De-novo assembly of a reference genome for a critically threatened Aotearoa New Zealand tree species, Syzygium maire (Myrtaceae)

Abstract

Aotearoa New Zealand’s swamp forests have experienced significant habitat loss in fewer than two hundred years. Many of the country’s tree species are endemic with sparse to no genetic information available to underpin conservation strategies. Syzygium maire, Aotearoa’s only endemic Syzygium species, is a culturally and ecologically important component of swamp forest habitats. Unfortunately, populations of S. maire have been greatly reduced, heavily fragmented and are susceptible to the emergent pathogen Austropuccinia psidii (myrtle rust), posing eminent danger of a further decline of the species. We sought to develop genomic resources to inform conservation management of S. maire. To this end, we used long read, high accuracy sequencing technology to produce a highly complete reference quality genome for S. maire. The genome sequence was named ‘Ngā Hua o te Ia Whenua’ by the local Māori tribe where the tree used for genome sequencing grows. We assess whether genome-level divergence with other Myrtaceae may have followed geographic isolation of the species. We detect conservation of large scale synteny between three Syzygium species and Eucalyptus grandis, providing support for the stability of Syzygium genomes across evolutionary time. We annotate genes implicated in fungal pathogen defence, identifying several hundred putative NLR genes, including putative homologs of previously identified Austropuccinia psidii resistance genes. Finally, we evaluate the genetic relationships of individuals of a small, isolated population of trees. We find evidence of high levels of kinship and inbreeding within small and isolated S. maire populations, informing local-scale conservation strategies for the species. Our findings enable practical conservation actions and provide resources for larger scale studies of S. maire and other Syzygium species in the future.