A population genomics analysis of the Aotearoa New Zealand endemic rewarewa tree (Knightia excelsa)

Abstract

Rewarewa (Knightia excelsa, Proteaceae) is a tree species endemic to Aotearoa New Zealand, with a natural distribution spanning Te Ika-a-Māui (North Island) and the top of Te Waipounamu (South Island). We used the pseudo-chromosome genome assembly of rewarewa as a reference and whole genome pooled sequencing from 35 populations sampled across Aotearoa New Zealand, including trees growing on Māori-owned land, to identify 1,443,255 single nucleotide polymorphisms (SNPs). Four genetic clusters located in the northern North Island (NNI), eastern North Island (NIE), western and southern North Island (NIWS), and the South Island (SI) were identified. Gene flow was revealed between the SI and NIE genetic clusters, plus bottleneck and contraction events within the genetic clusters since the mid-late Pleistocene, with divergence between North and South Island clusters estimated to have occurred ~115,000–230,000 years ago. Genotype environment analysis (GEA) was used to identify loci and genes linked with altitude, soil pH, soil carbon, slope, soil size, annual mean temperature, mean diurnal range, isothermality, annual precipitation, and precipitation seasonality. The location of the SNPs associated with these environmental variables was compared with the position of 52,192 gene-coding sequences that were predicted in the rewarewa genome using RNA sequencing. This new understanding of the genetic variation present in rewarewa and insights into the genetic control of adaptive traits will inform efforts to incorporate the species in restoration plantings and for marketing rewarewa honey based on provenance.