Carabid beetle males and their twisted genital orientation

Some male carabid beetles stow their tackle pointing to the left, others point to the right. They’re just born that way.

Jim Liebherr and I recently published a paper in Insect Systematics & Evolution about this. The abstract starts with…

“Western Australian populations of Mecyclothorax punctipennis (MacLeay) exhibit chiral polymorphism for male genitalic asymmetry. The plesiomorphic genitalic enantiomorph, wherein the male aedeagal median lobe is left side superior when retracted in the abdomen, is rotated 180° to a right side superior position in 23% of males from Western Australia.”

Okay, let me back up and break that down for those of you not spending a seemingly inordinate amount of time fiddling with beetle genitalia. For male beetles in general, much of their genitalia and reproductive tract is composed of multiple segments, most of which is made of tough and often relatively stiffly sclerotized cuticle (the bits from the tubes leading to the testes on out). Despite this bloke’s hard and often medieval-looking equipment the Sheila beetles need not worry as they sport equally rugged cuticular genitalia. Beetles and most other insects retract, more or less telescoping, their genitalia inside the abdomen when not in use. Fortunately, since otherwise they would be walking around dragging their junk in the dirt, which is never a good thing.

In carabid beetles, not only do they telescope the segments inside, they add a twist. Typically the penis (referred to as the aedeagus) and associated parts (the parameres, and genital ring for those that care about details) are rotated 90 degrees clockwise (relative to the way the beetle faces). When deployed this apparatus turns back to its true orientation and docks with the female from above and behind. This is the typical state in carabid beetles.

A couple of Pterostichus doing the wild thing to show how the genitalia is everted and interacts. image by A. Seago.

A couple of Pterostichus doing the wild thing to show how the genitalia is everted and interacts. image by A. Seago.

So what did we find? During more or less routine inspection of beetles in museum collections we (Jim, really) noticed that some individuals had the tip of the penis sticking out to the left, others to the right. A more detailed survey of specimens revealed that some retracted their genitalia left side up and some right side up. Being right side up is relatively rare in carabids (something like 1% of species). The vast majority of species are left side up and nearly all are either left or right side up, not mixed, within a single species (but see this post by DRM). So this was odd.

Mecyclothorax punctipennis male genitalia. A- a right side up male and B- a left side up male. You'll have to read the paper to get all the abbreviations.

Mecyclothorax punctipennis male genitalia. A- a right side up male and B- a left side up male. You’ll have to read the paper to get all the abbreviations.

Within this species the different forms of the whole structure are built, from the ground up, as mirror images.  Females, on the other hand, are uniform in their shape.

Even more striking is that all the right side up beetles (the rare form) were only found mixed in with lefties in populations in Western Australia. What is going on out in sandgroper country? We don’t know for sure, but hypothesize that this is a result of genetic drift coupled with small population size and isolation between east and west Australia. I have been to the Nullarbor Plain and can vouch for it as a barrier no one wants to walk across.

Mecyclothorax punctipennis population samples; boxes associated with localities include number of left superior males, number of right superior males (and fraction right superior males in brackets)

Mecyclothorax punctipennis population samples; boxes associated with localities include number of left superior males, number of right superior males (and fraction right superior males in brackets). Purple with a square icon means populations with only left side up males. Red and blue are mixed.

Genitalia get a lot of attention as it is right there, central in the process of reproduction, presumably under strong selection (e.g. female choice), and so it is interesting to see what appears to be a major change in genital structure, that doesn’t fit expectations of selection. You can get a copy of the paper here. Enjoy.

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About Kip Will

I'm an insect systematist with expertise in carabid beetles, who is always happiest in the bush.
This entry was posted in Carabids, Coleoptera, Entomology. Bookmark the permalink.

One Response to Carabid beetle males and their twisted genital orientation

  1. James C. Bergdahl says:

    Don’t forget crediting Phil Darlington.

    As I posted at David Maddison’s blogsite that you refer to on this subject, Phil is the first one I know of who mentioned many years ago the likelihood of polymorphic genitalia within carabid species, although he was not able to give any examples to the best of my knowledge. If evolution creates a variety of states of wing development within species, and other morpho-features, why should we not expect evolution to produce polymorphic median lobes in some carabid species?

    This of course would cause much confusion in in the discovery of a “natural” taxonomy of closely related species, since species delineation is usually closely tied to the morphology of median lobes and the assumption that they are more-or-less monomorphic!

    In my opinion, Phil Darlington also deserves much more credit for the development of the “theory of island biogeography” and the “taxon cycle” than Edward O. Wilson has ever given him, even though they both worked at Harvard University’s Museum of Comparative Zoology simultaneously, and no doubt Wilson learned much from him.

    James Bergdahl
    Spokane, WA, USA

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