Genomic evidence for a hybrid origin of the critically endangered shrub Callistemon kenmorrisonii (Myrtaceae) and persistence involving extreme clonality

Abstract Narrow range endemic species are often threatened by stochastic events due to their localised distribution. Many of these species also have small population sizes where demographic and genetic threats are problematic. The critically endangered Callistemon kenmorrisonii (Myrtaceae) is a narrow range endemic from East Gippsland, Victoria, Australia, with stochastic events (fire and flooding) and genetic factors being of concern. A DArTseq approach was undertaken to determine the genotypic and genetic diversity of each of the two known populations, as well as address uncertainties regarding a hybrid origin for C. kenmorrisonii . The analyses used 6,326 filtered Callistemon single nucleotide polymorphisms (SNPs) to reveal unexpectedly low genotypic diversity (using co-ancestry and pairwise genetic distance approaches) with only six genotypes identified out of 83 C. kenmorrisonii samples. Large genets were observed in both populations, and one population was found to be monoclonal. Additionally, ex situ seed grown progeny were found to predominately share a single maternal genotype, suggesting possible apomictic reproduction in the taxon. Principal coordinates analysis (PCoA), STRUCTURE and NEWHYBRIDS provided three lines of evidence supporting a C. subulatus x C. citrinus hybrid origin for C. kenmorrisonii. The combined results support C. kenmorrisonii as a potential microspecies arising through hybridisation of two common, widespread species. Combined with small populations and localised occurrences, the relative lack of diversity within and among populations is of conservation concern. It is proposed that further studies seek to confirm the species’ mode of reproduction.