The mysteries of hermaphrodite slug sex

Why does hermaphrodite slug sex stop at Preston? Professor Steve Jones tells Beki Hill why sex with yourself can sometimes be advantageous.

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Beki Hill: Sex! Well, hopefully that’s sparked your interest – today we’re talking about something Woody Allen alluded to in the comedy Annie Hall – sex with someone you really love… yourself. I think we can all guess what Allen’s character in the film meant, but Professor Steve Jones from the genetics department at UCL has a slightly different take on it…

Professor Steve Jones: So if you’re both male and female why the hell not do it to yourself?

BH: That’s right, this time on the Elements podcast we’re talking about hermaphrodites. Hermaphrodite slugs to be precise. I’m Beki Hill and I went to speak to Professor Jones about his work exploring genetic diversity. This describes the differences between the fundamental parts of every living being – their DNA. A huge contribution to genetic diversity is the ability to reproduce - even small differences can have huge effects on a population over time. Professor Jones explains that this is due, in part, to the two-fold cost of sex:

SJ: Now every time a woman or a female has sex or has a baby, she is copying somebody else’s genes; the male’s genes, at her own expense so she’s also producing sons who are about to do the same thing.

BH: All the way back when, Charles Darwin found that some plants and animals whose ancestors did have sex, had just about given it up entirely.

SJ: One of the ways you can study giving up sex is in hermaphrodites, so if you’re both male and female why the hell not do it to yourself? Because then you’re guarantee to have sexual reproduction and you’re guaranteed to copy your own genes. And what Darwin found was that in pants there was often a complicated mechanism to stop that happening.

BH: We normally think that this sort of self-fertilization can lead to the accentuation of genetic flaws – think about the problems caused by in-breeding in families from brother-sister or even father-daughter matings. But can this be advantageous in some animal communities? To find out more we have to get back to the intriguing matter of hermaphrodite slug sex. Why would anyone want to know more about this slimy topic? Well their mating actually has a very interesting story to tell…

SJ: In many parts of the world, down here in the deep South, in London, they have sex. Boy-girl meets girl-boy and if you look at the DNA and proteins of slugs down here they’re variable, the same as we are. But if you go to the North of England you find that all the slugs are the same; you look at DNA and it’s all the same. You do some experiments and it turns out that up there actually boy-girl meets girl-boy, they’ve given up on – they’re doing it to themselves. So if they all self fertilise, the male part and female part get together, very soon, just like an inbred line of mice, you end up with a strain in which they’re all the same. This actually turns out to be well know in plants – there are many many in the North of England and Scandinavia which are all the same, but in the tropics that doesn’t happen.

BH: Professor Jones explained that this is all to do with the kind of enemy you face and for slugs this seems to depend on where you live –be that a Northern town or the southern tropics…

SJ: If you go to Preston or to Glasgow, to give you the real rough environment, who are your enemies? They’re the cold and starvation, shortage of food, and under those circumstances they’re always the same every year it’s going to be cold and shortage of food and certain genes or combinations of genes are going to be particularly good at dealing with that, and if you’ve got that combination of genes it pays, as it were, to keep it by not having sex and jumbling it all up and having to start again.

But if you go to the tropics: who are enemies? It’s not generally shortage of food and cold – it’s disease and parasites and predators. Now if your disease organisms are having sex, like the malaria parasite which has super-sex and recombines massively (and that’s one of the reasons it’s very hard to deal with). If your enemies are having sex you yourself are stuck you cannot afford to give it up. And what we call an arms race starts, you have to recombine and generate new combinations and mix up genes or sooner or later your enemies are going to draw a hand of cards and kill you off.

There are classic examples of where that’s happened – the classic is of the Irish potato famine of the 1860s where potatoes were planted throughout Ireland from just one strain; one clone in fact, and everybody was perfectly happy for several years and then one year there was a mutation in a fungus which killed the entire crop off and thousands of people starved.

BH: So, sometimes sex with yourself can be hugely damaging, but at other times it can keep your species alive. Or maybe the real lesson is that Northerners are more likely to do it on their own?