Relationships Among Powered Flight, Metabolic Rate, Body Mass, Genome Size, and the Retrotransposon Complement of Volant Birds

Avian genomes are of interest because the rapid metabolic rate associated with powered flight requires small cells which constrain genome size. Consequently, flying birds tend to have small genomes relative to other vertebrates such as mammals. It thus stands to reason that flying birds should have smaller genomes than ground-dwelling birds with lower metabolic rates. Small genomes could be condensed but uncompromised in a number of ways, including smaller intergenic intervals, shorter introns, and/or a reduced transposable element (TE) complement. We evaluated genome size in light of the orthologous TE complement among 41 flying (FY) and seven ground-dwelling (GD) bird species to determine if a preponderance of deletions in orthologous TEs might explain the compact genomes of flying birds with high metabolic rates. We measured, across multiple loci in all 48 species, the lengths of 50 contemporary orthologous chicken repeat 1 (CR1, a non-LTR retrotransposon) copies relative to inferred ancestral CR1 sequences. We found genome sizes in GD birds were not different than those in FY birds, but the mean lengths of orthologous CR1 loci were significantly shorter in FY birds than in GD birds. Moreover, we observed a negative correlation between basal metabolic rate and length of orthologous CR1 loci. Finally, we observed positive correlations between body mass and both genome sizes as well as length of orthologous CR1 loci, which we expected given that body mass correlates negatively with metabolic rates. Our results support the contention that metabolism helps shape the avian TE complement and thus indirectly contributes to the compact genomes of birds.