Research

Johnston Part of Group Studying How Genome Size Affects Reproductive Fitness in Seed Beetles

October 6, 2015 by Rob Williams

Callosobruchus_maculatus_dorsal — Seed Beetle (*Callosobruchus maculatus*) female. Photo by Simon Hinkley & Ken Walker

Beetles can teach us a lot of things about genetics, especially with regard to differences in genome size within and between species. Professor Spencer Johnston worked with a team of scientists to test how the genome size affects reproductive fitness in seed beetles.

Led by Goran Arnqvist of the University of Uppsala in Sweden, the researchers have presented evidence suggesting that natural selection may be a more important determinant of genome size than chance events (genetic drift). In the study published in the September 2015 issue of the Proceedings of the Royal Society of London, they found no evidence that genome size is determined by random processes.

Johnston said they conducted a phylogenetic comparison of 12 species of seed beetles to describe the tempo and mode of the genome size evolution in the group, as well as to test if there was any correlation in evolution between the size of genome and body size at an interspecific scale.

The group also used a second group of 18 distinct genotypes of the main model species, Callosobruchus maculatus, to characterize interspecies variation in genome size and to ask whether genome size shows correlated evolution with life history and sex-specific fitness.

There was a reason to study this particular organism. These beetles are considered pests because their larvae infest seeds of legumes, such as soybeans and cow peas, doing considerable damage.

The researchers found that within the groups of seed beetles studied, the genome size was directly and positively related to successful reproduction in both males and females. The results suggest that variation in genome size may be much more important than previously believed.

Dr. Johnston said that novel analytic methods used in this study could be adapted to study other insects and could possibly help find new and improved methods of controlling the beetles and other pests.

“I have long sought evidence that genome size variation was adaptive. This is of interest, because we often find genome size differences among populations, including mosquitos and other very serious pest insects,” Johnston said. “Here we find that the populations with larger genomes have higher lifetime fecundity in females and greater competitive success as males. What I find interesting is that we really do not know why this should be so. It opens a whole new area for control efforts.”

The article can be found here: http://rspb.royalsocietypublishing.org/content/282/1815/20151421

Entomologists Discover New Way For Humans to Avoid Being Bitten By Mosquitoes

July 2, 2015 by Rob Williams

Michael Sanders - 1 - Smaller — Michael Sanders stands with the mosquito cage used in the experiments. Photo by Rob Williams.

Summer is usually a time for get-togethers, barbecues, and spending time outdoors. It also means a time where mosquitoes are the most active.

A group of scientists, led by Dr. Jeff Tomberlin, is looking at a possible way of making humans unattractive to these biting insects by outsmarting mosquitoes simply by using the chemical communications systems bacteria on the skin.

Tomberlin, along with Drs. Craig Coates, Tawni Crippen from USDA and Tom Wood from Pennsylvania State University, and former TAMU graduate student researcher Ms. Xinyang Zhang have discovered that they can disrupt mosquito attraction to a potential blood-meal by manipulating microbial communication (aka, quorum sensing).

Quorum sensing is a communications system between bacterial cells that allows cells to communicate amongst each other for various functions. The cell-to-cell communication is used in controlling or preventing such processes such as swarming or reproducing. To communicate, bacteria produce compounds that contain specific biochemical messages. The more these compounds are made, the more concentrated the chemical message becomes until it creates a group response, thus creating a behavior. The behaviors are likely to occur when the chemical messages are very concentrated, which makes it easy for other organisms to listen to the “conversation”.

Tomberlin said that this had derived from his previous research in forensics that determine why blow flies were attracted to dead animals. Like with blow flies, Tomberlin said that mosquitoes are influenced by several factors. For mosquitoes, these include, but are not limited to, the volume of carbon dioxide exhaled, our body temperature, and body odor including those odors associated with the microbes on our skin. He also said that the insects use chemoreceptors on their antennae to “listen in” to various communications systems of microbes on our skin.

He said that this “quorum sensing” of microbes ability has always occurred in nature, and the mosquitoes have evolved the ability to perceive these pathways via natural selection over time. The mosquitoes benefit from this ability by selecting a blood host based on the information received by the bacteria.

Tomberlin noted that if they can find the right code that the bacteria are producing to signal unattractiveness, this could be used to keep mosquitoes from biting us.

