Category: Fall 2016 Colloquium

The Odor of a Blood-Sucking Nightmare

11/22/2016

It can be a lonely world for a bed bug researcher! Dr. Mark Feldlaufer began his presentation by extending an open invitation to visit his bed bug research lab at the United States Department of Agriculture-Agricultural Research Center in Beltsville, Maryland. He assured us that the offer rarely gets takers, and it is really not surprising, all things considered. As parasites that feed on people and cause itchy welts, bed bugs give people the heebie-jeebies. Just a picture or two of the bed bugs (Fig. 1) was enough to have several attendees visibly cringe, as they imagine the marks and persistent itches that usually follow after bed bug bites. However, Dr. Feldlaufer’s fascinating research, which aims to improve bed bug detection and control, may contribute to a future where everyone can rest assured that there truly are no creatures lurking under the bed.

Figure 1: Close up of bed bugs in varying life stages. The smallest insects are young nymphs, while the largest insects are wingless adults. Adult bed bugs are approximately 0.18 inches or 4.5mm long (Photo courtesy of: USDA)

Bed bugs (Cimex lectularius), a group of true bugs (Hemiptera) in the family Cimicidae, are infamous for specializing on the blood of birds and mammals including humans. Like little vampires, they pierce the skin and suck blood from their prey. Bed bugs have plagued humanity for centuries. The many colorful nicknames for bed bugs (“crimson ramblers,” “mahogany flats,” “wall lice,” “chinches,” “brown backs,” “red coats,” etc.) highlight the fear, attention, and often even phobia, that they cause. Unsurprisingly, Dr. Feldlaufer most commonly fields the questions: “Do I have bed bugs?” and “How can I get rid of them?” To better answer these questions, he uses chemical ecology to develop tools for bed bug detection and control.

Figure 2: A woman training Brea, a German shepherd search and rescue dog, to use her refined sense of smell to locate missing persons by detecting their odor. Photo Credit: Sgt. John Crosby (Wikipedia Creative Commons)

Detecting bed bugs can be difficult. They are cryptic insects that hide most of the time and emerge at night to feed on blood. They often hide together in cracks and crevices, a behavior called “aggregation.” They prefer to be sandwiched between objects. Thus, they are rather difficult to see, but, thankfully, not more difficult to smell. We are accustomed to seeing trained dogs sniffing around to find drugs, explosives, or missing people (Fig. 2). However, what many may not know is that trained dogs are also one of the most effective methods to detect bed bugs (Pfiester et al. 2008). In a controlled experiment in hotel rooms, trained dogs were 98% accurate in locating live bed bugs. However, this method can vary significantly in effectiveness depending on the dog’s mood, how tired it is, and potential distractions. But the most significant biases often stem from human error, such as the attitude of the trainer. The dog is often able to sense if a trainer strongly believes that there may or may not be bed bugs in a particular location and is far more prone to falsely detect or fail to detect the target (Lit et al. 2011).

Figure 3: Bed bug colonies in mason jars. Bed bug frass is visible on the paper towels, and shed skins pile at the base of the jars. (Photo courtesy of: Dr. Mark Feldlaufer)

In a setting where there are high numbers of bed bugs, their presence can be apparent even without trained dogs. While developing, bed bugs shed their skin, which look like cream-colored crumbs, and their frass (fecal matter) which look like sooty black stains (Fig. 3). It is also possible, although unusual, to see a bed bug fleeing the scene. Nonetheless, in those critical times when an infestation is just beginning to establish, populations are small and difficult to locate. This is when the merits of bed bug detecting dogs become clear. Properly trained dogs (and handlers) can be used to detect infestations early when populations are smaller and easier to control.

often do not come in contact with them. It is impractical, dangerous, and against labelled usage, to coat every surface and crack of a home or business in insecticide, so bed bug chemical treatments often fail. The silver bullet control method appears to be heat treatments, but, like silver, heat treatments are expensive. Homes are heated to between 120-140⁰F for several hours to kill all bed bug life stages, which tend to cost anywhere between $2,500-$7,000. Though bed bugs themselves do not discriminate against the rich or poor, our methods of remediation tend to do just that. The most effective way to get rid of bed bugs is unaffordable to many.

In the course of his bed bug research, Dr. Feldlaufer has worked on other projects to help to detect and control bed bugs early in their infestations with a number of students, including our very own Dr. Kevin Ulrich. The team found that bed bugs consistently avoided a common chemical insect repellent, DEET, the main ingredient in most mosquito repellents. They did not, however, respond with greater avoidance to higher dosage. Although this may seem like a quick and easy fix, wearing DEET to bed may just cause bed bugs to merely move on to other surfaces (e.g. the couch). On the other hand, they also found that aldehyde compounds produced by bed bugs elicit a strong attractive response to other adult and immature bed bugs. These aldehydes may be one way bed bugs attract each other to form aggregations (Ulrich et al. 2016), suggesting that the aldehydes may be used to develop an inexpensive and more discreet option to lure and trap bed bugs. Dr. Feldlaufer currently aims to develop and test reduced risk pesticides that show promise in killing bed bugs.

