The evolution of genome size: what can be learned from anuran development?

Differences in nuclear DNA content in vertebrates have been shown to be correlated with cell size, cell division rate, and embryonic developmental rate. We compare seven species of anuran amphibians with a three-fold range of genome sizes. Parameters examined include the number and density of cells in a number of embryonic structures, and the change in cell number in the CNS during development. We show that genome size is correlated with cell proliferation rate and with developmental rate at different stages of embryonic development, but that the correlation between genome size and cell size is only evident at later stages. We discuss the evolution of genome size in amphibians. Our discussion takes into account data that reportedly support two conflicting hypotheses: the "skeletal DNA" hypothesis, which claims a selective role for differences in genome size, and the "junk DNA" hypothesis, which claims that differences in genome size are a random result of the accumulation of noncoding DNA sequences. We show that these supposedly conflicting hypotheses can be integrated into a more complex and inclusive model for the evolution of genome size.