Powdery parasites pursue pedunculate oak

Studying disease transmission is tricky for many reasons. Most humans frown on what might seem like the easiest experimental protocol – release a disease into the environment and watch to see how it spreads. For his doctoral dissertation in 2006, Ayco Tack settled on a different experimental protocol – bring the potential hosts to the disease. In this study, staged in Finland, the hosts were pedunculate oak trees, Quercus robur, and the disease was the powdery mildew parasite, Erysiphe alphitoides. Almost 10 years later, Adam Ekholm continued research on the same system, with Tack as his co-supervisor.

Ayco Tack

Trees on the move. Credit: Ayco Tack.

But before moving trees around, the researchers first needed to see how the disease moved around under field conditions.  Within a tree stand, powdery mildew success will depend on how many trees it occupies, how many trees it colonizes in the future, and how many trees it disappears from (extinction rate). The researchers measured these rates over a four year period (2003 – 2006) on 1868 oak trees situated on the island of Wattkast in southwest Finland. They also measured spatial connectivity of each tree to others in the stand. In this case connectivity is a measure of the distance between a tree and other trees, weighted by the size of the other trees. So a tree that has many large neighbors nearby has high connectivity, while a tree with a few distant and mostly small neighbors has low connectivity. Results varied from year-to-year, but in general, the researchers found higher infection rates, lower extinction rates, and some evidence of higher colonization rates in trees with high connectivity.

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Oak leaf infected with powdery mildew parasite. Credit: Adam Ekholm.

The importance of connectivity indicated that the parasites simply could not disperse efficiently to distant trees. But perhaps the environment might play a role in colonization rates as well. For example, fungi like powdery mildew tend to thrive in shady and humid environments. Thus a tree out in the open might resist colonization by powdery mildew more effectively than would a tree deep in the forest. To test this hypothesis, Tack and his colleagues placed 70 trees varying distances (up to 300 meters) from an infected oak stand. On one side of the oak stand was an open field, while the other side was closed forest. Thus two variables, distance and environment, could be investigated simultaneously.

Ayco Tack inspecting a potted tree_Tomas Roslin

Ayco Tack inspects an oak tree placed in an open field. Credit: Tomas Roslin.

The researchers collected infection data twice; once in the middle of the growing season (July) and a second time at the end of the growing season (September). Not surprisingly, infection rates were higher by the end of the growing season. In general, infection rates, and infection intensity (mildew abundance) were higher in the forest than in the field, indicating a strong environment effect. In the July survey, trees further from the oak stand had lower infection intensity, but as infection rates increased over the course of the season, the effects of distance diminished, particularly in the forest.

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Upper two graphs show the impact of habitat type on (a) proportion of trees infected and (b) mildew abundance. The lower two graphs are the influence of distance from parasite source on mildew abundance of trees set in (c) a forest habitat and (d) an open field. Mildew abundance was scored on an ordinal scale with 0 = none and 4 = very abundant.

Ten years later, Adam Ekholm, as part of his PhD dissertation that studies the effect of climate on the insect community on oak trees, added a third element to the mix – the influence of genes on disease resistance. He wondered whether certain genotypes were more resistant to powdery mildew infection. The researchers grafted twigs from 12 large “mother” trees, creating 12 groups of trees, with between 2 – 27 trees per group (depending on grafting success). Each tree in a given group was thus genetically identical to all other trees within that group.

Ayco Tack

Oak tree placed in the forest. Credit: Ayco Tack.

The researchers chose a site that contained a dense stand of infected oaks, but was surrounded by a grassy matrix that contained only an occasional tree. To study the impact of early season exposure, Ekholm and his colleagues divided the trees into two groups; 128 trees were placed in the matrix at varying distances from the infected stand, while 58 trees were placed directly in the midst of the stand for about 50 days, and then moved varying distances away. The researchers scored trees for infection at the end of the growing season (mid-September).

 

Trees that spent 50 days within the oak stand had much higher infection frequency and intensity than trees that were initially placed in the matrix. Some genotypes (for example genotype I in graphs C and D below) were much more resistant to infection than others (such as genotypes D and J). Finally trees further from the source of infection were less susceptible to become colonized over the course of the summer (data not shown).

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Proportion of trees infected (A) and proportion of leaves infected (B) in response to early season exposure to stand of oaks infected with the powdery mildew parasite (oak stand) or no early season exposure (matrix). Proportion of trees infected (C) and proportion of leaves infected (D) in relation to tree genotype. Genotypes are labeled A – L; numbers in parenthesis are sample size for each group.

