Genomic and transcriptomic data for the frog Platyplectrum ornatum

The diversity of genome sizes across the tree of life is of key interest in evolutionary biology. Various correlates of variation in genome size, such as accumulation of transposable elements or rate of DNA gain and loss, are well known, but the underlying molecular mechanisms that drive or constrain genome size are poorly understood. Here we study one of the smallest genomes among frogs characterized thus far, that of the ornate burrowing frog (Platyplectrum ornatum) from Australia, and compare it to other published frog and vertebrate genomes to examine the forces driving reduction in genome size. At ~1.06 Gb, the P. ornatum genome is like that of birds, revealing four major mechanisms underlying TE dynamics: reduced abundance of all major classes of transposable elements (TEs); increased net deletion bias in TEs; drastic reduction in the lengths of introns; and expansion via gene duplication of the repertoire of TE-suppressing Piwi genes, accompanied by increased expression of piRNA-based TE-silencing pathway genes in germline cells. Transcriptome data from multiple tissues in both sexes corroborate these results and provide insight into sex-differentiation pathways in Platyplectrum. Genome skimming of two closely related frog species (Lechriodus fletcheri and Limnodynastes fletcheri) confirms a reduction in TEs as a major driver of genome reduction in Platyplectrum and supports a macroevolutionary scenario of small genome size in frogs driven by convergence in life history, especially rapid tadpole development and tadpole diet. The P. ornatum genome offers a model for future comparative studies on mechanisms of genome size reduction in amphibians and in vertebrates generally.