American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Larger genomes linked to slower development and loss of late-developing traits”

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Molly C. Womack, Marissa J. Metz, and Kim L. Hoke (Dec 2019)

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Larger genomes linked to slower development and loss of late-developing traits

Frogs with lots of DNA take longer to grow up and may lose their ears

A metamorphosing <i>Boana lanciformis</i> (basin tree frog). <br />(Credit: Luis Coloma)
A metamorphosing Boana lanciformis (basin tree frog).
(Credit: Luis Coloma)

Genome size (the amount of DNA in an animal) varies widely among animals and can affect an animal’s appearance, how they develop, and how their bodies function. For example, increases in genome size have been linked with loss of toes in salamanders. Recently, researchers from Colorado State University were able to show that frogs with larger genomes take longer to develop from a swimming tadpole into a hopping froglet and often lack a middle ear bone as adults. The authors suggest that larger genomes, slower development, and smaller body sizes at the transition from tadpole to froglet (metamorphosis) may contribute to more than 39 mysterious losses of the middle ear among frog lineages. In amphibians more broadly, genome size is hypothesized to affect the formation of structures (bones, muscle, and other tissues) that appear late in development, closer to adulthood. However, few studies have linked larger genome size with changes in late-forming structures, especially outside of salamanders, which have much larger genomes than most animals. This new finding from Womack et al., appearing in The American Naturalist, concludes that increases in frog genome size, although less drastic than in salamanders, may affect development of late-forming traits such as middle ear bones. These results raise the possibility that the development of structures in other animals may be affected by changes in genome size.


Genome size varies widely among organisms and is known to affect vertebrate development, morphology, and physiology. In amphibians, genome size is hypothesized to contribute to loss of late-forming structures, although this hypothesis has mainly been discussed in salamanders. Here we estimated genome size for 22 anuran species and combined this novel dataset with existing genome size data for an additional 234 anuran species to determine whether larger genome size is associated with loss of a late-forming anuran sensory structure, the tympanic middle ear. We established that genome size is negatively correlated with development rate across 90 anuran species and found that genome size evolution is correlated with evolutionary loss of the middle ear bone (columella) among 241 species (224 eared and 17 earless). We further tested whether the development of the tympanic middle ear could be constrained by large cell sizes and small body sizes during key stages of tympanic middle ear development (metamorphosis). Together, our evidence suggests that larger genomes, slower development rate, and smaller body sizes at metamorphosis may contribute to the loss of the anuran tympanic middle ear. We conclude that increases in anuran genome size, although less drastic than in salamanders, may affect development of late-forming traits.