These findings illustrate how dispersal, host genotype and the environment influence the spread of a parasite under natural conditions. The parasite exists as a metapopulation – a group of local populations inhabiting networks of somewhat discrete habitat patches. Some populations go extinct while others successfully colonize each year, depending on distance from a source, tree genotype and environment. Ekholm and his colleagues encourage researchers to use similar experimental approaches in other host-parasite systems to evaluate how general these findings are, and to explore how multiple factors interact to shape the dynamics of disease transmission.

note: the paper that describes this research is from the journal Ecology. The reference is Ekholm, Adam; Roslin, Tomas; Pulkkinen, Pertti and Tack, Ayco. J. M. (2017). Dispersal, host genotype and environment shape the spatial dynamics of a parasite in the wild. Ecology. doi:10.1002/ecy.1949. The paper should come out in print very soon. Meanwhile you can also link to Dr. Tack’s website at www.plantmicrobeinsect.com Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Highly disturbed birds

About 50 million years ago, the fast-moving Indo-Australian plate crashed into the Eurasian plate, giving rise to the Indian peninsula, and beginning a process of faulting and folding that ultimately formed our present day Himalaya Mountains. This process continues today, with the Himalayas still rising about 5 mm per year. The region is very variable, with tremendous glaciers and snowfields at high elevations, and forests and grasslands at lower elevations.

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The lead Author, Paul Elsen, stands in front of the Tirthan Valley.  The highest peaks range up to 4900 meters.

The variation in elevation, climate and soils make the Himalyan region in northern India a mecca of biological diversity, hosting over 10,000 identified plant species and about 1000 bird species. As in most of India, human population growth is putting enormous pressure on the forested regions, partly as a source of wood for heating and cooking, which has led to extensive deforestation. In concert, substantial forested areas are being converted to farms or pastures to feed the growing population. Paul Elsen and his colleagues wanted to know how these transformations of forests to cropland and pastures were affecting bird population across the region. They were particularly interested in how birds survived the winter, a period of climatic stress and food scarcity, when many of the birds descend from their high elevation breeding grounds to lower elevations that are nearer to human populations.

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Chestnut-headed Tesia, an altitudinal migrant found in high elevation forests in summer, and in forests and agricultural lands in winter. Credit: Prashant Negi.

The researchers set up three transects across four different landscapes (total of 12 transects), representing four levels of disturbance. The undisturbed landscape was primary forest in the Great Himalayan National Park. A second disturbance type – low intensity – retained a mixture of community forest used for timber and fuel, and also included some small agricultural plots. A third disturbance type – medium intensity – had small wooded areas, but was dominated by mixed agriculture including orchards and a variety of crops such as grains, beans and garlic. The final disturbance type – high intensity – was used as pasture, had mostly grasses and very few trees or crops.

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Four land-use types. Credit Paul Elsen

The basic research protocol was literally a walk in the woods. Elsen walked (slowly) along the same trail in each transect three times during the winter season, and identified and counted all of the birds. Other researchers identified, measured and counted the plants growing along the transects.

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Lead field assistant, Lal Chand (left), and co-author Kalyanaraman Ramnarayan (right) conduct plant surveys near the top of the world.

 

Elsen was stunned by what his team discovered. Before beginning this study, he had spent about a year in the Himalayas within intact forests doing other PhD-related research. His travels into surrounding villages showed significant bird activity, but he assumed these birds were primarily species associated with humans or more open habitats. He expected decreasing bird diversity and abundance with increasing agricultural intensification, where the bird communities in intact primary forest would be teeming with species in high densities, and the areas with mixed agriculture and intensively grazed pastures would have just a few species. The data below paint a contrasting picture.

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Mean and standard error of (a) bird abundance and (b) number of bird species per site across the four land-use types.

Primary forest hosted the fewest number of birds and the fewest species of birds. Among the three disturbance levels, low- and medium-intensity had greater abundance and diversity than did the high-intensity disturbed sites. At least in the winter, low- and medium-intensity disturbed landscapes can be beneficial to bird populations. Elsen suggests that birds are attracted to the tremendous amount of food available in the agricultural lands, such as fruiting trees and shrubs, even in winter. Some birds can consume these fruits, while other birds consume the yummy energy-rich insects that are attracted to the fruit. There are also plenty of seeds available for granivorous birds. But high-intensity disturbed landscapes lack these benefits, leading to fewer forest-adapted bird species, which are replaced by open-country or generalist bird species.

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Pastoralist and his goats in a high-intensity disturbed site. Credit: Prashant Nagi.

 

The researchers caution that we still don’t know have a clear picture of how birds use different landscapes during the breeding season, although preliminary data indicate that more species are unique to primary forests during breeding season than in winter, and that fewer species inhabit intensively grazed pastures during breeding season than in winter. Consequently, Elsen and his colleagues recommend a holistic conservation approach, which recognizes the importance of conserving large portions of intact primary forest, while at the same time preserving landscapes with low- and medium-intensity agriculture.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Elsen, P. R., Kalyanaraman, R., Ramesh, K., & Wilcove, D. S. (2016). The importance of agricultural lands for Himalayan birds in winter. Conservation Biology 31 (2): 416-426. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.

