Sequencing a Juglans regia × J. microcarpa hybrid yields high-quality genome assemblies of parental species

Abstract

Members of the genus <i>Juglans</i> are monecious wind-pollinated trees in the family Juglandaceae with highly heterozygous genomes, which greatly complicates genome sequence assembly. The genomes of interspecific hybrids are usually comprised of haploid genomes of parental species. We exploited this attribute of interspecific hybrids to avoid heterozygosity and sequenced an interspecific hybrid <i>Juglans microcarpa</i> × <i>J. regia</i> using a novel combination of single-molecule sequencing and optical genome mapping technologies. The resulting assemblies of both genomes were remarkably complete including chromosome termini and centromere regions. Chromosome termini consisted of arrays of telomeric repeats about 8 kb long and heterochromatic subtelomeric regions about 10 kb long. The centromeres consisted of arrays of a centromere-specific <i>Gypsy</i> retrotransposon and most contained genes, many of them transcribed. <i>Juglans</i> genomes evolved by a whole-genome-duplication dating back to the Cretaceous-Paleogene boundary and consist of two subgenomes, which were fractionated by numerous short gene deletions evenly distributed along the length of the chromosomes. Fractionation was shown to be asymmetric with one subgenome exhibiting greater gene loss than the other. The asymmetry of the process is ongoing and mirrors an asymmetry in gene expression between the subgenomes. Given the importance of <i>J. microcarpa</i> × <i>J. regia</i> hybrids as potential walnut rootstocks, we catalogued disease resistance genes in the parental genomes and studied their chromosomal distribution. We also estimated the molecular clock rates for woody perennials and deployed them in estimating divergence times of <i>Juglans</i> genomes and those of other woody perennials.