Patterns of Reproductive Isolation in Eucalyptus-A Phylogenetic Perspective

Matthew J Larcombe; Barbara Holland; Dorothy A Steane; Rebecca C Jones; Dean Nicolle; René E Vaillancourt; Brad M Potts

doi:10.1093/molbev/msv063

Patterns of Reproductive Isolation in Eucalyptus-A Phylogenetic Perspective

Mol Biol Evol. 2015 Jul;32(7):1833-46. doi: 10.1093/molbev/msv063. Epub 2015 Mar 16.

Authors

Matthew J Larcombe¹, Barbara Holland², Dorothy A Steane³, Rebecca C Jones⁴, Dean Nicolle⁵, René E Vaillancourt⁴, Brad M Potts⁴

Affiliations

¹ School of Biological Sciences, University of Tasmania, Hobart, TAS, Australia matthew.larcombe@utas.edu.au.
² School of Physical Sciences, University of Tasmania, Hobart, TAS, Australia.
³ 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.
⁴ School of Biological Sciences, University of Tasmania, Hobart, TAS, Australia.
⁵ Currency Creek Arboretum, Currency Creek, SA, Australia.

PMID: 25777461
DOI: 10.1093/molbev/msv063

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.

Keywords: DArT markers; Dobzhansky–Muller incompatibilities; Eucalyptus globulus; hybridization; reproductive isolation; speciation.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Confidence Intervals
Crosses, Genetic
Eucalyptus / classification*
Eucalyptus / genetics*
Hybridization, Genetic
Models, Genetic
Phylogeny*
Reproductive Isolation*
Species Specificity