Molecular systematic analysis of the genus Notelaea (Oleaceae): SNPs from high-density DArT-sequencing unravel the mystery of the species limits of threatened species of Notelaea

Abstract

Conservation decision-making and prioritization of management actions for taxa at high risk of extinction require a clear understanding of systematic relationships and species boundaries. Taxonomic uncertainty surrounds two threatened species of native olive (genus Notelaea) endemic to Australia. Notelaea ipsviciensis is known from only one small population and is listed as critically endangered under Australian and Queensland legislation, while Notelaea lloydii is listed as vulnerable due to its restricted distribution in south-east Queensland, Australia. Notelaea ipsviciensis occurs in sympatry with N. lloydii, at the only site where it is found, and exhibits intermediate morphology between N. lloydii and Notelaea ovata raising speculation that it is a natural hybrid of the two. To address this uncertainty, we have reconstructed the molecular phylogeny of the genus Notelaea using both single-gene sequencing and single nucleotide polymorphism (SNP) data. While the chloroplast DNA markers were not found to be informative, the genome-wide SNPs used in this study have unequivocally resolved the long-standing question of the hybrid origin of N. ipsviciensis. Results of the phylogenetics and hybrid analyses of SNP data show that N. ipsviciensis is a natural hybrid of N. lloydii and N. ovata, and the taxon's circumscription needs to be re-evaluated. Our results also revealed unclear species boundaries among numerous other Notelaea species, including the threatened N. lloydii, indicating that further investigation and taxonomic revision may be necessary.