The genome of the symbiont of O. gallaeciana was assembled using long reads from Nanopore sequencing, utilizing Flye v2.8.3 with “--meta” option. The generated assembly was polished four times with Racon v1.4.13 with (-m 8 -x -6 -g -8 -w 500) option and then further polished once with Medaka v1.0.3 (https://nanoporetech.github.io/medaka) with the r941_min_high_g344 model using the MinION raw reads. Subsequent polishing with Illumina short reads was performed using ntHits v0.1.1 (https://github.com/bcgsc/nthits) and ntEdit v1.3.2 with the default settings. Duplications (heterozygous regions) were purged with PURGEhaplotigs v1.0.3 and this ended up in the final genome assembly.