When we last left off, we were exploring the tangled web of microbial evolution, where not all genes stay where they’re ‘supposed’ to, thanks to lateral gene transfer (LGT). This gene flow has been crucial in allowing bacteria to gain new traits from unrelated bacteria in their environment and not just from their direct ancestors. Now we understand that the traditional concept of a family tree doesn’t fully capture microbial evolution, and instead, it resembles a tangled network, with genes being shared between different species.
But here’s something interesting: LGT doesn’t treat all genes equally, meaning that not all genes are transferred with the same likelihood. Some genes are found to move frequently between different species, while others almost never do. However, this isn’t random, and there are biological reasons behind it. Two main ideas try to explain this: the complexity hypothesis, which suggests that highly connected genes (those that interact with many other proteins) are harder to transfer. We can imagine this with a simple analogy. If we think of some genes like special puzzle pieces, then we know that some pieces, like corners, can somehow fit into different puzzles. But a very detailed and weirdly shaped piece only fits into one specific spot. In the same way, highly connected genes don’t transfer easily and hence only work well in their original cell, because they need to interact with a bunch of other specific genes. Move them into a new cell, and they might not fit in, or work properly because these specific interactions may not exist. The balance hypothesis adds that even if the gene makes it into a new cell, it must be expressed at the right level. Making too much or too little of a protein could disrupt the cell balance, kind of like when you add too much yeast when baking bread.
So, genes that don’t rely heavily on other genes to do their job are much more likely to be transferred. Examples include genes that are involved in adaptive functions, which are traits that help microbes survive under specific environmental pressures. A classic example is antibiotic resistance genes. These genes allow bacteria to survive the presence of antibiotics, by breaking them down, pumping them out of the cell, or altering their targets so the drugs stop working. This makes them great candidates for transfer: if a bacterium picks up one of these genes at the right time, then it can gain a huge survival advantage.
On the other hand, genes involved in core functions, like DNA replication, transcription, or central metabolism, tend to stay put. These genes usually work as part of tightly integrated systems, requiring cooperation with many other genes and proteins. Transferring just one gene (or a small part of the system) usually isn’t enough to make it work, unless the entire set comes along. This can often be too costly for the cell to maintain, but it isn’t impossible. In fact, microbes have literally defied these limits by occasionally acquiring entire gene clusters or operons, allowing complex functions to transfer as a whole.
However, the barriers to LGT likely differ depending on the type of gene, the recipient cell, and the mechanism of transfer, since what might be a successful transfer in one context might completely fail in another. I often wonder how many potentially useful genes never succeed simply because the right conditions or receivers aren’t present. To add to that, how much do interactions between microbes and their environment influence whether transferred genes persist, and how can we study this in a way that truly reflects natural conditions? When I say ‘environment’, I mean the whole context where the bacterium lives and evolves, not just the physical place but all the factors that influence its biology. Understanding these differences is key to fully grasping how genes spread and shape microbial evolution.
But you might still be wondering, how do these genes actually jump between cells? What carries them? And once a gene actually makes it into a new host, what happens next? Does it stick around, or does it get thrown out? These are questions many researchers, including myself, are working to answer. Much of the credit goes to a fascinating class of DNA known as mobile genetic elements (MGEs), which are crucial vehicles of LGT. There’s so much more to this story, and it only gets more interesting from here. I’ll explain more about this in my next post, and I hope you stick around!