Bed bugs can be a terribly menacing presence in anyone’s home, but thankfully Dr. Feldlaufer has dedicated much of his career to keeping us informed about these insects and their crafty activities. By pairing lures with reduced risk pesticides, Dr. Feldlaufer aims to develop options to safely, affordably, and discreetly ensure we can sleep tight assured that the bed bugs will not bite.

References:

Lit L., Schweitzer J.B., and Oberbauer A.M. 2011. Handler beliefs affect scent detection dog outcomes. Animal Cognition 14: 387.
Pfiester M., Koehler P.G., and Pereira R.M. 2008. Ability of bed bug-detecting canines to locate live bed bugs and viable bed bug eggs. Journal of Economic Entomology 101(4):1389-96.

Ulrich, K.R., Kramer, M. and Feldlaufer, M.F. 2016. Ability of bed bug (Hemiptera: Cimicidae) defensive secretions (E)-2-hexenal and (E)-2-octenal to attract adults of the common bed bug Cimex lectularius. Physiological Entomology 41: 103-110.

Author Bio:

Hanna Kahl is a master’s student at University of Maryland in Dr. Cerruti Hooks’ lab researching the effects of red clover living mulch on arthropod pests and pollinators.

Samuel Ramsey is a PhD student at University of Maryland in Dr. Dennis vanEngelsdorp’s lab researching Varroa destructor.

Take Time to Stop and Smell the Stink

11/18/2016

Skunk, cilantro, burnt rubber—just a few of the many scents you might associate with the pungent odor produced by stink bugs. Like many animals, these bugs produce a variety of chemical odors (called semiochemicals) that modify the behavior of recipient organisms in different ways. That “stink” that these bugs get their name from is just one example. Stink bugs (family Pentatomidae) represent an extremely diverse family of insects that includes both agricultural pests and beneficial predators. Some of the agricultural pest species can cause millions of dollars in damages to crops. At the USDA Agricultural Research Service in Beltsville, Maryland, Dr. Don Weber studies stink bug semiochemicals in hopes of being able to take advantage of their communication to monitor their populations. In particular, Dr. Weber studies pheromones – one kind of semiochemical used for communication within a species. For example, male stink bugs emit pheromones to attract females, or in other cases, both females and males. In other insects, pheromones can also act to signal danger, food resources, or aggregation sites.

Because semiochemical signals often influence insect behavior, synthetically replicating these scents equips growers with a powerful tool to manage agricultural insect pests. This strategy works most effectively if the chemical composition of the natural odor is identified and isolated, and if the behavior it elicits is fully understood. The invasive brown marmorated stink bug (BMSB) (Halyomorpha halys) and the harlequin bug (Murgantia histrionica) (Figure 1) both respond to traps baited with pheromone lures which can be used in order to monitor levels of these pests to determine whether or not to take action against them with insecticides (Figure 2).

Figure 1. Adult brown marmorated stink bug, Halyomorpha halys (left, photo credit to G. Hamilton, Rutgers) and adult harlequin bugs, Murgantia histrionica (right, photo credit M.J. Raupp)

With stink bugs, there are some challenges that researchers such as Dr. Weber have faced in using pheromones as lures. Pheromones released by male stink bugs are not exclusively sex pheromones;females, males and nymphs alike are attracted by the male pheromones. This indicates to researchers that those pheromones might have some of the other roles mentioned above, such as signaling aggregation sites.

Yet another piece to this complicated smell puzzle stems from cross-species attraction, meaning that the deployment of a pheromone by one stink bug species can attract another stink bug species. Scientists can monitor BMSB by using MDT, the aggregation pheromone of a completely different stink bug species (the Asian brown-winged green bug, Plautia stali). However, because the pheromone is most attractive to BMSB in the fall, which is after most crop harvests have finished or already starting, so this is not very helpful for most growers1.

To produce better attractants for BMSB, researchers successfully identified the specific aggregation pheromone produced by BMSB adult males. The pheromone (commonly called murgantiol) is comprised of two different isomers, or molecular conformations, with a specific ratio. This ratio is important to know because there can be variation between species, and even between individual bugs, for what ratio is most attractive. Dr. Weber and colleagues determined which isomer ratio of murgantiol was most attractive separately to BMSB, and then combined this pheromone with MDT to see if a blend of the two was attractive to the bugs. The two pheromones together were much more attractive than either alone – with an added advantage of this blend being attractive to the bugs all season long, unlike MDT by itself. These pheromones also do not need to be extremely pure, which is good news for keeping the cost of lure production low.

