Genetic Analysis of Various North American and European Bat Species: A Potential Intervention Method for White Nose Syndrome

Abstract

Since its introduction in 2006, White Nose Syndrome (WNS) has ravaged the eastern North American bat population, and since then not much progress has been made to slow down the spread and infection. Pseudogymnoascus destructans, the fungus that interrupts the homeostasis of bats during hibernation and eventually causes the host bat to starve to death, is an invasive species likely introduced from Europe. European bats are able to manage the infection without death, while up to 100 percent of bats in North American hibernacula locations can be killed over one winter. There isn’t a consensus around whether or not North American bat species will be able to survive the fungus, but most scientists see the issue as urgent and place great value in searching for an effective intervention method for an animal that plays a major role in managing pest and vector populations. The first step toward finding an intervention method is comparing European and North American bats to figure out why the former are resistant to P. destructans; it could be genetic, behavioral, or even environmental. While all of these possibilities have potential, genetic differences have already been shown to exist, it could reveal further paths for research, and the opportunities this type of research would provide are the most feasible. The genome analyses of many bat species have not yet been done in terms of how they respond to WNS.

 

Research Proposal

INTRODUCTION

Human history is plagued by microbial invaders that change the course of events such as wars and invasions. The famous black plague killed one-third of Europe’s population, plunging countries like Britain, France, and Germany even deeper into the Dark Ages (Spyrou et al., 2016). Columbus and other European explorers brought smallpox, typhus, cholera, and influenza that weakened otherwise powerful indigenous populations, making them easy to invade (Mann, 2019). But what isn’t discussed as much are the plant and animal species that have died or are dying out from their own invisible enemies.

In the early 1900s on the Australian Christmas Islands, a native rat went extinct entirely because of a protozoa called Trypanosoma lewis carried by a flea, passed on by rats aboard ships passing by. On the east coast of the US in the 1930s, American chestnuts went functionally extinct after being a staple for the people and ecosystems, making the Great Depression even more desperate. The tree had caught a fungus from an imported Japanese chestnut tree that made it virtually impossible to reproduce (Kwong, 2019). Chytridiomycosis (Chytridiomycosis, 2018) infects the skin of amphibians and has caused the decline or extinction of over 200 species. Research suggests that infectious wildlife diseases are occurring at higher rates in recent years (Gallana et al., 2013), meaning tragedies like these won’t be going away. These extinctions have changed our lives forever, and humans have since looked for ways to bring these species back. It turns out that many North American bat species are also falling to an invisible enemy that is easy to detect, but difficult to combat.

Bats’ tendency to transfer diseases, such as Ebola and Covid-19, to humans, along with the fact that they live in caves and are active at night has caused them to be portrayed as dirty or evil in Western cultures. However, bats play an important role in ecosystems and their extinction will cause problems. First of all, bats offer importance just by existing, as they account for about twenty percent of all living mammalian diversity (Puechmaille et al., 2011). They are the only mammals to fly and their use of echolocation to navigate is unique. If anyone is fortunate enough to hear their squeaking at dusk, they may be reminded of a blood-sucking vampire or even the afterlife. But an alternative, more educated outlook will portray bats as mysterious creatures that humans can learn from.

Bats are of practical importance as well. For example, certain species of bats pollinate foods we eat, like agave, and manage insect populations. In fact, some houses have bat boxes placed nearby so people can enjoy an evening on the porch without the threat of mosquitoes ruining their night. This insect control is more than convenient. Bats saves the US $3.7 billion on pest control each year, making it in our economic interest to look out for their well being. Furthermore, bats play an important role in ecosystems: their feces provide vital nutrients in cave ecosystems, they spread seeds, and they serve as food for larger birds. And while bats are known to transmit diseases, they happen to eat a lot of disease vectors as well.

White Nose Syndrome was introduced to a small cave in New York state in 2006, and over the past seventeen years, P. destructans has spread in all directions and killed around 5 million bats, reversing the previous trend of bat populations steadily increasing (Langwig et al., 2012). As of 2011, at least nine species of North American bats have been infected by P. destructans and six have manifested the symptoms associated with WNS (Puechmaille et al., 2011). These are all signs of an extremely invasive species. P. destructans evolved along European bats, so even though European infection rates are still high, the survival rate is as well. Reports go back for decades of European bats with the signature white fuzz around their nostrils, yet this fungus was never a sign for alarm. A study focusing on European bats (Wibbelt et al., 2010) pointed out that European bats may be resistant due to immunological or behavioral factors. For example, European bat caves tend to be smaller, but it is important to point out that it has been found that hibernacula size does not predict WNS infection nor survival rates. However, there are few findings on what definitively makes European bats resistant and if those characteristics can be imitated in North America.

