Evolution of genome size in recent and fossil salamanders

Modern salamanders possess giant genomes, and directly correlated with this, the by far the largest cells among tetrapods. While no entire genome of a salamander has been sequenced thus far, recent studies have shown that large introns and novel genes contribute to the enormous genome sizes. The extreme cells size impacts many aspects of salamander biology and has been suggested to be closely associated with the enormous plasticity in life history pathways and their high regenerative capacity that includes limbs, eyes, spinal cord and other complex tissues. In the current study, we sectioned long bones (N>18, mostly femora) of members of all modern salamander families with known genome sizes as well as long bones of members of several fossil amphibian lineages to investigate if 1. the correlation between genome size and cell size that has been established based on leucocytes holds with respect to osteocyte size and 2. investigate when within the long evolutionary history of the clade the large genome sizes evolved. For outgroup reference, we sampled femora of a number of modern amniote and frog taxa with known genome sizes. Understanding the correlation between osteocyte lacuna size and genome size in modern taxa is vital for an investigation of the genome sizes in fossil taxa. In order to minimize errors caused by variation in location and orientation of sections, we aimed to sample homologous elements in the same location and the same plane of sectioning. Our results show that genome size and osteocyte size are tightly correlated in salamanders. Moreover, the large genomes of urodeles were already present in stem-group salamanders (Karauridae) and most likely evolved early in the evolutionary history of the salamander lineage, possibly as early as Paleozoic dissorophoid temnospondyls. This provides new insights into the deep time genomic evolution of urodeles and a novel dataset for understanding salamander origins and the evolution of central aspects of their biology, such as life history patterns, miniaturization, and developmental rates.