Figure 2. Pheromone-baited pyramid trap used to monitor BMSB. (Image courtesy of Dr. Don Weber)

Harlequin bug males produce murgantiol as well – so the same types of lures can be used to monitor both pest species, though the two species have different ideal isomer ratios5. Harlequin bugs, however, specialize in feeding on cruciferous plants – unlike BMSB, which are unspecific in their preferred hosts (polyphagous). Harlequin bugs were found by Dr. Weber to also be highly attracted to pheromones mixed with mustard oils, which are chemically derived from their host plants’ defensive compounds6,7. Adding these oils to murgantiol therefore could enhance trap performance for harlequin bugs, much like MDT for BMSB.

While Dr. Weber and his colleagues uncovered a lot of information about stink bug pheromones, there is still room to further our understanding. Now that researchers have evidence for which pheromone blends work most effectively for the two species, there is a lot of fine-tuning to be done about how exactly to use them in traps, and the most economical way to produce them. Dr. Weber and his colleagues also have plans in the works to investigate using pheromones for other pest-combating purposes, such as attracting insect predators of pest species like stink bugs, and genetically engineering plants to produce insect pheromones and act as trap crops. Overall pheromones offer an exciting approach to manage not only stink bugs, but many different pest insects, to better protect our agriculture.

To learn more about Dr. Weber’s research or contact him, visit his USDA homepage. For more information on BMSB or Harlequin bugs, you can visit the University of Maryland Extension pages on BMSB and Harlequin bugs.

Authors: Elizabeth Brandt, Aditi Dubey, Morgan Thompson

Resources Cited

Sugie et. al. 1996 "Identification of the Aggregation PHeromone of hte Brown-Winged Green Bug, Plautia stali Scott (Heteroptera: Pentatomidae)" Applied Entomology and Zoology 31:427-431
Funayama 2008, "Seasonal fluctuations and physiological status of Halyomorpha halys (Stal)(Heteroptera: Pentatomidae) adults captured in traps baited with synthetic aggregation pheromone of Plautia crossota stali Scott (Heteroptera: Pentatomidae)," Japanese Journal of Applied Entomology and Zoology, 52:69-75.
Weber et al. 2014 "Synergy of Aggregation Pheromone With Methyl (E,E,Z)-2,4,6-Decatrienoate in Attraction of Halyomorpha halys (Hemiptera: Pentatomidae)" J. Econ. Entomol. 107:1061-1068.
Leskey et al. 2015. "Behavioral Responses of the Invasive Halyomorpha halys (Stal) to Traps Baited with Stereoisomeric Mixtures of 10,11-Epoxy-1-bisabolen-3-OL" J. Chem. Ecol. 41: 418-429.
Zahn et al 2008. "Identification, Synthesis, and Bioassay of a Male-Specific Aggregation Pheromone from the Harlequin Bug, Murgantia histrionica" J Chem Ecol (2008) 34: 238-251.
Fahey et al. 2001 "The chemical diversity and distribution of flucosinolates and isothiocyanates among plants." PHytochemistry 56:5-51.
Weber et. al., unpublished data.

Varroa and Deformed Wing Virus: How the Vector Changed the Virulence of the Virus

11/2/2016

Recombination – the exchange of a section of genomes between viruses – is a well-known mechanism of evolution in enteroviruses, RNA based viruses infecting mammals (Fig. 1). This is an important source for the emergence of novel variants of viruses. Last week, Dr. Eugene Ryabov spoke at the University of Maryland about his research interests on this phenomenon. In particular, he focused on investigating the importance of recombination in viruses that infect invertebrate hosts, such as the honey bee (Apis mellifera) and how it can influence the development of diseases in this host.

Figure 1.Recombination between viral RNA genome. Credit: Abbie Smith, scienceblogs.com

Figure 2. Deformed Wing Virus on an adult bee. Note normal shape of wings on the bee to the right. Credit: Saskatchewan Beekeepers Association

In the 1980s a vicious parasitic mite from Asia, the Varroa mite (Varroa destructor) was introduced to the United States. The mites feed on honey bee pupae by sucking their hemolymph (blood) and are known to transfer various viruses, such as Deformed Wing Virus (DWV) (Bowens-Walker et al. 1999).
DWV is a RNA virus of the family Iflaviridae. It owes its name to the wing deformities (shrinking and crumpling) that it causes in infected honey bees (Fig. 2). These bees die shortly after they emerge, and it has been strongly associated with winter colony death (Highfield et al. 2009). This virus has become ubiquitous in the United States due to the various modes of transfer, such as the mite, food, feces, queen to egg, and drone to queen.

