Tracing the Naturalization of Golden Oysters in the United States

My master’s thesis research dealt with understanding the origin and spread of naturalized golden oyster mushrooms in the U.S. using population genomics.  The full thesis is available for download here: https://minds.wisconsin.edu/handle/1793/79004

SKIP AHEAD

A BIT OF BACKGROUND

In the 1990s, the golden oyster mushroom (Pleurotus citrinopileatus) was near impossible to find on the American market.  It’s native to subtropical hardwood forests of eastern Russia, northern China, and Japan, and has long been a popular culinary mushroom for cultivation across Asia.  Its sunny yellow caps make this species a show-stopper at farmers markets, and its popularity State-side has steadily risen over the years among specialty mushroom growers.

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Today, it’s reasonably common to find golden oysters at American farmers markets during the summer, as it requires temps consistently above 60º F to fruit.  In Growing Gourmet and Medicinal Mushrooms, Paul Stamets writes: “With the onset of commercial cultivation of these mushrooms adjacent to woodlands in North America, it will be interesting to see if these exotic varieties escape.”  The evidence generated by my thesis research suggests that Stamets was spot on.  The first recorded sightings of wild golden oysters in the US appear around 2012, following the rise in American cultivation of the species around 2000.  To my knowledge, naturalized golden oysters have been observed in Delaware, Illinois, Iowa, Maryland, Massachusetts, Michigan, Minnesota, New York, Ohio, Pennsylvania, and Wisconsin.

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ARE GOLDEN OYSTERS INVASIVE?

The term “invasive” can be defined in many ways.  Ecologist Dr. Elena Litchman defines invasive microorganisms as those which proliferate in a new range and impact local communities or ecosystems.  This description leaves little doubt that golden oysters could indeed be considered invasive.  We would be rather negligent to assume that the rapid spread of any non-native species would be inconsequential to native ecosystems.  The golden oysters’ swift spread suggests that the native decomposers are being outcompeted and displaced.  Climate change and habitat loss already pose major threats to fungal biodiversity, so it’s critical that we understand the role non-native fungi play in deepening those threats.  Fungi provide vital ecosystem services as a part of diverse microbial communities, and ecological studies have shown that losses to biodiversity, or reductions in species richness, have negative effects on ecosystem function.

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ABOUT MY RESEARCH

Although outdoor cultivation is common practice, this is the first ever case of escape with rapid and significant spread of a cultivated mushroom.  I have used genomic data (the entire DNA sequence of every specimen, rather than targeting specific gene regions) to analyze the ancestral lineages of specimens collected in the U.S., offering insight into the mechanisms behind their invasion and spread.  This helps us to understand the role we humans have played in the mediation of the spread of golden oysters.

My research aimed to answer the following primary questions: (a) Are wild strains genetically similar to commercial strains used for cultivation in the U.S.?  (b) Was there likely one introduction event, or multiple?  (c) Do the populations appear to be spreading clonally, or is there evidence for recombination?

The full analysis examining my 2017 dataset is available for download here: https://minds.wisconsin.edu/handle/1793/79004

The success of this research hinged upon the contributions of golden oyster specimens from citizen scientists like you!  Sincere thanks to those who have shared their collections for this work.  Scroll on for details on what you can do if you spot golden oysters growing wild outside of their native range.

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WHAT YOU CAN DO

Keep your eyes peeled!  Golden oysters have bright yellow caps when young, which fade to nearly white with age, and have a characteristic depression in the top of the cap.  The white gills run down the length of the white stems, which are usually highly branched.  Like other oyster species, they grow in clusters, and seem to prefer dead elms and other hardwoods.

User-generated databases of field observations like iNaturalist are excellent platforms to make your sighting known to scientists and the public alike.  GPS coordinates and photos increase the quality of your observation record, which are easy to include when you use your phone to submit an observation through the iNaturalist app while in the field (the iNaturalist community can also help confirm your ID).  Access to this growing database is free, so you can also use the site to view other golden oyster observations and monitor their spread yourself.

Please also consider contributing wild specimens for future research if you spot golden oysters growing wild in the U.S.  The Osmundson Lab at UW-La Crosse, where my thesis research was conducted, is continuing to collect specimens on an ongoing basis.  Detailed instructions for specimen submission follow.

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SPECIMEN SUBMISSIONS

If you’d like to contribute a specimen for future research, please send both dried mushrooms and a spore print.  Please do not send any fresh mushrooms!  If we wind up using your specimen, your name will be acknowledged in any publication that comes out of it.

Instructions for drying:

Larger specimens are best, but little mushrooms are just fine.  Drying mushrooms is best done using a food dehydrator on a low temperature setting.  If you don’t have a food dehydrator, you can use an oven on its lowest temperature with the door propped open a few inches – even better with a fan pointed at the door.  Care must be taken with ovens because the goal is to dry the mushroom without cooking it, which would damage the DNA.  Drying time will vary.  Let the mushrooms continue drying until they are crispy; if they are leathery or flexible at all, they have a bit more to go!

Instructions for taking a spore print:

Place a few caps on aluminum foil and cover (upside down bowls or Tupperware work nicely).  Leave them for a minimum of a couple hours, or overnight.  Then, remove the cover and the mushrooms (discard), taking care not to touch or smear the spore print.  There are often puddles of condensation that have formed; make sure to allow any moisture to dry before folding up the aluminum foil and include with the dried sample.  It may be hard to see, but the spore print will be a pale pink color.

Information to include:

Please include detailed information about where and when the mushrooms were collected.  GPS coordinates are preferred, otherwise, the street address or the name of the area/park/woods with city and state is helpful information.  Please also include the date it was collected and the type of tree it was growing on, if you know.

Specimens can be sent to:

Receiving Stores
UW-L Biology Department
ATTN: Dr. Todd Osmundson
855 East Ave. North
La Crosse, WI 54601

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THANK YOU

This research wouldn’t be possible without your assistance in the field and your willingness to contribute. You are truly helping to advance the science, and we’re very grateful for your help!

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