In mid-May 2014, Dr. Nathalie Nagalingum (Royal Botanical Garden Sydney) and I sampled along an altitudinal transect in the Blue Mountains for flies and bracken fern (Mt. Tomah and Mt. Victoria), and then yesterday we traveled to mid-Queensland to obtain more samples of flies and ferns for population genomics studies. In bracken in the north, hydrogen cyanide production is lower than in the south, and so we are interested in the reciprocal evolutionary response in the flies from a detoxification perspective. Allele frequencies at every locus will be estimated for the flies in the north and south and along the elevational gradient to identify regions of the genome that are being shaped by Darwinian evolution in the context of this arms race between the ferns and flies. We are on a shoestring budget, but we are delighted to be sampling in the fabulous subtropical bush of Australia.
We are nearing the end of our field trip! We have made it all the way to Gladstone in Queensland. Each day we drive northward in search of stands of bracken that are hosts to flies.
When we see a large stand, we sweep the stand with our nets to search for the adult flies, examine the stems and fronds for signs of larvae (lesions and galls), harvest the fern leaves for DNA and RNA analysis, and sample the rhizomes (underground stems - see our last post).
Below Dr. Nagalingum is sampling ferns, while Dr. Whiteman examines the contents of a net after sweeping the stand for flies.
Once we find a stand, we collect herbarium specimens and samples for DNA study. Below, Dr. Nagalingum has collected a fern frond and will later sample it for DNA to determine the population structure of the ferns across Australia.
At the end of the day, we make herbarium specimens. Below Dr. Whiteman is standing on the plant press to create a tight fit so that the plants can dry before being stored at the Royal Botanic Gardens in Sydney. The press contains the herbarium specimens, and Dr. Nagalingum tightens the straps of the press.
While at a stand of bracken we examine the fronds for evidence of damage from the flies. The stems and leaves are host to the fly larvae. Galls appear as unopened portions of the frond, and lesions are blackened portions of the stem.
In the three images below, first we see a frond with a gall; next, Dr. Whiteman unfurls a gall to search for a larva of Scaptodrosophila megagenys; and lastly Dr. Nagalingum has discovered a lesion in the stem, inside are larvae of Scaptodrosophila notha.
To collect adult flies, we sweep the bracken with nets, and search its contents for flies. Sometimes we find beetles, ants, wasps and butterflies. Since we only want the flies, we return the other insects to the wild. To collect the flies we use an aspirator - it's a human-powered vacuum to suck up the insects! Below Dr. Lapoint is using the aspirator to collect the flies caught in his net.
At Nambucca Heads in New South Wales, we were very successful and collected many flies! Above you can see all of the flies, which are stored in vials. At the end of the day Dr. Lapoint sorted through these flies (first image below), and isolated the flies that are specialized on bracken (second image below).
We also examine the lesions for larvae. At the Nambucca Heads site we found lesions with larvae in it (below).
In Gladstone, we were fortunate to have the assistance of Dr. Whiteman's nephews (Griffin and Rory Whiteman) for extracting the larvae from lesions!
Today we regrouped back in Sydney at the Royal Botanic Garden. Simon Goodwin and Maureen Pheelan, our gracious colleagues at the RBG, facilitated the planting of about 40 rhizomes that we collected so far from Victoria and New South Wales. These will be added to Dr. Thomson's living collection of bracken that he is donating to the RBG for continued scientific study.
Rhizomes are underground stems that produce the above-ground fronds of bracken. Rhizomes can easily be propagated from small fragments collected in the field. Aspens growing in the mountains of North America are largely the product of rhizomatous growth. This means that a field of bracken or a mountainside of aspen are all genetically identical (more or less) and comprise a population of clones. This is one reason why bracken is so weedy and potentially problematic for livestock. Bracken also propagates sexually.
Our goal is to regrow these rhizomes in a common garden so that we can test whether the toxin production is due to heritable variation across latitude and not simply due to environmental variation. If the plants from different locations still produce different levels of toxins after being regrown in the common garden, it is likely that this variation is in part due to heritable variation.
This is one step towards identifying the genes and alleles at those genes that underpin variation in these traits, which are the targets of natural selection driving the remarkable cline in toxin production Dr. Thomson discovered in bracken in Australia.
Rhizomes are collected from living plants in the field (upper left, Dr. Nagalingum collected bracken at Wilsons Promontory), brought in bags to the RBG (Dr. Nagalingum and Simon discuss labeling details), sanitized to remove pathogens (Maureen holds a sanitized rhizome and Dr. Lapoint prepares to sanitize a series), potted and placed in a cart (Dr. Whiteman pushes the cart) and placed in the nursery where they will be tended to by RBG staff (Maureen and Dr. Whiteman place them on shelves). Hopefully, in a week or two, fronds will begin to emerge so that we can analyze their toxin levels.
We departed today for Leg 2 of our trip, up the coast of New South Wales and Queensland! Stay tuned...