However, asymptomatic bees are common and can have either low or high virus loads (de Miranda et al. 2012). The story is complicated by the fact that DWV is very closely related to a series of other RNA viruses, such as Varroa destructor virus-1 (VDV-1). And the story gets even more complicated… because of recombination.

Using next generation gene sequencing, Dr. Ryabov (currently a visiting scientist at the United States Department of Agriculture (USDA) Bee Research Lab in Beltsville, MD) and his colleagues at the University of Warwick, UK, decided to characterize the virus diversity in honey bees. They found that the genome of DWV-like viruses could be divided into three functional parts, or “modules”, any of which were sometimes crossed-over between DWV and VDV. They identified three distinct types of genomes: the 100% DWV genome, and two types of recombinants formed by the association of “modules” from DWV and VDV, which they named VDV-1DVD and VDV-1VVD (Moore et al. 2011).

So what was thought to be a single population of viruses is actually a group of variants (VDV-1DVD, VDV-1VVD and DWV). Dr. Ryabov then compared the levels of each variant in honey bee pupae and associated Varroa mites. They found that individual honey bees were usually infected by a mixture of the three variants. Of the three viruses, the recombinant VDV-1DVD’s levels in honey bee pupae was highly associated with its level in associated mites. This suggests this recombinant is more efficiently transmitted between the mite and the honey bee.

When Varroa acts as a vector for the DWV, it increases the levels of viruses in contaminated colonies, causes deformities in the affected workers (Fig. 2), and overall results in increased risk of mortality of the whole colony. But what remains to be determined is whether this is caused by 1) Varroa amplifying and introducing more virulent strains of the virus and/or by 2) Varroa suppressing the honey bee immune response.

To test those two hypotheses, Dr. Ryabov exposed Varroa-naïve honey bees (collected from a Varroa-free region in Scotland) to DWV either orally (in brood food) or through Varroa mite feeding. They monitored the change in DWV diversity and loads within the host as well as changes in honey bee expressed genes to identify potential antivirus immune responses (Ryabov et al. 2014). They detected changes in expression for a number of genes associated with the immune response of honey bees while in presence of the mites. This suggests that the second hypothesis should be further explored.

This study also showed that in Varroa-free colonies (controls), honey bees had highly diverse DWV, though at low levels. When the bees were infected orally, DWV levels remained low, but the composition of the DWV strains changed compared to the controls. When bees were infected through Varroa, two outcomes would happen. Honey bees either showed low levels of diverse DWV strains, or they developed high levels of a single specific variant of DWV or very closely related variants, even though the infecting mite contained a high diversity of strains. By inoculating honey bees through injections (which simulates Varroa feeding), the researchers observed high levels of replication for the recombinant strains containing VDV-1 derived structural gene block. This suggests that these particular strains have an advantage due to the route of transmission. All of this largely supports the first hypothesis that Varroa amplifies more virulent strains of the virus.

In conclusion, this example of the shift in virulence of the DWV – from a benign and asymptomatic virus to a serious disease – illustrates the importance of the process of recombination in the generation of various strains of viruses, and how the addition of a vector, and a new route of transmission, can increase the impact of a virus by altering the relative composition of its strains.

Bloggers:
Meghan McConnell is a Master’s student in Dennis vanEngelsdorp’s Lab. She is currently studying honey bees, with a focus on non-chemical control of Varroa mites.

Nathalie Steinhauer is a PhD candidate working in Dennis vanEngelsdorp’s Lab on honey bee health and management practices. Her projects aims to identify and quantify the effects of risk factors associated with increased colony mortality.

References:
Bowen-Walker, P. L., S. J. Martin, & A. Gunn. 1999. The Transmission of Deformed Wing Virus between Honeybees (Apis mellifera L.) by the Ectoparasitic Mite Varroa jacobsoni Oud. Journal of invertebrate pathology 73(1), 101-106.

Highfield, A. C., A., El Nagar, L. C. Mackinde, M. L. N. Laure, M. J. Hall, S. J. Martin, & D. C. Schroeder. 2009. Deformed wing virus implicated in overwintering honeybee colony losses. Applied and environmental microbiology 75(22), 7212-7220.

de Miranda, J. R., L. Gauthier, M. Ribiere, and Y. P. Chen. 2012. Honey bee viruses and their effect on bee and colony health. In D. Sammataro & J. Yoder (Eds.) Honey bee colony health: challenges and sustainable solutions. CRC Press. Boca Raton. 71-102.