It is apparent that intervention is necessary in the crisis bats are facing against P. destructans. An unprecedented disease will be difficult to manage.  Despite WNS being superficial (it infects only the skin of the bat), it has drastic consequences. Despite the disease being named after the fungal infection of the bat’s nose, it is the “digesting, and eroding, and invading of the skin” around the wings that causes the most trouble. While the membranous wings allow bats to fly, they also play a crucial role in homeostasis, but it is also where P. destructans strikes the most because of the abundance of skin membrane available. This destruction of the skin membrane interrupts homeostasis and robs bats of their moisture and energy, so they won’t last through hibernation (Cryan, Meteyer, Boyles, Blehert). In addition, the disease can attack the nerve cells in the wings, meaning that many bats spend the last of their lives crawling through the winter snow, desperate for food and water.

What makes WNS especially disheartening is that even if a bat survives the initial infection, the long term impacts will continue to threaten the bat, as an individual and as a species (Secord et al., 2015). The illness robs them of their energy and has been shown to decrease reproductive success. In addition, just because a bat survived an initial infection once doesn’t mean it will have immunity the next year. In fact, research suggests that bats aren’t even able to regain their energy by the next winter, making them especially vulnerable to WNS (Davy et al., 2016). These results somewhat align with a Midwestern study (Langwig et al., 2015) that found that in caves that were initially infected in the winter of 2012-2013, survival rates were generally higher and declines were limited to larger sites. In contrast, in the winter of 2013 to 2014, two bat species declined by up to 99 percent and had a larger impact on more types of caves. The bats that survived the initial infection had not recovered enough to overcome the second. Going back to the earlier study focusing on physiological long lasting impacts, it is likely that infection negatively affected a bats ability to reproduce, making natural recovery seem less and less promising.

Not only is it the physiology of North American bats that makes them vulnerable to WNS, but it is their behavior and the environment. When bats hibernate, their immune function and metabolism decrease in regulation and their body temperature drops, making them the perfect prey for P. destructans. Additionally, before WNS, bats evolved different strategies to survive hibernation, like selection of humid areas and dense clustering to conserve energy and decrease moisture loss. These behaviors can be harmful-close proximity, moisture, and heat means that entire caves can be wiped out over a few months, destroying whole ecosystems. Such behaviors could be a result of genetics, environment, or both, and identifying differences between European and North American bat behaviors here could help identify actions humans could take to make hibernacula hostile to P, destructans.

According to a 2011 study (Puechmaille et al., 2011), there are competing hypotheses behind why manifestation of WNS looks so different between North American bats and European bats. 

1. Bats in Europe have been observed with WNS as early as the late 1970s in Estonia, and they have evolved to coexist. North American bats lack the mechanisms necessary to survive symbiotically with this fungus.
2. The North American fungus has evolved to become an even more virulent strain of P. destructans, or the fungus didn’t actually come from Europe, but is something totally different and more deadly from another area. This inaccuracy of information has proven this to be a reasonable hypothesis because many still relevant WNS studies incorrectly refer to the fungus as Geomyces destructans, as scientists initially erroneously named the genus. The thought of a more virulent strain evolving in North America is a scary one, as it could potentially pose a threat to all bats. 
3. P. destructans isn’t the pathogen actually causing North American bats to die, it is simply taking advantage of already immunocompromised individuals and something else is killing them. It is important to point out that a more recent 2017 study (Secord et al., 2015) stated that they “propose that damage to the bat wing, a physiologically dynamic membrane, brought about by G. destructans is sufficient to directly cause mortality.” It is clear that WNS causes bats to periodically arouse from torpor, or the state of hibernation, but it is unclear if this is the cause or prerequisite for mortality. While studies to back this up are hard to come across, it would explain why a genetically identical fungus is found in dead North American bats, but not European ones.

