Genomic analyses of hybridisation and clonality reveal uncertain species boundaries in two endangered narrow-range endemic Callistemon from Far East Gippsland, Australia

Narrow-range endemic species are often at the forefront of conservation efforts; however, a hybrid origin can significantly influence how they are classified and subsequently affect their conservation listings. The endangered narrow-range endemics, Callistemon forresterae and Callistemon nyallingensis from East Gippsland, Victoria, Australia, are both suspected to have arisen through hybridisation, and to reproduce clonally. We used 13,635 SNPs from DArTseq to assess their proposed hybrid origin and to quantify clonality and their genetic diversity. Collectively, principal coordinates analysis, STRUCTURE and NEWHYBRIDS provided strong support for the hybrid origin of C. forresterae. Varying levels of hybridisation were observed amongst all C. forresterae genotypes and are considered suggestive of a hybrid swarm. Conversely, no evidence of hybridisation was observed in C. nyallingensis, instead a strong genomic association with Callistemon subulatus was revealed. Pairwise relatedness estimates obtained using COANCESTRY revealed that C. forresterae and C. nyallingensis primarily reproduce asexually, with extreme clonality observed in all sampled populations of both taxa. From the three C. forresterae sub-populations (86 sampled stems) only nine genotypes were observed, and large genets comprised of many stems were found in each population. Only a single genotype was found amongst the C. nyallingensis population. Genetic diversity estimates were restricted by monoclonality in C. nyallingensis, while two of three C. forresterae populations exhibited heterozygote excess. The genomic data for C. forresterae and C. nyallingensis indicates that taxonomic revision may be necessary, however this would need to be underpinned by additional morphological studies supplemented with assessment of reproductive isolation and ploidy.