Moore, J; Jironkin, A; Chandler, D; Burroughs, N; Evans, DJ; Ryabov, EV (2011) Recombinants between Deformed wing virus and Varroa destructor virus-1 may prevail in Varroa destructor-infested honeybee colonies. Journal of General Virology, 92(1): 156–161. DOI:10.1099/vir.0.025965-0

Ryabov, EV; Wood, GR; Fannon, JM; Moore, JD; Bull, JC; Chandler, D; Mead, A; Burroughs, N; Evans, DJ (2014) A Virulent Strain of Deformed Wing Virus (DWV) of Honeybees (Apis mellifera) Prevails after Varroa destructor-Mediated, or In Vitro, Transmission. PLoS Pathogens, 10(6): 1–21. DOI:10.1371/journal.ppat.1004230

Alien Invaders

10/25/2016

Aliens are invading the forests of the United States! Not the green, bug-eyed aliens from outer space; no we are talking about the, well… green, bug-eyed aliens from Earth. With the globalization of trade, insect introductions leading to invasive pest problems have steadily increased over the last few centuries, causing massive economic and environmental devastation in the systems where these pests permeate. These invaders are especially difficult to manage when they are pests of our native North American forest trees due to the large spatial scale associated with them, making pesticide applications impractical.

Dr. Kris Abell, one of the UMD Department of Entomology’s newest post-doctoral associates, has investigated biological control efforts to combat two invasive forest pests, the elongate hemlock scale and the emerald ash borer. Biological control methods for controlling a pest involve using predators and parasites from the pest’s native range to create a natural population balance in its introduced range.

During his time as a PhD student at the University of Massachusetts, Amherst, Dr. Abell followed up Dr. Mark McClure’s work on biological control of Elongate Hemlock Scale (EHS), Fiorinia externa Ferris. EHS is an invasive insect pest from Japan, which attacks hemlock trees. Feeding by EHS scales damages the hemlock’s needles, turning them from green to yellow.

Elongate hemlock scale on hemlock needles. The oblong brown insects are the adult female scales, the smaller round yellow insects are 1st instar scales. Photo credit: Kris Abell

Encarsia citrina adult wasp measuring less than 1 mm in length. Photo credit: Kris Abell

The best way to understand a pest is to observe it in its native range, so with that, McClure went to Japan to study EHS. McClure’s research found that one species of wasp, Encarsia citrina, is a parasitoid, an insect that develops in and eventually kills its host, of EHS in both Japan and in its new range in North America. Wasps lay eggs in the 2nd instar of EHS, and wasp larvae hatch and feed on the scale, which kills the scale insect and produces more E. citrina. McClure’s research indicated that EHS is less abundant and has much higher parasitism rates by E. citrina in Japan than the EHS at his study sites in Connecticut. He hypothesized that differences in climate between the two locations, Japan

having a warmer climate than Connecticut, created an asynchronous relationship between the host (EHS) and the parasitoid (E. citrina) in Connecticut. This means that the scale and parasitoid are developing at different times of year, preventing the wasp from being able to effectively attack the scale in its introduced range. With the colder climate of Connecticut, it was hypothesized that the EHS scales developed more slowly. Wasps, as a result, would have fewer suitable 2nd instar hosts to parasitize. Dr. Abell tested this by observing scale abundance and parasitism by E. citrina at three distinct latitudes in the U.S. (Connecticut [“coldest”], Pennsylvania, North Carolina [“warmest”]), hypothesizing that he would find more parasitoid-host synchrony as he moved further south where warmer temperatures would allow for multiple generations of scales.

Ultimately, Dr. Abell did not observe any increase in synchrony between EHS and E. citrina at any of his three field sites. Instead he found continuous reproduction of EHS, and all life stages were present throughout the year. This led Dr. Abell to Japan to better understand how EHS behaves in its native range. While surveying hemlock scales and their associated parasitoids, Dr. Abell found 11 new species attacking EHS in Japan, some of which may have potential as classical biological control agents.

After Dr. Abell finished his work on EHS, he moved to Michigan State University where he studied another forest pest, the emerald ash borer (EAB), Agrilus planipennis Fairmaire, an invasive beetle species introduced to the United States from Asia. The adult females lay their eggs in bark of ash trees (Fraxinus spp.) and the larvae burrow under the bark where they feed and develop. Feeding damage results in girdling and ultimately the killing of the trees. There are a few parasitoid wasps that are known to attack EAB at different stages of its life cycle in Asia. Two of these parasitoids that have been introduced to the U.S. are Tetrastichus planipennisi, a wasp that attacks EAB in its larval stage, and Oobius agrili

Emerald ash borer adult: David Cappaert, Michigan State University, Bugwood

a wasp that is less than 1mm in length that attacks EAB eggs. Research done by Duan et al. in 2013 indicated that T. planipennisi was effectively established in Michigan and is a strong disperser. However, they observed that there was no parasitism of EAB in larger trees. In a study done by Dr. Abell, it was determined that the bark thickness was preventing this small wasp from attacking the EAB larvae. The ovipositor (egg-laying mechanism) of T. planipennisi is too short to reach the EAB larvae underneath the thick bark.

