Evolutionary relationship of the NBS-LRR gene family in Melaleuca and Eucalyptus (Myrtaceae)

Myrtaceous plants such as Eucalyptus and Melaleuca are economically and ecologically important. Many species, including M. alternifolia and M. quinquenervia, are known to be susceptible to the exotic rust fungus Austropuccinia psidii. Identifying the molecular basis of resistance against A. psidii would assist in incorporating resistance to this pathogen for industries producing essential oils and for re-afforestation. Nucleotide-binding sites and leucine-rich repeat (NBS-LRR) genes have been identified as important candidates in several studies. It is unknown whether resistance against A. psidii is conserved among, as well as within species of Myrtaceae. Therefore, this study aimed to identify whether candidate NBS-LRR genes that may have contributed to defence in M. alternifolia and M. quinquenervia are closely related to candidate NBS-LRR genes within the A. psidii resistance loci of E. grandis and E. globulus. NBS-LRR sequences of M. alternifolia, M. quinquenervia and E. grandis were subjected to phylogenetic analysis. Although no candidate NBS-LRR genes for A. psidii defence from M. alternifolia or M. quinquenervia were found to be orthologous to NBS-LRR genes within the E. grandis Ppr1 locus, one candidate NBS-LRR gene for defence against A. psidii in M. quinquenervia was orthologous to a candidate NBS-LRR within Ppr2 from E. globulus. Phylogenetic analysis of multiple species of Myrtaceae also revealed that NBS-LRR genes within Ppr1 may not be the result of recent gene duplications and were highly diverse.