We have finished the first leg of our trip! After Wilson Promontory, we headed north through Victoria, collecting ferns and flies along the way. Mostly we worked the edges of forests, particularly where controlled burnings had taken place or through power line rights of ways. Our goal was to collect enough material for population genetic analyses of ferns and flies. Below are some photos of us in the field after we left the beautiful Wilsons Promontory and headed up the coast.
Sometimes, our sampling brought us close to the road! Above, Dr. Nagalingum samples bracken near a busy road in southern New South Wales.
Dr. Lapoint samples flies on the border between Victoria and New South Wales.
Above, Dr. Whiteman stands in Victoria, just before crossing into New South Wales.
Dr. Lapoint examines contents of a net after sweeping from bracken in southern New South Wales.
Success! Two specimens of fern-feeding Scaptodrosophila that Dr. Lapoint collected from a sweep of bracken in southern New South Wales.
Female Scaptodrosophila megagenys (left) and Scaptodrosophila notha (right) collected from southern New South Wales, both of which use bracken during their larval stage. Note the egg guides on the ends of their abdomens, which are highly sclerotized (dark and hard) with serrated margins (if one looks closely). They use these to create wounds in the bracken while laying eggs.
Dr. Whiteman examines contents of a sweep net from a site that was recently burned (note the bracken have regenerated from below ground rhizomes) in Victoria, near Bemm River.
Dr. Whiteman in the background sweeping, bracken with galls of Scaptodrosophila megagenys in the foreground.
Dr. Lapoint and Dr. Whiteman sweeping bracken.
After a productive day in the field, terror strikes! Dr. Lapoint has found a terrestrial leech crawling up his leg!
So, we pulled over and Dr. Lapoint calmly (not really) removed this creature from his person.
A close up photo of the land leech. One has to admit that it is beautiful (right?) in its own way. But then again, we think even parasites have intrinsic value (see this paper on parasite conservation).
Yesterday, May 27, we drove down to the southern-most tip of the mainland of Australia (Wilsons Promontory, known by locals as the Prom). Jutting out into the Tasman Sea, this beautiful landscape of gum trees, tea trees and wattles sits protected from human development as a national park. We were fortunate to be able to collect ferns and flies there. A photo of Squeaky Beach, near to where we collected ferns and flies is below.
We found bracken fern throughout Wilsons Promontory National Park and to collect flies, we used a sweep-netting method. Above, Dr. Whiteman is looking to see what he's collected near Squeaky Beach and Dr. Lapoint is in the background actively sweep-netting.
Above, a photo of a Scaptodrosophila notha adult female, with fern-piercing egg guides, that we collected May 28! Larvae of this fly mine the rachis of the ferns.
Above, Dr. Nagalingum and Dr. Lapoint gaze at the temperate rainforest canopy in Tarra Bulga National Park, another of our collecting sites in Victoria. Below, the sun sets on the Tasman Sea!
Before we started our journey, we met up with Dr. John Thomson, who discovered the cline in bracken defenses along the latitudinal gradient in Australia, as well as the ecological details of the herbivores that we are studying.
Pictured above are Dr. Noah Whiteman (left), Dr. Nathalie Nagalingum (center) and Dr. John Thomson (right) in the gallery of the Royal Botanic Garden Sydney. Dr. Thomson, emeritus professor, University of Sydney, led research on the bracken-fly interaction for decades and has generously shared his deep and detailed knowledge of the system.
See Dr. Thomson's webpage here:
Yesterday, Dr. Thomson brought in specimens of bracken that he collected near Sydney with lesions that he thought would contain some larvae of one of the two herbivorous Scaptodrosophila species that we are studying...
In the image above, Dr. Nagalingum has opened a lesion in the rachis (stem) of a bracken (Pteridium esculentem) frond containing larvae of Scaptodrosophila notha, which is a member of the model fly family Drosophilidae. Because the larvae harm the living tissues of the plant, they are unlike many members of the Drosophilidae, which typically feed on microbes associated with decaying plant material. Photo by Dr. Noah Whiteman.
Stay tuned for updates from our expedition to collect flies and ferns from across the cline in bracken chemical defense production. We will start in Melbourne, Victoria tomorrow, and end in Queesland on June 10!
We are especially intersted in understanding the agents and targets of selection associated with a cline in the chemical defense repertoire of the ferns.
A cline refers to a gradual phenotypic gradient within a species that is coupled with a gradual environmental gradient. Similarly, we are interested in identifying the mechanisms used by herbivores of bracken that evolved to overcome these defenses.
In the image below are galls on a leaf of bracken fern (Pteridium esculentum) caused by larvae of an herbivorous fly (Scaptodrosophila megagenys), taken at Royal National Park, New South Wales. This fly is completely dependent on bracken as its larval food source. Bracken is highly toxic to generliast herbivores suggesting that the fly has evolved counter-defenses.
Our goal is to conduct a population genetic study of the ferns and the two fly species in order to identify regions of the genome that have been the targets of positive Darwinian evolution caused by antagonistic interactions between plant and herbivore.