prokaryote that lacked all of the items listed above, but was by all other characteristics, a prokaryote? What would that tell us about the evolution of the bacteria?
Many times in the classroom we are ask students to take an evolutionary “leap of faith”. Since bacteria do not easily form fossils, then we may never really find the elusive link between the prokaryotes and eukaryotes. The evolution of the eukaryotes is a logical series of events, shown nicely by the endosymbiotic theory – but to be truthful, something tangible that we can point to in the classroom has been hard to come by. There has always been Giardia, the parasitic eukaryote (protistan) that lacks a mitochondria, but most evidence suggests that Giardia evolved from a eukaryote that possessed a mitochondria – so it is probably not our missing link.
However, scientists at the University of Queensland have identified a group of bacteria – called the planctomycetes, that are closer to a missing link than anything we have had in the past.
Planctomycetes are an interesting group of bacteria. They possess a form of intercellular compartments that appear to have specialized metabolic functions. One of these, called an anammoxosome (breaks down ammonia) that appears to have a similar function to the eukaryotic mitochondria. The DNA of a planctomycetes is contained within a membrane-bound nucleoid region – not quite a nucleus, but it definitely represents an internal compartment for the genetic material. Also, most of the planctomycetes lack peptidoglycans (a sugar-amino acid combination) in their cell walls. The presence of peptidoglycans is a defining characteristic of bacteria in general, and is the target of many forms of antibiotics – the fact that the planctomycetes are lacking this compound suggests that they are not a common form of bacteria. In addition, planctomycetes tend to (but not always) reproduce by budding instead of binary fission. Yeasts, a one-celled eukaryote, reproduce by budding. (Looking for a comparison of a planctomycete with a common bacteria?- download our PowerPointfor use in the classroom)
So what does this mean? As pointed out by the researchers at the University of Queensland, too often model organisms, such as E. coli, are used as the basis for lectures on bacterial physiology. And while there is a place for the model organisms, the exclusive use of them does not help our audience understand the wonderful complexity and diversity of the prokaryotes. When students see an E.coli under a microscope, they have a hard time making the connection of how this type of organism evolves over time to become a significantly complex eukaryotic cell. But if we show them that there are examples in nature of more complex prokaryotes, some of which may be the missing link, then we can help them make the connections they need in order to understand how complex life evolved from simple, one-celled organisms.
- Fuerst, J., & Sagulenko, E. (2011). Beyond the bacterium: planctomycetes challenge our concepts of microbial structure and function Nature Reviews Microbiology, 9 (6), 403-413 DOI: 10.1038/nrmicro2578
- Tree of Life (Web) entry for Planctomyctes
- Planctomycetes PowerPoint file comparing the planctomycetes with a common bacteria
- Classroom handout (pdf)