Eucalyptus globulus is grown extensively in plantations outside its native range in Australia. Concerns have been raised that the species may pose a genetic risk to native eucalypt species through hybridisation and introgression. Methods for identifying hybrids are needed to enable assessment and management of this genetic risk. This paper assesses the efficiency of a Bayesian approach for identifying hybrids between the plantation species E. globulus and E. nitens and four at-risk native eucalypts. Range-wide DNA samples of E. camaldulensis, E. cypellocarpa, E. globulus, E. nitens, E. ovata and E. viminalis, and pedigreed and putative hybrids (n = 606), were genotyped with 10 microsatellite loci. Using a two-way simulation analysis (two species in the model at a time), the accuracy of identification was 98% for first and 93% for second generation hybrids. However, the accuracy of identifying simulated backcross hybrids was lower (74%). A six-way analysis (all species in the model together) showed that as the number of species increases the accuracy of hybrid identification decreases. Despite some difficulties identifying backcrosses, the two-way Bayesian modelling approach was highly effective at identifying , which, in the context of E. globulus plantations, are the primary management concern. 1. Introduction Plants are well known for their propensity to hybridise [1, 2], and the role of hybridisation in animal systems is receiving growing attention [3, 4]. Natural hybridisation has been widely documented in plants [1], with hybrid zones often being used to investigate the mechanisms that underlie speciation [5–7]. These studies have demonstrated that barriers to hybridisation that evolve in allopatry are often incomplete, and hybridisation and introgression are still possible when species secondarily come into contact [5, 7]. Two consequences of human development have been the fragmentation of natural plant populations and the widespread movement of plant species around the world [8]. In many situations this has resulted in exotic species coming into contact with cross-compatible indigenous species, leading to human mediated exotic hybridisation [9–11]. This exotic hybridisation and potential for subsequent introgression may threaten the genetic integrity of native species [10, 11]. Given the genetic risk posed by exotic hybridisation, methods for detecting hybrid progeny are needed to enable quantification and management of the issue [11]. In some situations first generation ( ) hybrids can be detected based on intermediate morphology, but in
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