Berkeley -- A surprisingly large number of beetles are missing one of their testes, the male gonads of insects. As far as the researchers who discovered this can tell, the insects are not in any way bothered or impaired by this absence.
The discovery is striking because most animals are bilaterally symmetrical, which means the left and right sides of the body roughly mirror each other. This bilateralism extends to many internal organs, although some systems, such as the human heart and liver, develop or are positioned asymmetrically.
"We've got two lungs, two kidneys, and females and males have paired gonads. Even our brain has two hemispheres," said Kipling Will, assistant professor of insect biology at the University of California, Berkeley's College of Natural Resources. "Evolution has predominantly favored bilateral symmetry in animals, so when we see that the rule is violated, as in the case with these beetles, it gets our attention."
Will led a systematic survey of all major lineages of the beetle family Carabidae. The results of the survey will appear in the April print issue of the Journal of Morphology, but are available now online.
The researchers said that field observations such as this provide valuable clues to beetle biology and evolution.
The one-testis phenomenon, or monorchy, was first noted in beetles by French naturalist Leon Dufour in 1825. He found that Harpalini carabid beetles had a single testis, but he and other scientists considered the condition to be limited to this group. It would take another 180 years before researchers would conduct a more thorough survey, finding that two other major lineages also lack one testis.
The survey required detailed dissection and study of over 820 species, a representative sampling from the 37,000 species of carabid beetles estimated to exist. The researchers found 174 species, all members of the three lineages with only one testis. The researchers noted that except for this one anatomical distinction, the one-testicle beetles appear and behave no differently than their two-testicle counterparts.
"The beetles with one testis are mating normally and doing their beetle thing," said James Liebherr, professor of entomology at Cornell University. "It strikes me that carabid beetles are pretty well known to scientists, yet the loss of an entire organ across three major lineages was not fully comprehended until this study."
For reasons unknown, in almost all cases it is the left testis that has disappeared.
"You might say that these beetles lost all that was left, except for one small group that seems to have lost their right," quipped Will, who is also associate director of UC Berkeley's Essig Museum of Entomology.
While animals such as jellyfish and starfish are radially symmetrical, bilateral symmetry is, hands down, the dominant body shape in the animal world, thanks in part to the drive for forward motion.
That's not to say there is no precedent for such deviations from bilateralism. One well-known example is the male fiddler crab, which has an outsized claw on one side that is used to attract female crabs and fend off male competitors.
Still, the researchers said the complete absence of an organ, or absence asymmetry, is rare. When it does occur, there is likely a good reason for the organ loss. Snakes, for example, have one lung that is significantly reduced to accommodate a relatively extreme body shape. Most birds have only one functioning ovary, which some biologists believe helps optimize their bodies for flight.
That raises the question of how and why some beetles evolved to have just one testis. The researchers ruled out obvious factors such as flight advantages or major changes in body form.
They did notice that the monorchid beetles' accessory glands, which produce the bulk of the seminal fluid, were somewhat larger than normal. The researchers suggest that packing of the internal organs in the abdomen is a kind of competition for space that may lead to the loss of the functionally redundant testis.
"Male crickets will directly transfer fluid from their accessory glands to female crickets to provide nutrition to their egg cases," said Liebherr. "It may be that the male beetles are similarly providing other things than sperm to the females. But it's a chicken and egg question. We don't know what came first. Was the testis lost first, leaving more space for the accessory glands to grow? Or did the testis lose out to make way for a larger accessory gland? That's a subject for further study."
Whatever the ultimate cause, something drove the evolution of this absence asymmetry.
"We found monorchy in three distantly related groups of carabid beetles - Abacetini, Harpalini and Platynini - indicating that the loss of the testis occurred at least three separate times in the evolution of the beetles," said Will. "It seems unlikely that it was completely random."
Based on the geographic distributions of the beetle groups involved, Liebherr estimates that the origins of monorchy in the beetles occurred 90 to 100 million years ago during the Cretaceous period. "That era witnessed a dramatic increase in the diversity of organisms," he said.
He added that these findings illustrate the value of basic natural history.
"So many of the scientific discoveries today seem to occur primarily on the molecular and genetic level, so findings such as this are remarkable," said Liebherr. "A lot of the ideas that come to the lab bench start in the field. We still have much to discover by looking at whole organisms."
Other co-authors are David Maddison at the University of Arizona and José Galián at the Departamento de Biología Animal Facultad de Veterinaria in Murcia, Spain.
This research was funded by the National Science Foundation.