What makes an invasive species?

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Published on: March 12, 2013
B. sylvaticum seeds

Brachypodium sylvaticum is a grass species known as slender false brome, and is native to Europe, Asia, and north Africa. It is a common sight in UK woodlands, and grows all over the country. In the USA though, its tufts don’t mark convenient picnic spots in woodland but are destroyed if seen in a new region because this invasive species has colonised miles of Oregon’s woodland floor. An Oregon-based research team has sequenced the B. sylvaticum transcriptome and hopes to use it as a mode for the evolution of invasive species.

Highlighted paper: Samuel E. Fox, Justin Preece, Jeffrey A. Kimbrel, Gina L. Marchini, Abigail Sage, Ken Youens-Clark, Mitchell B. Cruzan, and Pankaj Jaiswal 2013. Sequencing and De Novo Transcriptome Assembly of Brachypodium sylvaticum (Poaceae). Applications in Plant Sciences 1: 1200011

Slender false broom was widely planted in the in the mid part of the 20th century in an attempt to seed mountain rangelands in Utah, Wyoming, and Idaho. It was also planted in experimental gardens in two Oregon cities. The two attempts to establish the species were independent, but microsatellite analysis suggests the plants originated from the samestock of accessions. At some point in the late 20th century, some of these accessions crossed and the hybrids spread rapidly across Oregon’s forests (Rosenthal et al. 2008, Mol Ecol 17:4657). Today in Oregon this aggressive genotypes have formed thick monocultures that completely cover the forest floor at the expense of native flora, and have spread to California and Washington too.

B. sylvaticum blades

Fox et al. sequenced the transcriptome of native B. sylvaticum plants from Spain and Greece, and invasive plants growing in Oregon, USA using RNA-Seq. They aligned the constructed transcriptome sequences to each other and to that of the related model grass B. distachyon. Nearly 400 000 single nucleotide polymorphisms (SNP) sequences were identified.

The researchers chose to study slender false brome because it is a new invasive species which is not established extensively in the USA. There is currently no model for studying invasive species, so no one knows the molecular features of species that survive, successfully establish, or invade in a new ecosystem. As Mitch Cruzan, coauthor of the paper and Associate Professor of Biology at Portland State University, said, “False brome is in the process of active range expansion and is wildly successful despite experiencing colder, wet winters and drier summers than plants in the native range, so it is a great system for studying ecological and evolutionary aspects of invasion.”

There are many reasons a species may ‘invade’ a new environment – the hybrid bluebell pandemic in UK woodland, for example, has very different causes to the false brome invasion in the USA. This data and future research based on it will enable scientists to identify the hallmarks of an invasive species. Some characteristics are already known, of course. B. sylvaticum is a perennial species; it is resistant to drought; and able to survive in a wide range of environments. However these characteristics are true of slender false brome all over the world, and only a limited number of hybrid accessions originating in Oregon have made it problematic invasive species. The genetic differences between these invasive accessions and those found elsewhere in the USA and in Europe will be novel insights into what makes an invasive species.

Image credit: B.sylvaticum seeds by Kristian Peters via Wikimedia Commons. B.sylvaticum blades by Pere prlpz via Wikimedia Commons

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