Molecular differentiation of the Murraya paniculata Complex (Rutaceae: Aurantioideae: Aurantieae)

Abstract

Background: Orange jasmine has a complex nomenclatural history and is now known as Murraya paniculata (L.) Jack. Our interest in this common ornamental stemmed from the need to resolve its identity and the identities of closely related taxa as hosts of the pathogen 'Candidatus Liberibacter asiaticus' and its vector Diaphorina citri. Understanding these microbe-vector-plant relationships has been hampered by taxonomic confusion surrounding Murraya at both the generic and specific levels.

Results: To resolve the taxonomic uncertainty, six regions of the maternally-inherited chloroplastal genome and part of the nuclear-encoded ITS region were amplified from 85 accessions of Murraya and Merrillia using the polymerase chain reaction (PCR). Clustering used maximum parsimony (MP), maximum likelihood (ML) and Bayesian inference (BI). Chronograms were produced for molecular dating, and to test the monophyly of Murraya rigorously, using selected accessions of Murraya and 26 accessions of the Rutaceae and Simarubaceae. Sequence data from the ITS and chloroplastal regions suggest that Murraya paniculata (sensu (Swingle WT and Reece CR, The Citrus Industry, p. 190-430, 1967)) can be separated into four distinct but morphologically somewhat cryptic taxa: Murraya paniculata (sensu (Mabberley DJ, Taxon 65:366-371, 2016)), M. elongata, M. sumatrana and M. lucida. In addition, Murraya omphalocarpa was identified as a putative hybrid of M. paniculata and M. lucida with two geographically isolated nothovarieties representing reciprocal crosses. Murraya is monophyletic, and molecular dating suggests that it diverged from Merrillia during the Miocene (23-5 Ma) with this Murraya group speciating and dispersing during the Middle Miocene onwards.

Conclusions: The accessions from Asia and Australasia used in this study grouped into biogeographical regions that match herbarium specimen records for the taxa that suggest natural allopatric distributions with limited overlap and hybridity. Murraya paniculata has been distributed around the world as an ornamental plant. The division of the Murraya paniculata complex into four species with a rare hybrid also confirms morphological studies.

Keywords: Molecular dating; Monophyly; Murraya; Phylogeny; Rutaceae.

Fig. 1

Bayesian inference tree based on the combined sequences of the six chloroplastal regions from accessions of Murraya and Merrillia. Murraya kwangsiensis and M. microphylla were used as the outgroup and posterior probabilities are shown above each branch. The model of nucleotide substitution used was GTR + G and the Markov chains were run for 5,000,000 generations (burnin = 1,250,000 generations). ‘sl’ small leaflet and ‘ll’ large leaflet forms of Murraya lucida from Australia

Fig. 2

Bayesian inference tree based on the ITS sequences of accessions of Murraya and Merrillia. Murraya microphylla was used as the outgroup and posterior probabilities are shown above each branch. The model of nucleotide substitution used was GTR + G and the Markov chains were run for 5,000,000 generations with a sample frequency of 10 and a burnin of 1,250,000 generations. ‘sl’ small leaflet and ‘ll’ large leaflet forms of Murraya lucida from Australia

Fig. 3

Bayesian inference tree based on the ITS region combined with 6 chloroplastal regions of accessions of Murraya and Merrillia. Murraya microphylla was used as the outgroup and posterior probabilities are shown above each branch. The model of nucleotide substitution used was GTR + G and the Markov chains were run for 600,000 generations with a sample frequency of 10 and a burnin of 150,000 generations. ‘sl’ small leaflet and ‘ll’ large leaflet forms of Murraya lucida from Australia

Fig. 4

Maximum clade credibility tree produced using the BEAST suite of programs based on the combined sequences of five chloroplastal regions. The values next to the nodes are the ages (Ma). The bars represent the 95% highest posterior density

Fig. 5

Maximum clade credibility tree produced using the BEAST suite of programs based on ITS regions. The values next to the nodes are the ages (Ma). The bars are the 95% highest posterior density