Asexuality Awareness Week is later on this month, so Aisling Brennan explores what asexuality means across many domains of life, including ourselves.
BEFORE we were complicated enough to enjoy the perks of multicellular forms, way back around 1.2 billion years ago, asexual reproduction was all we had. At least, that’s when some brave eukaryotic cell thought to jumble up their genes and thus recombination became a thing. In fact every sexually reproducing organism on the planet is descended from that one cell.
In the long and tumultuous history of evolution since then, asexual reproduction, and asexuality in general has taken on many forms and meanings. From bacteria far and wide to our own human brothers and sisters, asexuality has come a long way.
Anyone who’s enjoyed the delights of Leaving or Junior Certificate Biology knows the drill when it comes to asexual reproduction: single-celled organism (think your garden variety bacteria, prokaryotes, even unicellular fungi) decides to heed the call of the natural order of things and reproduce. Only instead of putting some of its genes into separate egg baskets, it just duplicates all of its genetic material and splits itself down then middle, making a clone.
If you’re a prokaryote – a bacteria or other such unicellular organism lacking the joys of proper cell organelles – the method of choice is binary fission. On the other hand if you’re baker’s yeast you’ll probably prefer budding. That is growing a little ‘daughter’ cell that is still attached to the ‘mother’ for a time. Sort of like those of us who annoy our parents by refusing to move out. And if you want to be particularly gruesome and parasitic there’s endodyogeny, in which two daughter cells grow inside the mother cell… and then eat their way out before dividing. Lovely.
This is also how we, as multicellular organisms, grow steadily from miniscule fertilised egg to (nearly) fully-grown college students. Cells in our bodies make more of themselves to mend and grow everything from our bones and brain to our often quite damaged liver. In fact, it’s when we start to look at the multicellular organisms that things start to get even more interesting.
Strawberries are a prime example. We know they have seeds, but on the sly they’ll send out runners – stalks that reach across a distance, bury themselves in the soil and spring up again as a genetically identical strawberry clone.
Sticking with plants and fungi, but also branching out into the realms of (the slightly more biologically relatable) animals, we have fragmentation. Essentially fragmentation does what it says on the tin: a new organism can grow from a fragment of the original organism. This can mean developing a fragment of your very own that you can send off into the cold, cruel world to grow into a fully-fledged individual.
Or if you’re a lichen (a symbiotic fusion of plant and fungi) you might want to save future generations from having to find the fungi or plant that’s right for them and simply flake off a bit of both to get them started. Alternatively, it can be a bit weirder, like in some planarian worms. When they want to reproduce they simply lop off their tail end, and it goes on to grow into a responsible adult planarian worm that don’t need no gametes.
Speaking of strong independent individuals, we can also look at agamogenesis, which is (broadly) any form of reproduction that decides to do without a male gamete. This might seem strange to us, but it is surprisingly widespread. Occurring not only in plants but also a variety of insects and invertebrates, amphibians, sharks, reptiles, and even the rare bird. In animals this is called parthenogenesis, which directly translated back to its Greek roots means ‘virgin creation’.
Parthenogenesis occurs in a variety of forms throughout the insect group Hymenoptera (bees, ants, other colonial insects), and helps in the production of new queens, exclusively ‘male’ or ‘female’ communities and sterile worker populations. At least two species of goblin spider (family Oonopidae) are thought to reproduce parthenogenetically (though this is mostly through it occurring in lab environments) and this could help explain why no males of these species have ever been found.
Unfortunately, being humans, we can have some difficulty relating or comparing ourselves to anything lacking a spinal column, so here’s some good old fashioned vertebrates to start hitting the asexuality message home.
Parthenogenesis has been observed to occur naturally in a variety of reptiles, including Komodo dragons, whiptail and rock lizards, geckos, boas, pythons, rattlesnakes and many more. Some, like whiptails, the brahminy blindsnake, mourning geckos, and others, are obligatory parthenogenetic (they don’t have a choice in the matter) and are thus unisexual.
Other asexual, and decidedly spine-possessing, animals include at least 3 species of shark, though it is a possibility in several others. The fact that they live way down where we can’t breathe tends to impact on how much we know about them.
One of the poster children for parthenogenesis in vertebrates is the New Mexico whiptail, part of this is because during their extensive study, it was found that although the species is unisexual and can reproduce asexually, mating behaviours were still seen. One lizard would act as the dominant and mount the other, but the behaviour served no biological purpose in the life cycle.
Or did it? Eventually research showed that the behaviour was due to hormonal cycles, with lizards that had just laid eggs showing more ‘male like’ behaviour. The link between hormone levels and mounting meant that lizards who were allowed to display this behaviour (versus those kept in isolation, where they could still quite happily reproduce) had a greater reproductive rate.
And that vertebral section brings us all the way from brave unicellular organisms to, you guessed it, us. Humans can’t reproduce asexually, but that doesn’t stop us from claiming the work ‘asexual’. Asexuality in humans refers to the subset of the population that doesn’t, or rarely does, feel sexual attraction. Estimates tend to hover around 1% of the human population being asexual, but regardless of statistics it refers to a whole spectrum of identities and experiences, unique to our species.
So from the smallest of microorganisms, to the weird and wonderful spread of the multicellular kingdom, to human kind itself, asexuality is everywhere!