Hybridization and the evolution of invasiveness in plants and other organisms

Abstract

Less than a decade ago, we proposed that hybridization could serve as a stimulus for the evolution of invasiveness in plants (Ellstrand and Schierenbeck Proc Nat Acad Sci USA 97:7043-7050, 2000). A substantial amount of research has taken place on that topic since the publication of that paper, stimulating the symposium that makes up this special issue. Here we present an update of this emergent field, based both on the papers in this volume and on the relevant literature. We reevaluate the lists that we presented in our earlier paper of reports in which hybridization has preceded the evolution of invasiveness. We discard a few cases that were found to be in error, published only as abstracts, or based on personal communication. Then we augment the list from examples in this volume and a supplementary literature search. Despite the omissions, the total number of cases has increased. Many have been strengthened. We add a list of cases in which there has been evidence that intra-taxon hybridization has preceded the evolution of invasiveness. We also provide a number of examples from organisms other than plants. We consider how our examples suggest mechanisms whereby hybridization may act to stimulate the evolution of invasiveness. Hybridization does not represent the only evolutionary pathway to invasiveness, but it is one that can explain why the appearance of invasiveness often involves a long lag time and/or multiple introductions of exotics.

Spontaneous hybrids between native and exotic Rubus in the Western United States produce offspring both by apomixis and by sexual recombination

“…As these hybrids are neoallopolyploids, additional evolution may occur through genomic rearrangements and epigenetic remodeling (Comai, 2005). Lag periods often exist between introduction and invasion, and it has been hypothesized that evolutionary changes occur during these lag times (Schierenbeck and Ellstrand, 2009). …”

“…These hybrids may have increased fitness due to dominance, overdominance and epistasis effects, creating a situation in which sexual reproduction is disadvantageous (Springer and Stupar, 2007). Hybridization has led to the evolution of many new invasive taxa, many of which are successful due to putative fixed heterosis as a result of asexual reproduction or allopolyploidy (Schierenbeck and Ellstrand, 2009). Hybridization between a native species and a naturalized exotic species can produce fitness benefits in addition to heterosis, as the native species may have adaptations specific to its environment, and the exotic species may have an overall high fitness and broad environmental tolerance that allows it to persist in the novel habitat.…”

How Do Urban Forests Compare? Tree Diversity in Urban and Periurban Forests of the Southeastern US

“…For example, Pyrus calleryana Decne. has escaped cultivation in many areas of the US as a result of hybridization, forming dense homogenous plant communities and outcompeting local species [23,24]. Pinus pinaster Aiton and P. halepensis Mill.…”

Human-induced hybridization among congeneric endemic plants on Tenerife, Canary Islands

“…In Germany, hybridization among species of Rorippa (Brassicaceae) occurs predominantly in man-made habitats such as drainage ditches (Bleeker and Hurka 2001). In addition to directly disrupting natural barriers, humans also increasingly transport species across them (e.g., Hodkinson and Thompson 1997;von der Lippe and Kowarik 2007;Wichmann et al 2009), resulting in problems of invasiveness that frequently involve hybridization between native and non-native (or genetically modified crop) species (Schierenbeck and Ellstrand 2009).…”