Natural history collections provide irreplaceable resources in many arenas, both scientific and societal. These collections, including the WVU Herbarium, provide information on biodiversity, ecosystems, species distributions, climate change, and most recently, genomes.
Both of the species studied—Degranvillea dermaptera and Wullschlaegelia calcarata—are native to remote areas in Central and South America, and as a result, are poorly understood.
However, their genome sequences provide crucial pieces of evidence in reconstructing the evolutionary history of the orchids, which are a mega-diverse family of plants containing around 30,000 species and 800 genera — one tenth of all flowering plants are orchids.
Like most other plants, orchids typically have leaves and conduct photosynthesis.
However, about 300 orchid species are leafless and have lost the ability to make their own sugars, instead being completely parasitic on fungi. We often think of fungi as parasites and plants as their victims, but mycoheterotrophic orchids have turned things upside down. In fact, all orchids are parasitic on fungi at the beginning of their life cycles, and their seeds differ from those in other plant families by containing no nutritional resources. Orchid seedlings obtain all their nutrition by stealing from fungi. Most orchid species, like pink lady slippers here in West Virginia, will eventually develop leaves and become photosynthetic as they mature.
Thus, in an evolutionary sense, it would seem easy for an orchid to skip photosynthesis altogether and remain as a parasite throughout its entire life cycle, in other words, as a “cheater.”
This evolutionary shift to leafless parasitism has happened at least 30 times independently in orchids over their 100 million-year history, more so than in any other family, making them ideal subjects to study radical lifestyle changes.
Mycoheterotrophic orchids are small, ghostly forms that live in dark forests, making them extremely difficult to find and work with. Recent advances in DNA sequencing technology, however, have unlocked a treasure trove of genomic information from historical collections housed in herbaria.
Millions of pressed, dried plant specimens are kept in herbaria worldwide, and serve as repositories of biodiversity. The WVU Herbarium alone contains about 185,000 specimens, whereas the largest collections in the United States like the New York Botanical Garden and the Smithsonian contain several million. This allows researchers at WVU and around the world to access genomic and other information from historical collections that would otherwise be difficult or even impossible to obtain, as in the case of species that are now extinct.
Barrett and colleagues focused on the plastid genome, which contains many genes that function in photosynthesis. The plastid genome of a typical photosynthetic plant is around 150,000 base pairs, but the genomes of these non-photosynthetic orchids are between 36,000 and 47,000 base pairs, only about a quarter to a third the size of their green, photosynthetic counterparts. Their genomes have experienced massive gene losses, structural rearrangements, and greatly accelerated mutation rates. It is a “use it or lose it” scenario, resulting in nearly all photosynthesis genes being completely lost over time.
Because of these genomic losses, mycoheterotrophs like Degranvillea and Wullschlaegelia are some of the hardest to classify in plant taxonomy. At the same time, these drastic evolutionary changes make mycoheterotrophic orchids among the most fascinating plants to study, and provide information on the limits of genome evolution, allowing Barrett and other researchers to create models of evolutionary trajectories for these and other parasites.
Coauthors on the papers included Cameron Corbett and Hana Thixton-Nolan, both Ph.D. candidates in the Department of Biology; Matthew Pace, assistant curator of the William and Lynda Steere Herbarium at the New York Botanical Garden; Aaron Kennedy at USDA-APHIS; and John Freudenstein, professor and director of the Ohio State University Herbarium.
The papers were published in American Journal of Botany and Annals of Botany.
For questions, contact Barrett at craig.barrett@mail.wvu.edu.
