(Wolbachia is the little bright dots within the egg cell) Wolbachia is an intracellular parasite; it lives inside the main body cells (somatic) of both males and females but only the germ line cells of females (the female’s eggs). Being only found in female eggs, not male sperm, it has a vested interest in ensuring that the insects give birth to more girls than boys. As such the tiny bacterium goes about its absolute best messing about in a range very nasty ways with male insects.
Wolbachia will stoop to any level in its war against the male gender (not in humans...for now), here is short list of the tricks it has up its sleeve (and easy to read pictures to boot!):
It kills males during larval development, basically meaning that more females will be born each generation and hence there will be more wolbachia.
It turns males into females, the process of feminisation, resulting in either full blown females or the even less appealing infertile pseudo female.
It manages to let females get pregnant and have female babies without any men involved! This process has the awe inspiring name of parthenogensis. In the Trichogramma wasp, this has been so successful that males aren’t needed at all, the wasps population is now made up nearly all females, the males are redundant and only a few are born each generation.
Using a trick known as cytoplasmic incompatibility, uninfected male bugs are unable to breed with infected females. This cunning trick means that uninfected males have a lower fitness than infected ones, and hence more infected offspring are produced each generation and again wolbachia wins out by spreading its evil man hating ways through the population.
Some strains can prevent reproduction entirely if it is not present. The insects have become so reliant on the wolbachia parasite that they simply cannot live without it. Some species of wasps are now obliged to have this nasty critter in their systems; if you experimentally remove wolbachia from these wasps with antibiotics then they can’t have anymore offspring.
Wolbachia illustrates a general trend across much of evolutionary history, the trend towards feminisation and eventually asexuality. Wolbachia is a super charged version of this principle that is crushing populations of male insects around the world as we speak, but other animals are far from free of similar effects. Humans are also showing a trend towards feminisation.
Basically having different sexes is hard work, it takes a great deal of time and energy. If you need to have lots of gene shuffling (what all the kids are calling it these days) it is a good idea as it produces lots of variation, but it’s not nearly as efficient as good old fashioned asexual reproduction (but substantially more fun!). Over time species that have become established in a particular environment tend to move towards asexuality (as they don’t need all the variation). This is where we come in, we are doing well and released from large areas of natural selection (due to our good diets, modern medicine and lack of man eating tigers), and so we don’t need as much variation. So it seems that males are a waste of valuable resources, all we do is generate variation, we are not needed for producing new offspring. Selection has kicked in and has begun to whittle away at the Y chromosome (which is tiny by comparison to the X anyway). Y chromosome size has declined dramatically over the last few million years. The data is staggering today 7% of males are infertile and ¼ of these cases are new and not traceable to their fathers. This is getting worse, one by one the 27 remaining active Y chromosome genes will disappear, reduced by the relentless onslaught of mutation, and then men will become extinct!
Indeed there is the distinct possibility of humans becoming parthneogentic in the future, with females capable of producing cloned or slightly genetically reshuffled offspring. In addition it would only take a few choice mutations to get females to reproduce sexually with other females. There are incompatibility issues at present, but it is entirely possible for the nuclei of two female eggs to fuse to produce female only offspring. With some clever artificial insemination this trick could be closer than we think.
To put it bluntly males may well be doomed, fortunately for us guys this is a long term problem (some people predicting 125,000 years for total elimination of human males), but spare a thought for our insectiod man friends, who may well be on the out much sooner.
Awesome post!
ReplyDeleteIs the decreasing size of the y chromosome the direct cause of decreasing male fertility rates? If not, is it by some other mechanism?
I have heard lots about decreasing fertility rates in developed countries, and heard talks from folks in the public health field that would have me believe it is due to endocrine disrupting chemicals. Is it possible that these two factors are at play, the genetic and chemical?
Question, though ... wouldn't basic Darwinian evolution select very strongly for males with healthy Y chromosomes? I'm thinking that even a slight difference in the production of healthy sperm could lead to a very large cumulative difference in chance of living male offspring, say, 3-4 generations down the road. Evolution can act on very small differences like that, and every now and then you'd get a favorable atavism or even new mutation helping out.
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