Cottonwood genes and spider hostels

Back in my working days, people that I met would sometimes ask me what I did for my research. I usually told them that I studied spider sex, which, while true, was a bit misleading, as my interests were actually slightly broader. But studying spider sex was a good fit for my disposition, because, more than anybody I know, I can stare at something for a very long time and not get bored. And when spiders have sex, there can be very long periods, when, to our eyes, nothing is going on. As it turns out there is a great deal of pheromonal communication going on, and considerable vibrational activity as well, but it is not that easy for humans (even abnormally patient ones) to detect.

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A spider and her three egg cases within a web she has built in a Taphrina blister. Credit Matt Barbour

The point is that I have a very soft spot in my heart for spiders, and was delighted to learn about an indirect process that provided a comfortable home for needy spiders. Heather Slinn got interested in her project while an undergraduate summer intern. Her colleague, Matt Barbour, pointed out that when he flipped over blistered black cottonwood leaves (Populus trichocarpa) he often found a spider hanging out in there. This observation led to her to study the relationship between cottonwood trees, cottonwood genetics, pathogens that make leaf blisters and spider occupancy rates.

Taphrina fungi form cup-like blisters in the leaves of Populous trees. But these trees vary in how susceptible they are to leaf blisters. The researchers wanted to answer three questions about this relationship. First, do spiders prefer to live in leaf-blisters as opposed to unblistered leaves? Second, are differences in tree susceptibility to Taphrina a result of genetic differences between the trees? And third can differences in Taphrina-resistance account for differences in spider density?

One of the keys to the experiment was establishing a garden with distinct clones of trees of known genetic makeup (genotypes). Slinn and her colleagues studied five different genotypes, with approximately 40 trees per genotype. They dutifully watered them throughout the summer, and then sampled up to 30 leaves from each tree for blister density, blister size, and spider residency, using a modified shop vac to suck up all of the spiders.

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Black Cottonwood garden. Credit Matt Barbour

The researchers discovered that blistered leaves were 35 times more likely to have a spider and/or spider web than were unblistered leaves. Clearly spiders found blistered leaves to be highly attractive homes.

But there were pronounced differences among the five genotypes in blister density, blister size, spider density and the probability that a spider was occupying a blister. Graph A shows that genotypes 1 and 3 (G1 and G3) had the lowest mean density of blisters (about 2 or 3 per meter of plant), while G4 averaged about 20 blisters per meter. Although G3 had relatively few blisters, it did boast the largest blisters (see graph B). The researchers concluded that blister density and size were both under genetic control, but not linked to each other.

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Mean (A) blister density (number per meter of plant) and (B) blister size, for the five tree genotypes.

But how did blister density and size influence spider residency? G4, the genotype with the greatest number of blisters per plant, also had the greatest number of spiders (graph C). But on a per blister basis, we can see that the two genotypes with the largest blisters (G2 and G3 – see graph B) also had the highest probability of housing a spider in their blisters (graph D). So when spiders make decisions about where to live, both size and number are important.

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Mean (C) spider density and (D) probability a blister hosts a spider, for the five tree genotypes.

It is unclear why the spiders are attracted to blisters.  Some spider species have been shown to be attracted to structural complexity, because that provides more or better attachment points for web strands and egg cases.  Depressed blisters may also give protection from abiotic factors such as wind and precipitation.

Like most good studies, this research raises a host of related questions. Why is there so much genetic variation in Taphrina-resistance within this tree species? Slinn suggests that there may be tradeoff whereby investment into Taphrina-resistance might compromise a plant’s ability to invest in other functions such as cold-resistance, or rapid growth and/or high reproductive rates. A second question is how does spider presence influence other species – for example does hosting a spider reduce the number of herbivorous insects that might attack the tree? A third issue raised by the authors is that plants infected with Taphrina may be weaker and more susceptible to herbivores. In that case, the spiders may be present in blisters because they are attracted to the herbivores that are eating the leaves. And finally, herbivores eating the leaves may transmit other diseases affecting our forests, such as dutch elm disease, chestnut blight and white pine blister rust. Unraveling this complex chain of events will keep researchers busy for many years.

note: the paper that describes this research is from the journal Ecology. The reference is Slinn, H. L., Barbour, M. A., Crawford, K. M., Rodriguez‐Cabal, M. A., & Crutsinger, G. M. (2016). Genetic variation in resistance to leaf fungus indirectly affects spider density. Ecology 98(3): 875-881. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.