During the experiments, the group used a mutant form of bacteria that could be found on our skin (Staphylococcus epidermidis) and removed the genetic mechanism that encodes quorum sensing. They then carried out several experiments using blood feeder containers covered either with the silenced or unmodified wild-type bacteria to test how attractive the feeders were to the female Aedes Aegypti, which is known to be the main vector for yellow fever, he said.

Tomberlin said that each of the feeders was fitted with a paraffin film containing a millimeter of rabbit blood that was injected between the flask and the film. The feeders were then kept at average body temperature via warm water pumped through the flask and placed in mosquito cages containing 50 mosquitoes each for 15 minutes.

Some of the different scenarios tested included placing each of the feeders in separate cages, then putting both types in the same cage at the same time. Based on the results, Tomberlin said that they believe that inhibiting bacterial communications could lead to newer, safer methods for deterring mosquitoes than conventional methods that include DEET.

Tomberlin also said that this discovery of manipulating bacterial conversations has other applications, including blocking communications between bacteria in the lungs of patients with cystic fibrosis that lead to new treatments and helping to reduce pipeline corrosion that could be caused by microorganisms.

Professor Co-Authors Article Featured In Science about Bee Eusociality

May 26, 2015 by Rob Williams

Several scientists including the Department’s own Dr. Spencer Johnston have published results in a recent edition of Science Magazine on how genetics can affect the eusociality of bees.

Led by Karen Kapheim, the team studied the genomes of ten bee species that had varied social complexity that represented multiple independent transitions in the evolution of social structures in the insects. Johnston said that the scientists wanted to answer the question on how genetics played a role in determining behavior in social insects, such as bees.

“Scientists have many questions they would like to ask of the genome sequence. Among these is, ‘How is behavior determined?’” he said. “The honey bee, with its many complex behaviors seemed an obvious place to start in an effort to answer this question. The honey bee genome sequence is relatively small, complete and accurate. The genes have largely been identified and the information gained to date is readily available at a nationally known website for genetic data.”

Johnston said that comparative genetics seemed to be the way to address the behavior question. “The idea was, let’s sequence other bees with easily compared behaviors and see if we can find genome sequences that change in concert with the behavior,” he said.

The group compared 10 species of bees ranging from solitary species to higher, more complex social structures, such as those found in honey bees. Johnston said that he was engaged early in this process to measure the number of nucleotides in bees that seemed good candidates for this comparative study.

His findings had a profound influence on the direction of the study. He found that the small honey bee genome is the exception.

“Other bees can have up to 4 billion letters in their genomes. That meant a great deal more effort and that extra effort limited the number of species that could be compared,” he said “With that hurtle addressed, the sequences were generated for four bees with differing levels of social behavior, and comparable sequences from four other bees were added to the study as they became available.”

The comparison of the genome sequence and the level of social behavior provided answers that were fascinating. It isn’t the sequence itself that changes coincident with behavior.

The study found that the more social insects a human trait that to control the expression of genes by adding a simple CH3 (Methyl) group to one of the letters (usually a methyl group will be added to a C that is followed by a G). Johnston said that the more social the bee, the more genes that contain a methylated C and that the honey bee appeared to takes this methylation one step further than other organisms. “As honey bees age, they take on increasingly risky jobs. Only the oldest bees take the high risk jobs leaving the hive to forage and scout. Younger bees tend the queen and maintain the hive,” Johnston said. “What does that behavioral change have to do with these methylated Cs? Coincident with this change in behavior is a change in methylation. Older bees have less and less of their genes controlled (usually this also means a reduction in activity) by methylation of a CG couplet. Methylation appears to be a signature of social behavior.”

Johnston also said that he and the scientists also studied additional changes with sociality. They found out that the number of interacting genes (the gene network) grows increasingly complex as bees become more social. In the most extreme form (eusociality), where workers give up their own fertility to tend a sister (the queen) and her brood, he said that the level of networking is extensive.

“The increasing level of networking we observe as insects become more social raisesmany interesting questions. Does this same phenomenon occur in humans? Are altruists characterized by an increasingly complex gene network?” he said. How about other vertebrates? Is the complexity of the gene network one of the ways we differ from other primates? What about our favorite pet? Has domestication changed the level of networking? Some would even ask, do cats even have such a network?”

In the long run, Johnston said that the study of eusociality also is a study of humans and that further research needs to be done to learn more.

“We know a great deal about the sequence of letters in the human genome, but we have much more to learn. The sequence is inherited, but the changes we identified here are not in the sequence itself,” Johnston said. “The environment plays a role. Our future may depend upon knowledge of that role.”