A research team conducted a comprehensive analysis of phylogenetic relationships among Australian citrus species and 13 international accessions, shedding light on their genetic diversity and evolutionary history. By examining complete chloroplast genomes and single copy nuclear genes, they revealed significant differences between the two sets of data. Notably, Australian species formed a distinct clade based on nuclear genes, with Citrus inodora and Citrus australasica showing a close relationship. This study enhances our understanding of Australian citrus phylogeny, emphasizing their unique status within the genus and serving as a valuable genetic resource for future citrus breeding programs and evolutionary studies.
Citrus, a member of the Rutaceae family, encompasses various genera and species, of which native Australian limes are a notable subset. These species, including Citrus australasica, C. australis, and C. glauca, exhibit resistance to Huanglongbing (HLB) and possess desirable traits for breeding. Despite their significance, few studies have comprehensively examined their evolutionary relationships compared to widely cultivated citrus species.
A study (DOI: 10.48130/TP-2023-0021) published in Tropical Plants on 21 December 2023, elucidates the genetic connections among Australian citrus and international accessions counterparts.
The research employed phylogenetic analysis based on 86 single copy nuclear genes, utilizing MrBayes and RAxML methods, which yielded congruent results in high supports. The nuclear phylogeny delineated two major clusters, with Australian wild limes forming a distinct clade (Clade I). Within Clade I, sub-clade A exhibited a close relationship between Citrus inodora and Citrus australasica, with significant node support. Notably, different cultivars of C. australasica showed variations in their genetic relationships. Sub-clade B included other Australian citrus species, with Citrus glauca being the most distantly related. Furthermore, the analysis highlighted a separation between Australian citrus and non-Australian species. However, the chloroplast genome analysis revealed a conflicting topology with the nuclear phylogeny, indicating potential reticulate evolution or hybridization. Australian citrus species displayed similarities to citrons in their chloroplast sequences, suggesting reticulate evolution. Citrus australasica formed a monophyletic clade divergent from other Australian species, whereas C. inodora, C. australis, and C. glauca shared chloroplast similarities with other native limes. The incongruity between chloroplast phylogeny and current geographical distribution suggests complex evolutionary pressures shaping Australian citrus species. Additionally, close relationships were found between C. australis and C. glauca chloroplast genomes, indicating potential chloroplast sequence similarities among these species.
According to the study's lead researcher, Robert J. Henry, “ The results were used to assess the congruence between the chloroplast and nuclear phylogenies and the previous relationships among Australian taxa with the results derived from this study. Improved understanding of genetic relatedness may help explain the evolutionary drivers that have created such a unique clade of citrus species on the Australian continent.”
In summary, this study revealed a distinct Australian citrus clade, emphasizing the close relationship between Citrus inodora and Citrus australasica. The findings suggest potential reticulate evolution and a shared origin with Citrus medica. Future research should explore the genetic basis of their environmental adaptation and conservation implications. This study illuminates the unique evolutionary pressures shaping Australian citrus diversity.
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References
DOI
10.48130/TP-2023-0021
Original Source URL
https://doi.org/10.48130/TP-2023-0021
Funding information
This project was funded by the Hort Frontiers Advanced Production Systems Fund as part of the Hort Frontiers strategic partnership initiative developed by Hort Innovation, with co-investment from The University of Queensland, and contributions from the Australian Government and Bioplatforms Australia.
About Tropical Plants
Tropical Plants (e-ISSN 2833-9851) is the official journal of Hainan University and published by Maximum Academic Press. Tropical Plants undergoes rigorous peer review and is published in open-access format to enable swift dissemination of research findings, facilitate exchange of academic knowledge and encourage academic discourse on innovative technologies and issues emerging in tropical plant research.