So, what have I actually been doing during the last few weeks when I haven’t written much here. One thing is reading about gynogenesis for a presentation at a lab meeting. I will discuss this phenomenon in a few posts, starting today.
There is a class of words that are, basically, a negation of a perceived norm. Almost all these words (atheist, invertebrate, asexual, and so on) are entirely devoid of meaningful content (1), as they merely describe a group that is lacking something, and there is no reason to assume that objects that are united in not having a certain character are similar in any way whatsoever. My favorite examples has always been “invertebrate”, because it is easy to illustrate. We could easily draw a generalized vertebrate (jawed skull, spine, post-anal tail, shoulder and pelvic girdle, ribs) that would capture most of the relevant characters of most of the vertebrates (though few of them are present in all of them).
But it is impossible to do the same for “invertebrate”. The various groups of invertebrates are so dissimilar:
Do they have an exoskeleton? Some do, some don’t.
Do they have fins or other appendages? Some do, some don’t.
Do they have a distinct mouth and distinct anus? Some do, and some don’t.
Do they have eyes, antennae, or other sense organs? Some do, some don’t, and the ones that do are usually vastly different.
“Invertebrates” are not even consistently symmetrical, containing both bilaterally, radially, pseudoradially, and entirely asymmetrical (there’s another of those words!) organisms. Some are colonial, some are not. Some have elaborate larval stages, some do not. There is no cohesion of any sort in “invertebrates”.
The same goes for “asexual”. This is what sex looks like (excuse my crappy drawings):
(F means “female genome”, M means “male genome”)
As simply as possible, two individuals meet, some part of their body undergo meiosis in which their genome is divided into two equal halves called gametes (one chromosome of each pair in each half), these fuse, either internally or externally, to form a zygote, which then develops into a new individual, and the same cycle is repeated in that generation. Very simple, very elegant, but has some unique problems (see below). “Asexual reproduction”, meanwhile, is usually seen as this:
An individual does not undergo meiosis, but simply produces identical copies of itself somehow (this process is called “cloning”). There are a number of different ways that even this simple system happens (which I’ll come back to in a later post), and this system, called parthenogenesis, is perhaps the most common asexual system, but asexual reproduction is not limited to this system. Today, I will present another asexual system, which is very close to the parthenogenetic system, but differs in one vital aspect. This system is gynogenesis.
Gynogenesis is, according to a quite recent review, a system that combined the worst aspects of sexual reproduction with the worst aspects of “asexual reproduction”. I’ll ignore the details of this for now, and just show this:
Sexual reproduction has short-term disadvantages, but long-term advantages, while for parthenogenesis, these are reversed. I don’t know that either of the phenomena I listed in the top left box actually have both long- and short-term advantages, but they are the best guesses for candidates I could think of. However, gynogenetic organisms certainly combine many of the worst aspects of both systems. This is what it looks like:
Females produce eggs without undergoing meiosis, so the eggs are clones, and so far, the system very much resembles parthenogenesis. However, in order to initiate embryogenesis, the process by which the egg undergo division and develop into an embryo, the eggs need to be activated by sperm. These sperm do not fuse with the egg in the same way as in sexual reproduction, but are dissolved after having initiated embryogenesis (I’ll get back to some exceptions in a later post).
What is not obvious in the picture, perhaps, is the most interesting feature of this system: all the gynogens are female, and the sperm therefore necessarily has to come from a different species.
There are about 80 known species-pairs of gynogens and sperm-donating sexual species, and the gynogens are typically hybrids between two sexual species. Examples include several fishes (Carassius auratus, Poecilia formosa, Hypseleotis sp., some Poeciliopsis), some insects (Ips bark beetles, Muellerianella and Ribautodelphax leafhoppers, Alsophila moths), some Ambystoma salamanders, and some other organisms.
The “worst of two worlds”.
The problem with “asexual reproduction” is that, if all your offspring are clones, they cannot very easily respond to environmental or parasitical changes, as the genome will be more or less fixed. There is also a process called “Muller’s ratchet”, which is the idea that, to the extent that mutations do happen in asexual lineages, these will (as are probably most mutations) be at best neutral, and at worst negative. A sexually reproducing organism can get rid of these deleterious mutations, to some extent, by meiosis and recombination, but in a clonal organism, the bad mutations will accumulate, and eventually drive the asexual organism to extinction. However, the advantage is that an asexual organism can reproduce more quickly, and do not need to find a mate. A single copepod capable of cloning can easily fill a lake in one summer, which is impossible for a single copepod that is not capable of asexual reproduction.
The problem with sexual reproduction is, of course, the reverse: you need to find a mate, impress this mate to make him/her/it mate with you, and in the process, half your genome is lost to the next generation, and paired with a genome that you essentially know nothing about. However, you can “easily” evolve countermeasures for parasites, environmental changes, and so on.
Gynogens, therefore, get all the bad stuff from asexual reproduction, as they cannot get rid of deleterious mutations, and they cannot respond quickly to environmental and parasitical threats. However, as embryogenesis needs to be initiated by sperm, they still need to find a mate and entice him to give them sperm, so they don’t have that advantages of asexual reproduction. They make it even harder for themselves, of course, in that the mates they need to find and mate with are of a different species, which are invariably sexual and therefore will gradually evolve to become incompatible with the gynogen without the latter being able to respond.
Another problem with sexual reproduction, from the female’s point of view, is that half of your offspring will be males. This may not seem very problematic (it solves the problem of finding mates, for instance!), but the inherent problem with this becomes apparent when we look at a gynogenetic-sexual pair.
Let us assume that the amount of sperm that is produced by males in a population is not a limiting factor, but that the population is limited by resources or shelter, for instance. This would give us a constant population in that locality over time, but a large surplus of offspring could be produced every single year. Let us assume that survival to reproductive age is the same in gynogens and sexuals, and that fertility rates in both sexual and asexual females are the same. This would mean that the ratio of sexuals and gynogens in any given year is dependent only on the ratio of gynogens and sexual females the previous years (as males do not replace themselves in the population independently). We’ll also assume that the system is closed, so there is no migration in or out of, say, the pond. Let us finally assume that males cannot tell gynogens and sexual females apart.
We start with a population consisting of a third each of gynogens (red), sexual males (blue), and sexual females (green):
In the next generation, the gynogens will have received half the available sperm, and the sexual females will have received half. But the sexual females have produced 50% males and 50% females, so the ratio will look like this:
As we have unlimited sperm but fixed population size, the next generation will look like this:
We now have 50% gynogens, and only 25% each of the sexual sexes. The same process is repeated in the next breeding season, and we get:
In fact, this system will quite rapidly end up in a state where it is very likely that, by chance, all available males mate only with gynogens:
(The math may be off as I did this tabel while eating breakfast…)
If this happens, the sexual specie will of course become extinct that year, while the gynogen will persist until the next breeding season, and then become extinct. Supposedly, this is the case regardless of the starting percentages, but the proportion of gynogens will always increase, and drive the system (at least locally) to extinction. But some of these systems have persisted for 100,000 years. This suggests that the very simplistic set of assumptions above are not realistic, and that there are aspects that were not accounted for.
So how do these systems work?
Well, that is what I’ve been studying for the last few weeks, and I find it extremely interesting. I’m planning to go through some of what is known, using a well-studied model system, in the next few days. Next time, I’ll write about why the system is not driven to extinction, then about why the gynogens persist at all, and then we’ll see. There are plenty of interesting asexual systems to continue with.
(1) Some of the words, like “anarchism”, have subsequently been filled with content, but still share many of the same characters that the other words in this category have.