“Social” Chromosome Discovered

Researchers identify a chromosome in ants that influences colony social structure and, much like the mammalian Y sex chromosome, doesn’t recombine.

By | January 16, 2013

Flickr, Dani P.L.The Y chromosome, apparently, is not as special as we thought. Another example of a non-recombining chromosome that regulates a choice between two phenotypes is described in a study published today (January 16) in Nature. Fire ant colonies are organized either around a single queen or several, and the presence of a non-recombining “social chromosome” in some worker ants dictates which structure a colony adopts.

“It’s very exciting work,” said Gene Robinson, an evolutionary biologist at the University of Illinois at Urbana-Champaign, who was not involved in the study. The results help us “understand behavioral plasticity and alternative behavioral forms in as broad a genomic context as possible.”

Fire ant (Solenopsis invicta) colonies are organized by one of two different social structures. In monogyne colonies, a large queen accumulates a large amount of body fat, mates with one male, and flies off to establish a new colony she initially feeds with her own body stores. In polygyne colonies, new queens don’t grow large and fat, and stay with their home colonies their whole lives. Worker ants only tolerate queens matching their own colony’s social type, and distinguish between them based on odor cues.

Which of these social structures a fire ant colony adopts depends on what’s known as a supergene—a set of linked genes that together produce a complex phenotype. When worker ants—females with two sets of chromosomes—carry two copies of the supergene marked by the B allele for the odorant-binding protein gene Gp-9 (Gp-9BB), the colony accepts only one Gp-9BB queen. Colonies with mixed worker genotypes—i.e., both Gp-9BB and Gp-9Bb ants—accept several Gp-9Bb queens.

But it’s not just the Gp-9 gene that matters; all the other genes in the supergene likely play a role in the colony’s social phenotype as well. As such, scientists speculated, this system only works if “those genes can’t recombine,” explained Ken Ross, an evolutionary biologist at the University of Georgia, who did not participate in the research.

Laurent Keller, an evolutionary geneticist at the Université de Lausanne in Switzerland, and colleagues decided to look more closely at the differences between the B and b variants in males, who only carry one set of chromosomes, and therefore only one supergene allele. Keller’s group found that, within the Gp-9b supergene, a section is inverted compared to Gp-9B variants. This inversion prevented recombination between Gp-9B and Gp-9b supergenes, though Gp-9B supergenes do recombine with each other.

Keller dubbed the chromosome carrying the Gp-9 supergene the “social chromosome”—and noted its similarity to the mammalian Y. Both chromosomes help direct a complex phenotype that relies on the co-expression of genes that evolved together. And just as the Y chromosome doesn’t recombine with the X, the social chromosome carrying a Gp-9b allele doesn’t recombine with the Gp-9B-carrying chromosome.

“It takes quite a few genes to make a male or female,” said Keller, but an “in-between” genotype doesn’t work well. “It’s the same with social forms.” A solitary queen who hadn’t built up enough body fat wouldn’t be unable to establish a new colony on her own, and fat build-up would be a waste in queens who simply return to their home societies where food supplies are plentiful.

It’s not clear whether other supergenes, such as those that control mimicry in butterflies, act in a similar fashion, though Keller thinks it likely that other complex behaviors with two possibilities might have similar “Y-like” chromosomes. But Ross argued that if such non-recombining chromosomes were more common, more would probably have already been identified.

Keller and his lab are extending their studies to closely related ant species to see whether they have their own social chromosomes. It will also be important to determine which genes in the region are actually contributing to the social phenomenon, noted Ross. “Does each contribute in some small way? Or it could be one of them?” Untangling this question, he added, will likely take “several lifetimes of scientific research.”

J. Wang et al., “A Y-like social chromosome causes alternative colony organization in fire ants,” Nature, doi:10.1038/nature11832, 2013.