Most studies since the publishing of this paper have taken on the first hypothesis, yet a comprehnesive genetic analysis of P. destructans, North American bats, and European bats has not yet been completed and could yield any of the above results.

The geographical and historical separation of North American bats and European bats makes pursuing the genetic information of multiple bat species a promising opportunity for answers. And of course, there are certain environmental and behavioral factors that may make some bats more vulnerable to WNS than others, but changing bat behavior could prove to be difficult and has a high risk for failure. For this reason, the more practical difference between European bats and North American bats to pursue would be in genetics, which has shown to be effective in the past. The lack of research detailing the genetics of bats prompts me to ask: What genetic differences in bats are present that make North American bats more vulnerable to White Nose Syndrome, and how can those differences be utilized to help bats recover from this epidemic?  

 

METHODS

This study will require the genomic sequencing of North American bats and European bats, and then analyzing what common similarities lie in the two groups and then compare those similarities across geographic boundaries. This study would be conducted by taking samples of DNA from bats killed by WNS, that survived WNS, and that never had WNS. This process would occur in North America and Europe. Priority of genome analysis in bats would be given to the skin (Brown Adipose Tissue, essential to hibernation) and glands that produce hormones involved in hibernation such as leptin, melatonin, and GCs (Willis & Wilcox, 2014). Here, gene expression will be important because while all somatic cells have the same genetic information, DNA is expressed differently in different parts of the body. In addition, priority would also be given to parts of the DNA that impact hibernation and the immune system. By comparing genetic traits between infected and noninfected bats between North America and Europe, we could shed light on why there is a difference in survival rate between geographic locations, whether that be because of hibernation techniques, or immune responses, or something else all together.

 

DISCUSSION

The most obvious result of this research is that a significant genetic difference could be found between different species of bats, and hopefully this difference can be used to make North American bats more resistant to WNS. Some of the further reaching possibilities for intervention may include a virus that attacks P. destructans or changes the bat’s DNA, or a vaccine that promotes WNS immunity. A simpler solution may be to analyze what enzymes, hormones, or behaviors are different as a result of genetics and make those factors play less of a role. The research proposed here is only a jumping-off point for further analysis of human intervention of WNS using bat genetics.

It is possible that there would be no significant genetic differences between bats, but this too has implications. Perhaps there is some weakness in P. destructans that scientists could take advantage of. This avenue has already been explored, as researchers have found that the fungus lacks an enzyme that protects it from UV rays (Palmer et al., 2018). However, this method of sanitizing bats requires a lot of work before implementation and P. destructans should be further explored for weaknesses. The process of synthesizing and comparing this many genomes will be long and costly, but it could lead to answers on how to save multiple species of bats, making it worthwhile.

Perhaps the answer lies in the genome of the fungus, but the difference arises geographically. This corresponds with hypothesis 2 explored in the earlier mentioned 2011 study. It must be confirmed that the North American version of the fungus did in fact originate from the European version and not some other fungus all together. If the North American fungus is confirmed to originate from Europe, have there been any significant mutations since its introduction? What if the North American fungus is actually a different, more dangerous species all together that happens to have similar symptoms, like white fuzz on the nose, to P. destructans found in Europe?

In addition, recent changes in the environment could also play a role in the vulnerability of North American bats to WNS. From 1989 to 2014, the biomass of insects caught in traps used in a study dropped by 75% (Goulson, 2019). Since insects are the main food source for bats, perhaps there is less food around and bats are able to store less energy before the start of hibernation. If this were the case, WNS would likely kill bats in less time because homeostasis is already interrupted. It is important to note that this study was conducted only in highly industrialized countries. In addition, this wouldn’t explain why bats in Europe don’t die, while those in North America do. More research should be conducted on various insect biomass across geographical regions and the impact on bats.

