from elephantiasis to sperm competition

Well, it’s not too great a leap, is it? I thought of this post because over on the Sciblogs copy of my last item we started talking about sperm competition. We got there via Drosophila bifurca.

The male D.bifurca produces the longest sperm of any animal – an amazing 58mm. Most of that’s tail. Each sperm is rolled into a tiny ball within the male’s seminal vesicle (part of his reproductive tract) & then, when he mates, fired like a pea from a peashooter into the female’s vagina.

As you can see from the following image, in D.bifurca the male & female gametes are of broadly similar size (in technical terms, they are isogamous). 

a, Scanning electron micrograph (SEM) showing a single, 6-cm D. bifurca spermatozoon dissected from the seminal vesicle, where sperm are individually rolled into compact balls. b, SEM of a single D. bifurca sperm (copied six times) next to an SEM of a D. bifurca ovum at the same magnification. Micrographs by R. Dallaifrom Bjork & Pitnick, 2006.

And these long sperm are produced in very long, relatively large testes: in D.bifurca they make up around 11% of the male’s body weight. There’s a trade-off here, though; such big sperm are costly to make & so relatively few are produced. A male bifurca produces only 6 sperm for every egg that a female makes.

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The testes of Drosophila bifurca fruit flies make up 11 percent of the dry body mass of the male. In this image, a male is "surrounded" by an uncoiled testicle dissected from a male of the same size. Credit: Romano Dallai

Normally sperm are much much smaller than the eggs that they fertilise (anisogamous), as in the following example of a human sperm & egg. In animals with anisogamous gametes the male produes huge numbers of these tiny sperm, which increases the odds that one of them will actually make it to the egg. In these circumstances there’s significant male: male sexual selection, via sperm competition – the male producing sperm that swim longer, or are more active, is more likely to fertilise the female & so any genes relating to sperm size, stamina (if that’s the right word!) & activity will tend to be selected for. What’s more, in these circumstances eggs will be quite rare & thus ‘valuable’, so there’ll be strong male:male competition to be the one to fertilise them.

Bjork & Pitnick note that in an animal like D.bifurca, where sperm & egg are much closer in size, & males produce fewer sperm, you’d thus predict a reduction in competition. So bifurca provded an excellent test case for this prediction.

Now, when I first heard about D.bifurca (via an item in the book Blue genes & polyester plants, the story went that much of an individual sperm’s tail was left hanging around outside the female’s body, with only the head of the sperm making it anywhere near the egg membrane. This could be an example of sperm competition, where the sperm tail from a successful mating blocks the female’s vagina & makes subsequent fertilisation less likely. What actually happens is even more interesting: the sperm moves up into the female reproductive tract until it’s entirely housed within her body: her reproductive tract is around 60 mm long, lying like a loosely coiled spring within her abdomen.Bjork & Pitnick argue that the exceptionally long sperm of male birfuca are the result of intense sexual selectio, where only the longest sperm get the egg. Sperm competition may still be operating here, as it’s going to be harder to swim up to the egg if there’s already another sperm in there, blocking the way.

In some species sperm competition is a lot less subtle. In black-winged damselflies the tip of the male’s penis is shaped a bit like a brush. Females take multiple mates, & when the latest male comes along, before he actually inseminates the female he pumps his penis in & out of her reproductive tract, brushing out most of his competitiors’ sperm as he does so. Chimpanzees – which are promiscuous – simply produce huge amounts of sperm: the male getting the most sperm inside a female chimp will be the most likely to successfully fertilise her (always assuming that sperm motility & viability are similar in all males involved. (Those massive quantities of sperm are produced in commensurately-large testes. A pair of testes in a male chimp weigh around 120g, while in the much larger gorilla – where a single male has a harem of females & mates with them exclusively, relatively undisturbed by other randy males – the testes weigh in at only 30g.)

But wait, there’s more. The butterfly Cressida cressida takes sperm competition to even greater lengths. After mating successfully, the male applies a type of cement to his partner’s vaginal opening, blocking it up so that no other males can get in. Insect chastity belts, anyone?

A,Bjork & S.Pitnick (2006) Intensity of sexual selection along the anisogamy-isogamy continuum. Nature 441: 742-745  

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