Thille, a chromosome-level type B avocado genome, and the structural and allelic variation at candidate flowering-type loci
Avocado (Persea americana Mill.) is heterodichogamous: each flower opens first as female and then as male, and cultivars fall into two complementary flowering types, A and B, determined by the transcription factor SDMYB, whose dominant allele confers type A. Avocado genomes have so far come from type A cultivars. Here we report a chromosome-level genome of Thille, a type B cultivar used in breeding. From 72.2 Gb of PacBio HiFi reads (81x coverage) and Omni-C data, the assembly places 859.8 Mb, 96% of the expected 896 Mb genome, in 12 chromosomes with an NG50 of 84.5 Mb, a consensus quality of QV 69.95, and 97.3% complete BUSCO genes; repeats occupy 64.0% of the assembly, and 35,386 protein-coding genes were predicted. We compared Thille with the type A cultivars Hass and Puebla, a predominantly Guatemalan hybrid, and with a West Indian (WI) accession, a Lowland botanical group variety of unreported flowering type. The assemblies differ by large rearrangements, including an 18.8 Mb inversion on chromosome 12 specific to Hass. Using phased reads and haplotype assemblies, we reconstructed the alleles of candidate flowering genes from a type A versus type B Fst scan. Both WI haplotypes carry the recessive SDMYB allele found in Thille; based on this study and the previous study of SDMYB, we predict the WI accession to be type B. Six genes, including SDMYB, carry an allele shared by all three type A cultivars and absent from Thille and WI. The genome and HiFi reads are available under BioProject PRJNA1142556.