Population genetics may unlock the origins of living cycad specimens

Cycads, which as a group date to around 300 million years ago, are the oldest living lineage of seed plants. Having changed in appearance relatively little over that time, they are often referred to as ‘living fossils.’ At their peak during the Mesozoic, cycads were distributed almost worldwide. Today they are considered one of the most threatened plant groups in the world, largely due to human activities, but also due to natural disasters and failures to reproduce.

Over 80% of global cycad species are held as living collections in botanical gardens, and gardens have in some cases held those specimens for centuries following their collection on historic expeditions. Unfortunately, the wild origins of these ex situ living collections are often unknown or poorly documented. Without provenance information, the specimens have little use in current and future recovery and conservation programmes.

In a recent article published in Plants People Planet, authors Natalie Iwanycki Ahlstrand and Dennis W. Stevenson make the case for using population genetics methods to retrace the provenance of living collections housed in botanical gardens so that they might someday be used for recovery purposes. These methods, which use short DNA reads from throughout the target genome, have a greater sensitivity and universal applicability than other approaches such as DNA barcoding and microsatellite markers, the authors argue.

“Population genetics methods have been used to compare the genetic variation between cycads kept in botanical garden collections from wild cycad populations. Additionally, genetic investigations have been performed to compare the genetic diversity between threatened species held by different botanical gardens, such as for Brighamia insignia (Vulcan palm), a threatened plant that is known from only one location in the wild,” says Iwanycki Ahlstrand, a postdoctoral researcher at the Natural History Museum of Denmark.

The authors detail two case studies of cycads kept for centuries in botanical gardens without adequate locality information. The first is the Eastern Cape giant cycad, Encephalartos altensteinii, a specimen of which was brought back to Kew Gardens by Francis Masson in 1775 and remains there to this day as the world’s oldest potted plant. A later plant of the same species is held at Hamburg Botanic Garden and designated as the living type specimen. The species is designated as vulnerable, with at most ten thousand individuals – and possibly less than half that number – left in the wild. However, only a small minority of the 77 botanical gardens reported to hold living specimens have any locality information for them. Many may be linked to the very earliest expeditions of European collectors.

The second case study is that of Zamia acuminata, first collected by Danish botanist A.S. Ørsted during his time in Nicaragua and Costa Rica in the mid-19^th century. The type specimen is held in the herbarium of the Natural History Museum of Denmark, but has no date or collection number, and is not mentioned in the collector’s field notes. A locality appears to have been added after the fact or by a different person, and further appears to be incorrect, tying the specimen to Nicaragua rather than Costa Rica, as evidence suggests.

In both of these cases, population genomic methods could provide much-needed information on the collections’ origins. “One of the long term goals of retracing provenance would be to assist in future species recovery work in its original habitat, or, in an ecologically suitable habitat, in the case where the original habitat no longer exists or cannot be restored,” explains Iwanycki Ahlstrand. “Knowing the provenance will help horticulturists cross two suitable plant species, based on their genetic origins, in order to potentially create a more diverse gene pool for future plant reintroductions.”

“One of the benefits of keeping threatened species such as cycads in botanical garden collections is to help us study them over time and answer biological questions such as how long their lifespans are, or how frequently they can reproduce, under various growth conditions. These types of data will ultimately help us to conserve cycads in the wild.”