This week’s BEACON Researchers at Work blog post is by MSU graduate student Rohan Maddamsetti.
In his treatise on love, Symposium, Plato tells a myth of a time when men and women were one. People used to have one head with two faces, and four hands and four feet. Terrible was our might and strength, and the thoughts in our hearts were great; so Zeus punished us for our hubris by dividing each of us into two halves. After this trauma, each half, desiring to heal his or her state, came together, and throwing their arms about one another, entwined in mutual embraces, longing to grow back into one.
Why do men and women exist? Finding love is difficult, and since males need to find females (and vice-versa) to reproduce, the dating pool is half as large as it could be if we were hermaphroditic, like leopard snails. Reproduction would be even easier, if we could simply bud off clones of ourselves whenever we felt like it, as Hydra do. Why is there such diversity in how living things reproduce?
Evolutionary biologists have struggled to answer these questions, ever since Charles Darwin and Alfred Russel Wallace first originated our discipline. I certainly don’t know the full answer—however, there’s a lot of exciting research these days that I think will resolve these questions in the years to come. One theme of my research is the origin of sexual reproduction, which I’m defining here as combining genes from two parents to make an offspring. However, I’m attacking this problem from an unusual direction.
I largely study how genomes change over time in bacteria. I’m interested in both long, geological timescales (Escherichia coli and its close relative Salmonella enterica shared a common ancestor 120-160 million years ago) as well as short timescales that I can observe in the laboratory. A fascinating difference between bacteria and complex animals (cats, people, squid) is that bacteria exchange genes willy-nilly with each other, even without having to reproduce. This process, called horizontal gene transfer, is akin to being able to grow bark-like skin after picking up the necessary genes by touching a tree. Many dangerous pathogens have acquired antibiotic resistance from unrelated species this way. Horizontal gene transfer is pervasive. In one study, the authors examined the genomes of 20 different Escherichia coli strains and found that in sum, these bacteria contained a grand total of 18,000 different genes, but had only 2,000 genes in common! Even though bacteria divide asexually, they still exchange massive numbers of genes.
The reproduction of viruses and parasitic genes called transposons drive much horizontal gene transfer. Transposons are so named, because they copy and paste themselves in genomes. Hence, they appear to “jump around,” or in other words, transpose themselves. When these selfish genes reproduce by copying and pasting themselves, sometimes they carry along some extra genetic baggage, say genes allowing Japanese people to metabolize the carbohydrates found in seaweed. Based on this fact, I have been working on understanding how a gene-centered view of evolution in bacteria can explain how genomes are put together. In my current view, genomes are made up of cooperative networks of genes that co-evolve. In some sense, all living things are not simple entities, but communities made up of genes with their own individual interests. Usually, the genes in a genome work together nicely. Sometimes, however, the selfish reproductive interests of a few genes prevail, to the detriment of the entire organism. For instance, more than 10% of the human genome consists of a particular kind of jumping gene called an Alu element. When Alu elements spread, by copying and pasting themselves throughout the human genome, they can cause many different kinds of disease, including breast cancer, hemophilia, and type II diabetes. Bacteria often profit from the free exchange of genes. In more complicated organisms, like humans, perhaps the risk of breaking something complicated (such as brain development) outweighs the potential benefits of gaining a gene that can do something new.
So, perhaps sexual reproduction originated as a mechanism for preventing the genome from being hacked by rogue genes, or viral sequences. By finding a partner to share genes in order to produce an offspring, maybe complex organisms can find a partner with fewer parasitic genes, or can recombine genetic material in such a way to minimize the spread of parasitic genes. The alternative might be the gradual accumulation of genetic parasites over the generations, eventually leading to a “mutational meltdown,” when the load of genetic parasites overwhelms rest of the community of cooperative genes in the genome.
Why are there men and women? Why do some creatures duplicate themselves, while others have to put on spectacular displays to find mates? I am one of the many biologists—including several BEACON researchers—working to find the answers to these fundamental questions.
For more information about Rohan’s work, you can contact him at maddamse at msu dot edu.