This post was inspired by an interview I participated in with Pavini Moray on the Bespoken Bones podcast. The podcast will be available to listen to Spring 2019!
Today is St. Valentine’s Day, which, generally speaking, bugs me no end (I’m a miserable old git and I hate joy). However, it’s a pretty good excuse to talk about microbial sex, and the evolutionary impact that microbes have had on our sex lives (I’ll be keeping it general, don’t worry – this is not a confessional).
Rosie Redfield, who is a professor at the University of British Columbia, has written pretty extensively around this topic (she is also a total badass and her greater work is worth investigating if you’re at all interested in the smalls and microbial genetics). She has said in the past that she studies ‘why bacteria have sex, to figure out why we have sex’. It’s an interesting question – why DO we have sex?
Biologically speaking, sex is a way to mix the genes from two parents in order to produce a genetically distinct offspring. Parents that produce in this way (for example, humans), have two sets of genes (they are ‘diploid’): so you have two copies of every gene in your coding, one from your biological dad and one from your biological mum. Bacteria are haploid, and they have one set of genes instead of two. Generally, bacteria produce asexually, via binary fission. A copy of their genome is made and the cell splits into two, with the copy going to one of the ‘daughter’ cells. So, in theory, the daughter cells are clonal, identical, to the original cell (in practice this isn’t necessarily the case, but that’s another discussion). If you compare it to humans, this is a pretty efficient way of creating offspring. Take a population of 100 sexual reproducers, and a population of 100 asexual reproducers. With the system operating at full efficiency, 100 humans might be able to conceive 50 new offspring (if they’re feeling energetic enough). If you chop a clipping off of 100 tomato plants, you’ll get 100 new offspring. If you start with 100 cells of E.coli, within 20 minutes they will create an entirely new population of offspring…..20 minutes later those new offspring will produce offspring…..and then 20 minutes later…..you get the picture. Bacterial populations can grow exponentially.
So, evolutionary success is all about trying to get your own genes into an offspring, in order for those genes to perpetuate throughout the generations. Evolutionarily speaking, then, isn’t sexual reproduction a bit of a dead end? Having to combine two sets of genes instead of just passing on your own? Lots of beings reproduce asexually, many kinds of plants, animals and smalls. Clearly, sexual reproduction leads to diversity, but, to an extent, asexual reproduction does too (mutation is the spice of life). Bacteria also interact with external genetic elements, like viruses, that are constantly shuttling pieces of genomic information to and fro. Bacteria, in fact, can pretty much have sex on their own, via a genetic process known as ‘transformation’, although in many cases the DNA co-opted from the environment during transformation is used as food for the bacterium rather than a means to promote diversity.
However – bacteria also have sex in a way that is far more recognizable to us a being ‘sex’. So, how does this work (remember that haploid genome) and why do they do it? The how is far easier than the way. Under favorable circumstances, ‘conjugation’ takes place. A ‘male’ cell donates a chunk DNA to a ‘female’ cell via a structure known as a sex pilus. Members of mating pairs are characterized as being ‘male’ or ‘female’ only in as much as the male counterpart is the donating cell. Conjugation is, furthermore, not strictly heterosexual and in some cases two male cells have sex and switch bases (nucleiclly acidly speaking). Conjugation also crosses the boundaries of life and systems like this exist between some bacteria and plants.
Conjugation in bacteria is certainly an ancient undertaking, but it’s not entirely clear why they do it. Redfield notes that there is “no conclusive evidence that bacterial sex is for fostering genetic diversity…” rather, any diversity that arises seems to be a side effect of genomic repair and replication. And for those of you feeling any sense of superiority in regards to human sex versus bacterial sex – I have troubling news. The fact that ‘higher’ organisms reproduce sexually at all is quite probably due to other modes of DNA repair, also rendered necessary by another group of microbes.
Around 2.45 billion years ago, cyanobacteria started mediating the Great Oxygenation Event (aka the ‘oxygen catastrophe’ if you’re feeling dramatic). This was an unfortunate side effect of cyanobacteria developing the capacity to photosynthesize. There are two kinds of bacterial photosynthesis: anoxygenic, which produces stuff that isn’t oxygen, such as sulfur, and oxygenic photosynthesis. Oxygenic photosynthesis is what green plants do, and it produces oxygen. Green plants learned this trick from cyanobacteria. When cyanos started photosynthesizing and pumping oxygen into our planet’s early atmosphere, Earth was changed forever and the rest is history: mammals, humans, Starbucks.
Oxygen is super toxic, and at the time of the GOE life was particularly rough for other biota which were previously well-adapted to an anaerobic lifestyle. In-built into the code of DNA are some pretty robust mechanisms for repairing itself and most single-stranded errors can be handled. Oxygen, however, produces double-stranded errors and they can have serious implications for an organism. Scientists believe that oxygen stress and DNA damage are one of the reasons why, as eukaryotes developed, meiotic sexual reproduction became necessary. Meiotic sexual reproduction is where we started with this blog – two parents, combining their genes to produce offspring. This ‘recombination’ of two sets of every gene necessary for survival can repair double stranded errors produced by oxygen toxicity, and other mutagens.
So, why do humans have sex? Well, lots of reasons (duh) – some of which may well have been driven by the ancestors of contemporary bacteria. And why do contemporary bacteria have sex? The short answer is: we’re not entirely sure. But, hey. Why not?