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Avatar of: James V. Kohl

James V. Kohl

Posts: 492

January 17, 2013

Excerpt: The results help us “understand behavioral plasticity and alternative behavioral forms in as broad a genomic context as possible.” -- Gene Robinson

My comment: Dr. Robinson helped detail nutrient-dependent pheromone-controlled social behavior in the broadest genomic context possible, which is now detailed even further in this report on the "social" chromosome. For example, Elekonich and Robinson (2000) added invertebrates to our model of genetically predisposed hormone-organized and hormone-activated vertebrate social and sexual behavior. Their "from egg to adult" approach played nicely off our approach in From fertilization to adult sexual behavior.

In our review we did not limit ourselves to the molecular biology of invertebrates or vertebrates. We included what was known about the molecular epigenetics of  social behavior and sexual behavior in species from microbes to man. "Parenthetically it is interesting to note even the yeast Saccharomyces cerevisiae has a gene-based equivalent of sexual orientation (i.e., a-factor and alpha-factor physiologies). These differences arise from different epigenetic modifications of an otherwise identical MAT locus (Runge and Zakian, 1996; Wu and Haber, 1995)."

Wang et al (2013) make it clearer that epigenetic modifications of genes found in social chromosomes entered the continuum of adaptive evolution before the sex chromosomes. They also make it clearer that chemical ecology is the driving force of adaptive evolution.  We now have the ecological niche construction of "fat" queens for comparison to that of multiple queens with downstream differences in nutrient-dependent pheromone-controlled social niche construction, which determines differences in behaviors of individuals and differences in the behaviors of entire colonies. These epigenetic effects of nutrient-dependent pheromone-controlled genetically predisposed plasticity have also been detailed in the honeybee model organism.

That's why I was able to use the honeybee to clarify the roles of ecological, social, neurogenic, and socio-cognitive niche construction in the context of nutrient-dependent pheromone-controlled behavior in species from microbes to man. "The concept that is extended is the epigenetic tweaking of immense gene networks in ‘superorganisms’ (Lockett, Kucharski, & Maleszka, 2012) that ‘solve problems through the exchange and the selective cancellation and modification of signals (Bear, 2004, p. 330)’. It is now clearer how an environmental drive probably evolved from that of food ingestion in unicellular organisms to that of socialization in insects."

Now that genes on chromosomes in cells have been epigenetically linked to behavioral plasticity,  to alternative behavioral forms, and to socialization in ants, models of molecular epigenetics that extend across all species may become better accepted. For example, "It is also clear that, in mammals, food odors and pheromones cause changes in hormones such as LH, which has developmental affects on sexual behavior in nutrient-dependent, reproductively fit individuals across species of vertebrates."

The "bottom line" of molecular biology has not changed in more than 3 billion years: random mutations do not cause adaptive evolution. "Olfaction and odor receptors provide a clear evolutionary trail that can be followed from unicellular organisms to insects to humans." -- Kohl, J.V. (2012) Human pheromones and food odors: epigenetic influences on the socioaffective nature of evolved behaviors. Socioaffective Neuroscience & Psychology, 2: 17338. DOI: 10.3402/snp.v2i0.17338.

Avatar of: Roy Niles

Roy Niles

Posts: 115

January 18, 2013


Kohl, How does an odor "cause" anything if it has not evolved to serve the purpose of the function that it's supposedly altering?  These adaptions represent an intelligently orchestrated change, and odors in and of themselves have no intelligent methods of forcing a reaction that was meant to have intelligent results.

Yes, random mutations do not cause adaptive evolution, but neither do molecular forms that haven't evolved to accomplish those same purposive tasks.  Your system, if it had supposedly worked, would have been no better than any other random and ths essentially unintelligent process.


Avatar of: James V. Kohl

James V. Kohl

Posts: 492

January 18, 2013

Niles: Details on how the epigenetic landscape becomes the physical landscape via chromatin remodeling are beyond the scope of this report. Are they beyond your ability to grasp? If not, I may try to tell you more about them. So far, however, it seems you do not understand the fact there is no other model for what I have detailed. You wrote: "Your system, if it had supposedly worked..." when obviously the system's biology of my model does work, or it would not be exemplified in species from microbes to man.

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