Circumscription of Murraya and Merrillia (Sapindales: Rutaceae: Aurantioideae) and susceptibility of species and forms to huanglongbing

Abstract

As circumscribed by Swingle & Reece (1967), Murraya paniculata (L.) Jack [Rutaceae: Aurantioideae: Aurantieae] comprises Murraya paniculata var. paniculata (L.) Jack and three varieties, Murraya paniculata var. omphalocarpa (Hay.) Tan., Murraya paniculata var. ovatifoliolata (Engl.) Domin, and Murraya paniculata var. zollingeri Tan. The widely cultivated ornamental known variously as orange jasmine, orange jessamine or mock orange is often called Murraya exotica L. This latter epithet was regarded by Swingle & Reece (1967) as a junior synonym of Murraya paniculata and opinions of botanists on this issue have been divided for more than 200 years. My interest in the status of the ornamental form stemmed from the need for its status and origins to be resolved, as it is a transient host of a devastating disease of citrus known as huanglongbing. This disease is caused by phloem-limited putative species of bacteria 'Candidatus Liberibacter spp.' [I-Proteobacteria]. The vector of the disease, which throughout most of Asia and the Americas is caused by 'Candidatus Liberibacter asiaticus', is the Asiatic citrus psyllid (Diaphorina citri Kuwayama [Hemiptera: Psyllidae]), a species that is native to India and that may have evolved with a species of Murraya (sensu lato). Orange jasmine is the favoured host of the psyllid. My project focused on the taxonomic status of Murraya exotica as a species but also included the relationship between Murraya and Merrillia caloxylon. My research was based on the molecular biology, morphology and phytochemistry of accessions collected from Asia, Australasia and the Americas. I also tested these accessions for the presence of the HLB pathogens. Six regions of the maternally-inherited chloroplast genome (trnT-rps4, trnCGCA-ycf6, trnL-F, rps16, matK-5"trnK, psbM-trnDGUC) were amplified by PCR and sequenced. In addition, part of the internal transcribed spacer (ITS) region of the nuclear-encoded ribosomal RNA operon was also sequenced. The data obtained were subjected to phylogenic analysis using parsimony and Bayesian inference. The phylogenetic results derived from the chloroplast genome, as well as the nuclear ITS region, indicated that Murraya exotica is a species. This was supported by: a phylogenetic tree based on characters of basal and terminal leaflets; principal component and redundancy analysis of elliptic Fourier descriptors based on the shape of basal and terminal leaflets and a separate analysis of the dimensions of basal and terminal of leaflets, and the basal angles the leaflets; "discriminant function analysis of quantitative characters of basal and terminal leaflets; and phytochemistry of leaflet and bark ethanol and n-hexane extracts. The molecular and morphological studies also indicated that, in addition to Murraya exotica, other taxa formerly classified as Murraya paniculata comprise: Murraya paniculata (L.) Jack from Indonesia (syn. Camunium vulgare Rumph. and Murraya sumatrana Roxb.); Murraya asiatica n. sp. ineditus from mainland Asia; and Murraya ovatifoliolata (Engl.) Domin., comprising small and large leaflet forms of Murraya ovatifoliolata var. ovatifoliolata (Engl.) Domin. n. comb ineditus, and Murraya ovatifoliolata var. zollingeri (Tan.) n. comb. ineditus and two hybrids: Murraya × omphalocarpa Hay. ineditus, small and large leaflet forms, possibly with Murraya exotica as the male parent and a form of Murraya ovatifoliolata as the female parent; and Murraya × cycloopensis ineditus from Papua, possibly with Murraya exotica as the female parent and a form of Murraya ovatifoliolata as the male parent. I developed a key to the taxa based on leaf and leaflet characters. I found no evidence to reunite Merrillia caloxylon with the genus Murraya. I only detected 'Candidatus Liberibacter asiaticus' in Murraya exotica accessions from Brazil and China.

Date of Award	2011
Original language	English