Paleogenomic data suggest mammal-like genome size in the ancestral amniote and derived large genome size in amphibians.

An unsolved question in evolutionary genomics is whether amniote genomes have been expanding or contracting since the common ancestor of this diverse group. Here, we report on the polarity of amniote genome size evolution using genome size estimates for 14 extinct tetrapod genera from the Paleozoic and early Mesozoic Eras using osteocyte lacunae size as a correlate. We find substantial support for a phylogenetically controlled regression model relating genome size to osteocyte lacunae size (P of slopes <0.01, r²=0.65, phylogenetic signal λ=0.83). Genome size appears to have been homogeneous across Paleozoic crown-tetrapod lineages (average haploid genome size 2.9-3.7 pg) with values similar to those of extant mammals. The differentiation in genome size and underlying architecture among extant tetrapod lineages likely evolved in the Mesozoic and Cenozoic Eras, with expansion in amphibians, contractions along the diapsid lineage, and no directional change within the synapsid lineage leading to mammals.