Horizontal Gene Transfer: How parasites control insect host behavior

Published recently in the journal Current Biology, research from a study conducted by the RIKEN Center for Biosystems Dynamic Research has revealed that parasites likely manipulate the behavior of their host organisms using horizontal gene transfer. 

In general, many parasites control their hosts to ensure the survival and reproduction of their species. One example of such a parasitic species is the horsehair worm, born in water but migrating to dry land using aquatic insects. After being eaten by a mantis (terrestrial insect), the worm grows within the host, beginning to manipulate its behavior. Once fully matured, the horsehair worm prompts the insect host to jump into the water and drown to its death, allowing the parasite to complete its life cycle and reproduce. 

Interestingly, previous studies have indicated that the horsehair worm likely manipulates the host’s biological pathways to increase movement towards the light (and approach water) by mimicking molecules found in the hosts’ central nervous system. To better understand how they developed such a mimicry mechanism, researchers analyzed the whole-body gene expression in a Chordodes horsehair worm before, during, and after host manipulation. From this study, researchers found over 3,000 genes that were expressed more during manipulation and 1,500 that were expressed less. In contrast, gene expression in the mantis brain did not change whatsoever, indicating that the parasites were producing proteins that would manipulate the host nervous system. After further analysis, interestingly, it was found that over 1,400 hairworm genes that matched those found in their mantis hosts were absent in species with different hosts. From these findings, researchers concluded that the identified mimicry genes (linked with neuromodulation, light attraction, and circadian rhythms) were likely the results of multiple horizontal gene-transfer events from various mantis species during the evolution of horsehair worm species.

Considering that horizontal gene transfer is far more common in prokaryotes, it was rather interesting to see an application of this phenomenon in eukaryotic species. This was especially the case since it is a primary way through which bacteria evolve antibiotic resistance. In that sense, horsehair worms would likely make ideal model organisms to study the mechanism by which horizontal gene transfer occurs and allows molecular mimicry, advancing current scientific understanding of evolutionary adaptation.

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Parasitic worm gene used to track down new host

Partnered with bacteria, Steinernema carpocapsae invade and kill insect hosts. While looking for a new host S. carpocapsae use a variety of techniques, such as jumping or standing on its tail while waving its head, to lure in a new host. In the study scientists used a process call RNA interference (RNAi) to look for a link between genes and the behaviors used to find a new host. RNAi reduces the expression of genes so the scientists can examine their function. The scientists used RNAi to reduce a gene that codes for a molecule known as FLP-21. Taking away the FLP-21 hindered the ability of jumping and tail standing implying that regulation of these behaviors is linked to FLP-21.

Scientists have also discovered where in the body FLP-21 can be found. chemical and imaging techniques have shown that FLP-21 is located in the neurons in the worms head. Finding what causes the behaviors of host hunting has allowed scientists to conclude that S. carpocapsae can be an excellent model organism to study parasites in mammals without actually having to use infected mammals.

I thought this was a very interesting article. I think it's great to have an organism we can use to study parasitic infections in mammals without having the infect a mammal.

https://www.sciencedaily.com/releases/2017/03/170302143954.htm

 http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1006185

The Tapeworm:Complete mitochondrial genomes of Taenia multiceps, T. hydatigena and T. pisiformis: additional molecular markers for a tapeworm genus of human and animal health significance

Tapeworms come from the genus Taenia that is comprised of flat worms, many being parasitic. Tapeworm require two mammalian hosts for both transmission as well as life cycle completion. Typically transmission is as follows: egg->herbivore->carnivore where the cestode matures and releases its eggs. Infection by tapeworms in humans is caused by inadvertent consumption of eggs or larva typically found in fleas as well as undercooked meat. Infections in mammals can be life threatening if untreated. The picture to the left is of a tapeworm from a dog.

Mitochondrial genomes have been found to have great impact on the science community in understanding molecular variation amongst species and have provided utility in molecular and population genetics as well as evolutionary biology.The tapeworm genus Taenia has growth to have great importance in both human and veterinary biology because of the vast diversity of the parasite. With the completions of 7 total genomes of  T. multiceps, T. hydatigena and T. pisiformis scientists are able to compare and contrast variations within the genomes of the same genus. This research can help to develop genetic markers and used in what scientists are calling an "extended mitochondrial toolkit". Having completed 7 Taenia species genomes, it is possible to analyze amino acids present as well as estimate phylogenetic features for the genus which differed from previous estimates using only partial genes. With these new analyses, better molecular markers were discovered and now these mitochondrial markers are used for molecular ecology, population genetics as well as diagnostics.

Parasites drove human genetic variation

Around 100,000 years ago modern humans began spreading out of Africa, where they learn to adapt to the different climates,different ways to find food, as well as a new way to fight off new pathogens.In this article, research has shown that certain pathogens, like parasites worms had the biggest part in driving the natural selection of humans. However, genetic adaptation also could have  made humans more susceptible to autoimmune diseases.

       The scientist removed one at a time the different variables from their model in order to figure out which of the variables would have the strongest impact on their model. “What we show is that all three factors are important, but the strongest factor is the pathogenic environment,” says Rasmus Nielsen, a computational biologist at Berkeley, and a co-author of the study.

       The parasitic worms seemed to be the most strongest factor in the natural selection force, more so than bacteria or viruses. According to Rasmus Nielsen it makes sense because bacteria and viruses can evolve very quickly and may have the ability to rapidly circumvent any genetic advantages gained by the humans.  On the other hand, something that evolves slowly like worms give the humans time to solidify their defenses.

       I think this article gives a good inside view of how our bodies learn to adapt to the changing enivorment around us, who knows what will comes next that will make us, humans, evolve again.

Parasitic worms such as Schistosoma mansonicaused more genetic diversity in humans than did climate, diet, bacteria or viruses.