Under the bark of this ash tree we see the aftermath of developing EAB larvae: Serpentine galleries of EAB larvae that have fed on this tree. All of this damage results in girdling. Photo credit: Kris Abell

Once biological control agents like these wasps are released, it is important to continue monitoring them. Dr. Abell helped determine the best methods to monitor the establishment and range of the tiny egg parasitoid, O. agrili. Logs infested with sentinel EAB eggs were set out in the field to detect O. agrili. This method, while effective, did not accurately represent the parasitism, taking place on wild trees. Dr. Abell tested two other methods; visual searching and bark sifting. Visual searching is a labor-intensive method by which observers flake off pieces of ash bark to reveal EAB eggs. Once parasitized by O. agrili the eggs turn black, therefore any black eggs were brought back to the lab and were further analyzed to confirm parasitism by O. agrili. Bark sifting entailed shaving the outer bark off of the ash trees and sifting out the smallest pieces in the lab to look for parasitized EAB eggs.

The bark was also placed in emergence chambers to collect any parasitoid wasps that emerged from the bark remnants that were missed in earlier screening. After two years of testing these methods, Dr. Abell concluded that the bark-sifting method was a more effective way to measure the rate of O. agrili egg parasitism in the field because significantly more parasitoids were recovered with this method. Invasive insects continue to attack our forests today, therefore it is very important to continue to understand and utilize biological control methods to preserve our forests. Dr. Abell continues his work on EAB biological control in the Shrewsbury lab here at the University of Maryland where he is evaluating other introduced and native parasitoids and additionally an integrated approach that combines pesticides with classical biological control methods.

About the Authors:
Olivia Bernauer is a second year Master’s student in Dennis vanEngelsdorp’s bee lab working to better understand the floral preferences of Maryland’s wild, native pollinators.

Jackie Hoban is a second year Master’s student working on emerald ash borer biological control in Paula Shrewsbury’s lab.

Inversion Immersion: How Chromosomal Architecture Leads to Speciation

10/5/2016

What makes a species a species? What are the characteristics that make it unique enough to be distinguished from similar creatures? What are the genetic underpinnings that allow species to evolve to create a distinct entity?

These are the type of questions Dr. Carlos Machado is trying to answer. Answering them are key to understanding our planet's biodiversity. A traditional definition of speciation centers around the concept of reproductive isolation. While members of two different species may mate, if they either produce no offspring or sterile offspring, these two individuals represent different species. This is known as the biological species concept. However, what factors lead to reproductive isolation? One way is through physical separation of populations (e.g., a mountain range) or allopatric speciation. On the other hand, what about sympatric speciation, or speciation in overlapping geographical distributions?

Fig 1: The famous Drosophila fruit fly.

These inversions have a lower recombination frequency which basically means the genes found in these regions do not change that much. Inversions can be found in many species including humans. Dr. Machado showed that species differences, such as mating discrimination by females and courtship song differences, mapped (are localized) to these regions. Fixed chromosomal rearrangements were critical for sympatric speciation of these two species. When compared to Drosophila species that do not co-occur in the same area (allopatric species), such as D. simulans and D. mauritiana, differences mapped to many different regions.

Building upon the shoulders of early giants in genetics, such as Richard C. Lewontin, John Lee Hubby, and Theodosius Dobzhansky, Dr. Machado’s work utilizes two species of fruit flies with geographical overlap (or sympatric distribution) in their populations: Drosophila persimilis and Drosophila pseudoobscura. He examined the chromosomal differences in each species to identify what makes each species unique. In these chromosomes, there are regions called inversions (Figure 2) where a segment of a chromosome is broken off, reversed, and then rejoined.

Fig 2: An example of a chromosomal inversion. Notice the segment in the middle is broken off, reversed, and rejoined. Image taken from: http://evolution.berkeley.edu/evolibrary/images/history/inversion.gif

Dr. Machado’s work brings us a step closer to understanding the roots of speciation. Following from this work, his future research ideas center on correcting inversions using the latest genetic techniques, such as CRISPR (Clustered regularly interspaced short palindromic repeats), to study in further detail these regions that are responsible for species’ differences.

Jonathan Wang is a PhD student in Raymond St. Leger’s lab studying host-pathogen interactions.

Jen Jones is a PhD student in Bill Lamp’s lab, studying how socioeconomic factors influence the distribution of mosquito populations.

Food and the Environment: A Balancing Act

10/4/2016

As the world’s population continues to grow, a key issue facing society is how to balance feeding the world while protecting the environment. Dr. Kate Tully has worked on this problem in both East Africa and the United States, where the solution will require different approaches, in part due to the different use of crops. In developed nations, many grain crops are grown for animal feed and fuel compared to developing nations where most are grown for human consumption. As such, different goals and approaches need to be taken for a sustainable future.

