Endogenous Avian Leukosis Virus subgroup E elements of the chicken reference genome

The chicken reference genome contains two endogenous Avian Leukosis Virus subgroup E (ALVE) insertions, but gaps and unresolved repetitive sequences in previous assemblies has hindered their precise characterisation. Detailed analysis of the most recent reference genome (GRCg6a) now shows both ALVEs within contiguous chromosome assemblies for the first time. ALVE6 (ALVE-JFevA) and ALVE-JFevB are both located on chromosome 1, with ALVE6 close to the p arm telomere. ALVE-JFevB is a structurally intact element containing the ALVE gag, pol and env genes, and is capable of forming replication competent viruses. In contrast, ALVE6 (ALVE-JFevA) contains a 3352 bp 5’ truncation and lacks the entire 5’ LTR and gag gene. Despite this, ALVE6 remains able to produce intact envelope protein, likely due to a mutation in the recognition site for a known inhibitory miRNA (miR-155). Whole genome resequencing datasets from layers, broilers and three independent sources of wild-caught red junglefowl were surveyed for the presence of each of these reference genome ALVEs. ALVE-JFevB was found in no other chicken or red junglefowl genomes, whereas ALVE6 was identified in some layers, broilers and native breeds, but not within any other red junglefowl genome. Improved assembly contiguity has facilitated better characterisation of the two ALVEs of the chicken reference genome. However, both the limited ALVE content and unique presence of ALVE-JFevB suggests that the reference individual is unrepresentative of ancestral Gallus gallus ALVE diversity.