Toxins in the environment could also be weakening bats so they don’t last through hibernation. It has been hypothesized that the “lack of a sufficient immune response against fungal invasion during hibernation by bats in North America could be caused by immunosuppression due to high levels of toxins” (Puechmaille et al., 2011). Bats tend to forage in environments that come into contact with “wastewater-treatment plants, agricultural operations, and other point and nonpoint sources of contaminants” (Secord et al., 2015). This type of behavior has resulted in contaminants of emerging concern being found in greater and greater concentrations in the tissues of bats. Furthermore, it has been found that fat metabolism and mobilization of lipid-borne xenobiotics reduces immune function during torpor, which could make bats extra vulnerable to WNS. Additionally, “Antibacterials and antibiotics may directly interfere with the fungal-bacterial balance in bats that are coping with WNS”. Moreover, caffeine is increasingly being found in the environment and arouses bats out of torpor, possibly exacerbating the effects of WNS. Salicylic acid inhibits prostaglandins, which also play a role in hibernation. If these toxins were significantly impacting bats, research must be done on how the presence of these chemicals differs between North America and Europe.

Genome testing is likely the most promising avenue for protecting bats, but there is no guarantee that it will yield results. A 2017 study (Secord et al., 2015) stated that “The ability of G. destructans to invade the wing skin of hibernating bats is unlike that of any known cutaneous fungal pathogens in terrestrial mammals.” A disease of unprecedented damage that spreads quickly could easily wipe out several species of bats, but what are the chances of that happening, and what will be the consequences? Looking back at the species mentioned early in the introduction that went extinct due to a pathogen, it is likely that once bats are gone, we will miss them despite the overall societal lack of appreciation for them. Since the extinction of the American chestnut, scientists have tried to crossbreed it with a chestnut species that is resistant to the fungus. They have even tried to genetically modify the tree by implementing a gene of a specific grass that possesses an enzyme that can attack a fungus (Kwong, 2019). No research was found on bringing back the Christmas Island rat, but since extinctions like these, efforts have been made to sterilize cats on islands to protect what wildlife is left. While North American bats are not yet extinct, this possibility should be considered given the nature of P. destructans.

If the situation gets desperate, it is possible that North American bats could be cross-bred with European bats with the hope that the offspring will be fertile and able to tolerate WNS. Or, the North American bats could actually be genetically modified to produce an enzyme of some sort to defend themselves against WNS. This enzyme could come from another animal, probably a bat from Europe, and after much testing it could be introduced to North American bat habitats. However, it is important to note that genetic alterations of this sort are done primarily on plants and this intervention method would require large advancements in genetics. Unfortunately, both of these routes would still mean the extinction of North American bat species because the original DNA would be altered. In addition, there are risks that come with introducing new genetics into an ecosystem. 

If these routes are not taken and species of the North American bat do go extinct, there are several options from there. Of course, no further action could be taken, and without bats, insect populations wouldn’t be balanced which would lead to a cascade of effects. It is possible the European bats could be introduced as a replacement, but there are dangers to this as well. The subtle differences in continental bat populations are the difference between life and death when it comes to WNS, so imagine what other differences could lead to positive or negative impacts. All of these options have the potential to be beneficial, but it is necessary that work is done now to avoid extinction in the first place.

Of course, there is the possibility that even without intervention, bats could survive. As Ross MacPhee points out, “Most wildlife biologists are hoping that such diseases, although severe, will eventually accommodate and the species will pull through” (Viegas, 2008). Perhaps there are a few North American bats that will develop immune resistance, and they will survive to found a new population of bats, largely resistant to WNS. Or, once bat populations are so low, P. destructans will die out, allowing the remaining bats to slowly recover. Of course, these possibilities will cause a genetic bottleneck, and the resulting lack of genetic diversity could create more vulnerabilities. In addition, as mentioned in Conservation implications of physiological carry-over effects in bats recovering from white-nose syndrome, WNS could have long lasting effects on its victims and even their offspring; if bats are able to recover, it will take a long time to do so. 

One of the difficult aspects of WNS is how fast it spreads and how hard it hits. These qualities have left scientists stretching themselves thin and urgently searching for answers. However, this opens up the opportunity to explore many avenues of research, some genetic, some environmental, and some behavioral. Genetic analysis appears to be the most feasible opportunity for intervention: it could lead to vaccines, or a virus that wipes out P. destrucans, or something else. However, the other options mentioned also have potential, and the most practical intervention method is probably a mix of multiple research breakthroughs. Each and every possibility should be explored, and this analysis will open up even more doors and opportunities. Yes, bats may survive White Nose Syndrome, but we should do what we can to minimize the risk, because bats are an integral part of our world, and losing them would be a tragedy.

 

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