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.

Item Type:	Refereed Article
Keywords:	reproductive isolation, speciation, Dobzhansky–Muller incompatibilities, hybridization, Eucalyptus globulus, DArT markers
Research Division:	Mathematical Sciences
Research Group:	Applied mathematics
Research Field:	Biological mathematics
Objective Division:	Expanding Knowledge
Objective Group:	Expanding knowledge
Objective Field:	Expanding knowledge in the biological sciences
UTAS Author:	Larcombe, MJ (Mr Matthew Larcombe)
UTAS Author:	Holland, B (Professor Barbara Holland)
UTAS Author:	Steane, DA (Dr Dorothy Steane)
UTAS Author:	Jones, RC (Dr Rebecca Jones)
UTAS Author:	Vaillancourt, RE (Professor Rene Vaillancourt)
UTAS Author:	Potts, BM (Professor Brad Potts)
ID Code:	100325
Year Published:	2015
Funding Support:	Australian Research Council (FT100100031)
Web of Science® Times Cited:	34
Deposited By:	Mathematics and Physics
Deposited On:	2015-05-12
Last Modified:	2017-10-30
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