PZ Myers. 2005 Jan 21. Speaking of sex differences…. <http://pharyngula.org/index/weblog/speaking_of_sex_differences/>. Accessed 2008 Dec 01.

Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Friday, January 21, 2005

Speaking of sex differences…

…here's a real sex difference, reported in Science.

The odd group of insects called twisted-wing parasites, or more formally Strepsiptera, is easily overlooked. Spending most of their lives hidden inside other insects, the majority of the 596 known species have been identified only from adult males caught during their brief mate-seeking flight. "These are really, truly enigmatic insects," says David Grimaldi, entomology curator at the American Museum of Natural History in New York City. "They break all the rules."

The differences between males and females of the same strepsipteran species are extreme. Adult males are small, flylike creatures, whereas most adult females resemble grubs and remain inside their host, merely protruding their fused heads and thoraces when ready to receive a male's sperm. In one strepsipteran family, males and females actually parasitize different kinds of insects. "Everything you find about them is like they came from outer space," says population geneticist J. Spencer Johnston of Texas A&M University in College Station.

Don't take the "came from outer space" bit literally—they are terrestrial arthropods in good standing, even if their relationships to other arthropods are a little bit obscure. But they are definitely bizarre. They are parasites of other arthropods with an extreme sexual dimorphism—the ladies look rather like Jabba the Hutt, to me. Here are a couple of examples:

strepsipteran malestrepsipteran female
Adult male, left, and female, right.

The females lurk inside their hosts, with their heads and thoraces bulging out through the intersegmental membranes, and secrete pheromones that attract the more typically insectile adult males, which flit to the females and mate with their heads. The females have a specialized opening between their antennae/mouthparts called the brood passage that channels sperm back to their more conventionally located reproductive tract.

strepsipteran
Adult female Eoxenos laboulbenei, left, and abdomen of a wasp infested with Xenos vesparea, right.

The females retain a swarm of eggs (on the order of a thousand) and allow them to develop internally, and then release the motile larvae, called triungulins, back through the brood passage. That's right, they mate through the "face", and give birth back through the same place. The larvae then crawl or spring to new hosts by curving their abdomens ventrally, then snapping it back to hop forward. When they find an insect host, they secrete a digestive enzyme that dissolves a patch of host cuticle, and they burrow inward to molt into a grub that lives within the body cavity.

strepsipteran
Adult male Eoxenos laboulbenei, left, and larval (also called a triungulin) Halictophagus tettigometrae, right.

Strepsipteran parasites also do odd things to their hosts. Some species modify the host's secondary sexual features, flipping them into the form carried by the opposite sex, basically transgendering them. Others may secrete hormones that cause the gonads to atrophy, causing parasitic castration. They may also increase the life expectancy of their hosts, either by behavioral changes that reduce predation or directly, by physiological changes that reduce the host's sexual activity—sex is risky and exhausting, so minimizing the activity may boost one's lifespan (whether the host thinks it's worth it is an unanswered question.)

Proffitt F (2005) Twisted Parasites From "Outer Space" Perplex Biologists. Science 307(5708):343.

Posted by PZ Myers on 01/21 at 03:23 PM
Science • Organisms • 0 Trackbacks • Other weblogs • Permalink
  1. Aren't living things a kick? There is just so much to learn.
    #: Posted by  on  01/21  at  03:36 PM
  2. I prayed to The Creator for Science, and I got science. Damn! This is fascinating stuff. Thank you! (Oh, excuse me: Amen)
    #: Posted by  on  01/21  at  03:38 PM
  3. Begging ... Praying. Yea, pretty much the same thing.

    Amazing information. Much appreciated as always.
    #: Posted by  on  01/21  at  03:47 PM
  4. Cool.

    Isn't there also a radical male/female dimorphism in anglerfish? (I read about this long ago, so I could be mistaken.)

    - graefix
    #: Posted by  on  01/21  at  04:45 PM
  5. very nice. i had undergrad pop gen from spencer johnston. didn't understand a word he said or a formula he wrote, but he was extremely charming, robin williams-y.. i wonder what adaptive benefit these guys get from parasitically castrating their hosts.. seems like if they did it too much they would run out of hosts.. i dunno..
    #: Posted by  on  01/21  at  05:07 PM
  6. Here's another real<> sex difference. This one in the human brain. Here's the press release and the article is available online at NeuroImage, Haier et al. <I>in press.
    #: Posted by  on  01/21  at  05:32 PM
  7. Wow, thanks! I have always been a big, big fan of Strepsiptera, but it's so hard to get any solid information on them. (Most stuff can be summed up with 'Are they related to flies? Or are they related to beetles? Who knows, but they're weird!')

    That female E. laboulbeni looks a lot less booger-like than most females, BTW. Godfray writes that a few 'primitive' (and hence presumably marginally less weirded-out) Strepsiptera are true parasitoids of Thysanurans, and perhaps she is one of these.

    I suppose most of us will never have seen these things. The females don't get out much, and the males (once they are adults) live for mere hours. So the piccies are especially nice. Carl Zimmer's Parasite Rex (the US paperback edition) also has a cool close-up photo of a male Xenos peckii (just below the picture of a Manduca bearing its fateful cargo of braconid cocoons).

    Still, as far as sex difference goes, I'd say the Rhizocephala have them beat. But like Strepsiptera (and like a lot of other parasites and parastioids), these barnacles from hell also castrate the host. I've had the same thought as mccm, and I suppose it may be that castrating them keeps them from wasting lots of energy growing genitals that could be better spent (from the parasite's viewpoint) in nourishing the parasite.
    #: Posted by Mrs Tilton  on  01/21  at  05:37 PM
  8. </I>Also, do you have a cite for the Strepsiptera report?
    #: Posted by  on  01/21  at  05:42 PM
  9. Castrating the larval host often induces the host to molt through several more larval instars instead of pupating. This gives the parasite more time to develop, as well as a juicier meal once the babies hatch.
    #: Posted by coturnix  on  01/21  at  08:21 PM
  10. I just had to follow up here:
    http://sciencepolitics.blogspot.com/2005/01/what-are-gonads-for-among-else.html
    #: Posted by coturnix  on  01/21  at  10:22 PM