title: Patterns of Reproductive Isolation in Eucalyptus-A Phylogenetic Perspective.
abstract: We assess phylogenetic patterns of hybridization in the speciose, ecologically and economically important genus Eucalyptus, in order to better understand the evolution of reproductive isolation. Eucalyptus globulus pollen was applied to 99 eucalypt species, mainly from the large commercially important subgenus, Symphyomyrtus. In the 64 species that produce seeds, hybrid compatibility was assessed at two stages, hybrid-production (at approximately 1 month) and hybrid-survival (at 9 months), and compared with phylogenies based on 8,350 genome-wide DArT (diversity arrays technology) markers. Model fitting was used to assess the relationship between compatibility and genetic distance, and whether or not the strength of incompatibility "snowballs" with divergence. There was a decline in compatibility with increasing genetic distance between species. Hybridization was common within two closely related clades (one including E. globulus), but rare between E. globulus and species in two phylogenetically distant clades. Of three alternative models tested (linear, slowdown, and snowball), we found consistent support for a snowball model, indicating that the strength of incompatibility accelerates relative to genetic distance. Although we can only speculate about the genetic basis of this pattern, it is consistent with a Dobzhansky-Muller-model prediction that incompatibilities should snowball with divergence due to negative epistasis. Different rates of compatibility decline in the hybrid-production and hybrid-survival measures suggest that early-acting postmating barriers developed first and are stronger than later-acting barriers. We estimated that complete reproductive isolation can take up to 21-31 My in Eucalyptus. Practical implications for hybrid eucalypt breeding and genetic risk assessment in Australia are discussed.
authors.affiliation: School of Biological Sciences, University of Tasmania, Hobart, TAS, Australia matthew.larcombe@utas.edu.au.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Larcombe
authors.affiliation: School of Physical Sciences, University of Tasmania, Hobart, TAS, Australia.
authors.forename: Barbara
authors.initials: B
authors.lastname: Holland
authors.affiliation: School of Biological Sciences, University of Tasmania, Hobart, TAS, Australia Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Sippy Downs, QLD, Australia.
authors.forename: Dorothy A
authors.initials: DA
authors.lastname: Steane
authors.affiliation: School of Biological Sciences, University of Tasmania, Hobart, TAS, Australia.
authors.forename: Rebecca C
authors.initials: RC
authors.lastname: Jones
authors.affiliation: Currency Creek Arboretum, Currency Creek, SA, Australia.
authors.forename: Dean
authors.initials: D
authors.lastname: Nicolle
authors.affiliation: School of Biological Sciences, University of Tasmania, Hobart, TAS, Australia.
authors.forename: René E
authors.initials: RE
authors.lastname: Vaillancourt
authors.affiliation: School of Biological Sciences, University of Tasmania, Hobart, TAS, Australia.
authors.forename: Brad M
authors.initials: BM
authors.lastname: Potts
mesh_terms: D016001:Confidence Intervals; D003433:Crosses, Genetic; D005052:Eucalyptus / Q000145:classification* / Q000235:genetics*; D006824:Hybridization, Genetic; D008957:Models, Genetic; D010802:Phylogeny; D060047:Reproductive Isolation; D013045:Species Specificity
keywords: DArT markers; Dobzhansky–Muller incompatibilities; Eucalyptus globulus; hybridization; reproductive isolation; speciation