Species delimitation, hybridization and possible apomixis in a rapid radiation of Western Australian Leptospermum (Myrtaceae)

Species delimitation is challenging in rapid radiations because the typical markers of speciation are often obscured. Here, we use comprehensive sampling and genome-wide single nucleotide polymorphisms to assess species boundaries in a radiation of nine morphologically similar Leptospermum taxa that failed to be discriminated in previous phylogenomic analyses. Our data recovered clear separation of L. maxwellii, L. sericeum and L. inelegans as currently circumscribed. A phrase-named taxon, Leptospermum. sp. Peak Charles/Norseman, was not distinct from L. incanum, and we recommend their synonymization. Another pair, L. nitens and L. roei, were also indistinct and differ by a single morphological character that also varies in L. inelegans without taxonomic recognition. We recommend synonymization of L. nitens and L. roei and consistent treatment of this character as a non-diagnostic, variable trait. Difficulty arose in discriminating L. erubescens and L. oligandrum; we make three suggestions and recommend further morphological investigation to determine the most appropriate taxonomic outcome. As expected, hybridization was common across the complex, but, unexpectedly, many individual plants were genetically identical within, and sometimes between, populations of most species. We hypothesize that this is due to apomixis. Overall, this study demonstrates the value of population genomics in the integrative taxonomy toolbox for disentangling species in rapid radiations, while also offering insight to the evolution of this poorly known group of Australian Leptospermum.