‘Nebbiolo’ genome assembly allows surveying the occurrence and functional implications of genomic structural variations in grapevines (Vitis vinifera L.)

Abstract

Background: ‘Nebbiolo’ is a grapevine cultivar typical of north-western Italy, appreciated for producing high-quality red wines. Grapevine cultivars are characterized by possessing highly heterozygous genomes, including a great incidence of genomic rearrangements larger than 50 bp, so called structural variations (SVs). Even though abundant, SVs are an under-explored source of genetic variation mainly due to methodological limitations at their detection. Results: We employed a multiple platform approach to produce long-range genomic data for two different ‘Nebbiolo’ clones, namely: optical mapping, long-reads and linked-reads. We performed a haplotype-resolved de novo assembly for cultivar ‘Nebbiolo’ (clone CVT 71) and used an ab-initio strategy to annotate it. The annotated assembly enhanced our ability to detect SVs, enabling the study of genomic regions not present in the grapevines’ reference genome and accounting for their functional implications. We performed variant calling analyses at three different organizational levels: i) between haplotypes of clone CVT 71 (primary assembly vs haplotigs), ii) between ‘Nebbiolo’ and ‘Cabernet Sauvignon’ assemblies and iii) between clones CVT 71 and CVT 185, representing different ‘Nebbiolo’ biotypes. The cumulative size of non-redundant merged SVs indicated a total of 79.6 Mbp for the first comparison and 136.1 Mbp for the second one, while no SVs were detected for the third comparison. Interestingly, SVs differentiating cultivars and haplotypes affected similar numbers of coding genes. Conclusions: Our results suggest that SVs accumulation rate and their functional implications in ‘Nebbiolo’ genome are highly-dependent on the organizational level under study. SVs are abundant when comparing ‘Nebbiolo’ to a different cultivar or the two haplotypes of the same individual, while they turned absent between the two analysed clones.