While crop yield has increased in many parts of world since the 1960s, this trend has not occurred in sub-Saharan Africa. Dr. Tully is working in Kenya and Tanzania to determine what changes can improve yield without excessive fertilizer use. Africa as a whole plans to increase nitrogen-based fertilizer use, but there is an upper limit to the amount plants can use before the rest is lost to the environment.

Research plots in Kenya. The left plot received no fertilizer while the right plot received the maximal amount of 200 kg N/ha fertilizer. Photo Credit: Kate Tully

To determine an optimal amount of fertilizer, Dr. Tully planted corn in Kenya and Tanzania, fertilized them with inorganic nitrogen (N) treatments ranging from 0-200 kg N/ha, and assessed the ultimate fate of the nitrogen—was it stored in the soil, incorporated into the plant, or washed out in groundwater?

The effect and destination of the nitrogen depended on the climate, particularly precipitation patterns, and soil conditions in the field. The soil in the Kenyan plots contained high amounts of clay, slowing the loss of nitrogen in the soil and increasing nitrogen availability for the crops. Moderate amounts of nitrogen doubled yield, as most of the nitrogen was directly taken up by the plant. In Tanzania, the soil in the plots had a much higher proportion of sand, resulting in a build-up of nitrogen as ammonium in the soil and very little incorporation by the corn.

Large amounts of fertilizer nitrate were leached through the sandy soils to groundwater. Dr. Tully concluded that, in addition to using better overall management practices such as irrigation and increasing organic matter content, site characteristics should determine the optimal nitrogen fertilizer amount; in these cases, farmers in Kenya can increase yield by adding 75-100 kg N/ha while those in Tanzania should only use 50 kg N/ha. These levels can increase yield while protecting the environment from excess nitrogen leaching into waterways and gassing off into the air.

Back in the U.S., Dr. Tully turned her attention to soil nutrient dynamics on the Eastern Shore, Maryland. The dominant agricultural crops feed—corn and soybeans support their primary industry—poultry. Over the years, phosphorus from fertilizers and chicken litter has accumulated in the soil to the point of saturation. Phosphorus typically binds to soil particles with minimal nutrient leaching to the surrounding environment. Yet, as human-accelerated sea-level rise causes saltwater intrusion of coastal farmland, soil dynamics are changing. Saltwater can significantly alter soil chemistry, which may result in the release of bound soil nutrients as runoff. While nitrogen and phosphorus are essential nutrients for living organisms, too much can throw entire ecosystems off balance. Excess phosphorus in particular has led to algal blooms in fresh water estuaries. Over time algal blooms deplete life supporting oxygen resulting in ‘dead zones.’ But to what magnitude, rate, and extent are saltwater intrusions increasing the movement of phosphorus?

Microcosms of soil from the Eastern Shore are exposed to varying levels of oxygen, ionic strength (equivalent to salt levels found in seawater) and sulfate to quantify nutrient loss. Photo Credit: Dani Weissman

Dr. Tully is exploring such questions via laboratory experiments and field studies. Field collections of soil and soil water in addition to vegetation monitoring were carried out in five settings on several Eastern Shore farms: crop, crop edge, bare ground, neighboring ditches, and marshes that have been invaded by Phragmites, a large exotic perennial grass that thrives in high concentrations of nitrogen and phosphorus. Her findings show that phosphorus was released into ditch water adjacent to crops, which then flows into tidal creeks and eventually the Chesapeake Bay. Understanding effects of saltwater intrusion on nutrient release is critical to the health of the Bay, and crop productivity thus should be included in restoration models and crop planning. As in the case of Kenya and Tanzania, successful strategies for balancing complex food and environmental issues necessitate engaging all stakeholders, especially local farmers who are directly affected.

Read more of Dr. Tully’s work!

Tully K, Hickman J, McKenna M, Palm CA, Neill C. 2016. Effects of increased fertilizer application on inorganic soil N in East African maize systems: vertical distributions and temporal dynamics. Ecological Applications 26: 1907-1919. doi:10.1890/15-1518.1
Becca Eckert is a Ph.D. student in Bill Lamp’s lab. Her research examines how changes in light and nutrient availability affect macroinvertebrate growth and diversity in heterotrophic headwater streams as mediated by changes in leaf-associated algal quantity and quality.
Lisa Kuder is a Ph.D. student in Dennis vanEngelsdorp’s Bee Lab. Her research focuses on road ecology, specifically improving highway rights-of-way for pollinators.

