Reproduced with permission of S.B. Johnny.
(b) The life cycle of slime molds.
Source: Image of Physarum reproduced with permission of Jerry Kirkhart.
Researchers at Hokkaido University in Japan created an artificial map of Tokyo and its surrounding areas where local towns were represented as food (oat flakes), and mountains and other obstructions represented as light areas (the plasmodia avoid light). The slime mold created networks that were similar to the Tokyo rail network. This likely reflects the way in which the plasmodia reconfigures to make the most efficient connections between food, a form of energy minimization, similar to the objective of railway designers. These intriguing experiments, which we will not discuss further here, serve to illustrate that “single-celled” organisms can take part in complex behaviors more akin to complex multicellular organisms.
Although slime molds are eukaryotic, there are examples of cell differentiation in prokaryotes as well, for example, the formation of bacterial spores in Bacillus and Clostridium, and the formation of different motile and non-motile cells in biofilm-forming bacteria such as species of Bacillus and Pseudomonas. Some bacteria, such as the Myxobacteria, engage in similar cellular behaviors as eukaryotic slime molds, forming swarms of cells and specialized fruiting bodies.
The astrobiological significance of these observations is that separating organisms into “single-cellular” and “multi-cellular” is likely to be over-simplified. These observations also suggest that the transition from prokaryotes that are undifferentiated and single-celled to more complex multicellular eukaryotes with irreversible differentiated cellular structures may not be such a radical categorical transition as we like to think. You will often see this depicted as a “one-time” major evolutionary transition. It seems more likely that multicellularity is not a binary feature of life, but that intermediate states of complexity exist.
Discussion Point: When Did Multicellularity Evolve?
The discussion in this chapter should have convinced you that the title of this discussion point is probably too simple. Many prokaryotic and single-celled eukaryotes exhibit behavior that is multicellular, from the swarming activities of slime molds and Myxobacteria to the specialized cells found in some bacteria, such as spores. Nevertheless, it still remains a cogent question to ask ourselves why cells tend to form multicelled structures. Why isn't Earth covered in cells all individually going about their lives? One obvious answer is that energetic needs force collaboration. One microbe's waste can be another one's food, and thus associations will be selected for, as we will see in microbial mats in the next chapter. Cell differentiation, for example the formation of fruiting body cells and swarming cells in Myxobacteria, can be explained in terms of the efficiencies to be gained by specialization, where each particular cell can carry out a function better than a very general cell that must do everything. What about irreversible differentiation, for example in human cells? Imagine a version of yourself where your cells could all break down and spread out looking for food and then regroup? Why did cells give up this versatility? One answer could be that it was energetically favorable for cells to irreversibly commit to certain roles in an organism. Alternatively, perhaps the success of a complex organism like an animal in the environment removes the selection pressure for it to be able to disperse, and those capacities in cells were lost? There are clearly many fascinating questions to be answered. What do you think about the emergence of multicellularity and the steps that led to plants and animals?
5.12 Viruses
This chapter has focused on cellular structures. Insofar as they constitute self-replicating life forms, this is justified, but there are other entities on the planet that are linked to the biosphere and which play an immensely important role in ecosystems and biological interactions. Two of them are viruses and prions, which we now explore. Quite apart from their biological importance, they challenge the neat separation of material into “life” and “non-life,” and are worth considering from this perspective. You might like to explore Chapter 2 again where viruses were discussed in the context of ideas about the definition of life.
Discussion Point: Astrobiology and Viruses
The idea of sending missions to other planets to search for viruses may seem at first a little unlikely. However, consider the fact that viruses may have evolved multiple times and in some environments outnumber cellular life forms. If biospheres of cellular life always produce associated “particles” that replicate in those cells, and those particles are pervasive, then perhaps a search for such associated materials in addition to cellular life itself is a sensible objective of planetary exploration? Even if we never find another biosphere in which to search for viruses, contemplation of viruses in the cosmic context invites several other questions such as: Does a biosphere always produce non-cellular entities that replicate inside cells? In other words, are virus-like entities an inevitable by-product of a planetary biosphere? Discuss this idea in the context of what you know about viruses and their characteristics.
Berliner, A.J., Mochizuki, T., and Stedman, K.M. (2018). Astrovirology: viruses at large in the Universe. Astrobiology 18: 207–223.
Griffin, D.W. (2013). The quest for extraterrestrial Life: what about the viruses? Astrobiology 13: 774–783.
Viruses are not cellular structures and require cells for replication. Although many people associate viruses with animal disease, many (known as phages) infect bacteria and archaea, and they are known to be infectious agents in extreme environments such as volcanic hot springs and polar lakes.
Viruses are microscopic infectious particles that have a diversity of shapes and forms, from helical to bottle shaped. They are constructed of either single- or double-stranded DNA or RNA surrounded by a protein coat called a capsid. Some of them are surrounded by a structure called the envelope, which is made of lipid and often has glycoproteins within it. The envelope plays a role in facilitating access to the host cell. In some sense, although viruses are not cellular, they still exhibit compartmentalization of their structures. In the case of the Tobacco Mosaic Virus (TMV), the capsid forms a long, thin tube that surrounds the RNA (Figure 5.26). Viruses are typically 20–350 nm in size, much smaller than even prokaryotic cells. Their inability to replicate on their own has caused controversy about whether viruses can be considered “life.”
Figure 5.26 The structure of the Tobacco Mosaic Virus (TMV). The diagram shows its nucleic acid and protein coat. Also shown is a micrograph of the virus.
Source: Schematic reproduced with permission of Thomas Splettstoesser.
The differences in the genetic material of viruses are important. RNA viruses carry the enzymes needed to translate the RNA into proteins, meaning that they can often complete their life cycle in the cytoplasm of eukaryotes. Some RNA viruses such as the retroviruses (which include Human Immunodeficiency Virus, HIV) encode for the enzyme reverse transcriptase, which allows the RNA to be converted to DNA, which is then subsequently incorporated into the host genome as a provirus.
Viruses mediate important processes in the biosphere. They play an immensely influential role in the cycling of carbon in the biosphere by breaking apart or causing the lysis of bacterial cells in which they are reproducing, thus recirculating carbon in the oceans. They can mediate the transfer of genetic information from one cell to another as they infect and reproduce inside cells. Although they may not fit within a