Resurrecting the Work of Gordon Alexander: Grasshopper Communities and our Changing Climate

9/23/2016

Dr. César Nufio has examined a vast number of grasshoppers to understand the impacts of climate change on insects in the Rocky Mountains of Colorado. To be precise, the number of specimens he has captured and processed over the last 10 years has exceeded 180,000 from grassland communities found along a high plains to subalpine gradient. Nufio’s National Science Foundation funded research combines extensive field surveys with comparisons of museum collections, weather data, and laboratory and field experiments. The entire project started with his discovery of a collection of 25,000 pinned and label grasshopper specimens and three data notebooks at the University of Colorado’s Natural History Museum. These pinned specimens and notebooks, which are part of the Gordon Alexander Collection, were part of several field studies conducted over 50 years ago.

Gordon Alexander and assistants at the subalpine field site Brainard Lake (Roosevelt National Forest, 3200 m in elevation) on August 17th, 1958. Photo courtesy of The Gordon Alexander Project.

Dr. Gordon Alexander (1901-1973) was a professor at the University of Colorado and chair of the Biology Department for over 20 years. In the springs and summers of 1958-1960, Alexander conducted field surveys of grasshoppers found across the Front Range of Colorado. He was interested in determining which species were present where and when, and how grasshoppers adapted to living on mountains. Four of these surveyed sites, which were associated with high prairie, lower montane, upper montane, and subalpine meadows, were sampled on a weekly basis. During each of these surveys, Alexander record the species, and life stage of all grasshoppers that were captured. Fortuitously, each of these sites were associated with weather stations that have been collecting data since 1953. Despite the importance of Alexander’s surveys and data collections for serving as a baseline for addressing how insect development and communities have been affected by climate change over the last 50 years, the potential for such a collection was nearly forgotten.

In 2006, Nufio began resurveying the same sites that Alexander had nearly half a century ago. Nufio wanted to understand how climate had changed along the gradient and how this change might impact the timing of grasshopper life history events (when they hatched and how fast they developed), their elevational ranges and demography (population size, longevity, reproductive rates), and body sizes. All of these questions could be addressed comparing his recent findings to the observations made by Alexander.

To Nufio’s surprise, the sites had not warmed equally across the elevational gradient. Temperatures in the high prairie had not significantly changed since the 1950s but the montane and sub-alpine sites had warmed a great deal (more on temperature: Article). As such it was not surprising that the grasshoppers appeared to hatch and reach adulthood at roughly the same time they did 50 years prior at the lowest site. At the higher sites, however, temperatures had warmed by ~1.5°C during Nufio’s initial resurvey (2006-2008) and consequently, the grasshopper communities were found to have hatched and became adults much sooner than they did during Alexanders time by 2-4 weeks! (more on phenology: Article). During 2009 to 2011, seasonal temperatures across the mountains declined and the grasshoppers across the mountain no longer hatched early or reached adulthood earlier than they previously had. However, in 2012 which was the second warmest year in the last 118 years in Colorado, the communities at all of the sites advanced their development by nearly a month. One population even matured 52 days earlier than previously recorded! Interestingly, a second study examining flowering times at one of the sites showed that the grasshoppers have changed their timing to adulthood by twice as much as the plants had changed their flowering times. As grasshoppers are herbivores, changes in a plants phenology can have major impacts on the grasshoppers. For example, if a grasshopper population prefers a certain plant and they emerge before the plant is available then the grasshoppers will most likely experience starvation.

Grasshoppers go through five nymph stages (instars) (top five sketches) before becoming an adult. Generally nymphs and adults look similar in shape with the exception that adults have fully developed wings (bottom sketch). Some grasshopper species the nymphs and adults have strikingly different colors or patterns. Image: Utah State University

In addition to examining phenology, Nufio more recently was interested in the demographic changes among species over the elevational gradient. He observed that species with short wings showed a reduction in body length with no change in reproductive output with increasing elevation. Conversely, grasshoppers with long wings showed no change in body size but a reduction in reproductive output. Nufio also examined changes in weight, longevity, and reproduction over time in response to temperature using a caged field experiment. During a warm year, he found that females tended to be heavier, live longer, and laid more eggs (more on demography: Article). While these subalpine grasshoppers appear to be benefiting from warming, his surveys suggest that those at the bottom of the mountain may be negatively affected by warm years.

Grasshoppers in the historical collection assembled by Dr. Gordon Alexander. (Photos courtesy of Dr. Cesar Nufio)

Living grasshopper as part of Dr. Nufio's current research. Photo credit to Jeff Mitton.

Grasshoppers in the historical collection assembled by Dr. Gordon Alexander. (Photos courtesy of Dr. Cesar Nufio)

Fall Colloquium Schedule

9/9/2016

Please join us for as many as you can! Lunch will be provided for all attendees.

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