Integrative Phylogenomics of Amphibians

Abstract

Amphibians -- comprising approximately 8,700 described species of frogs, salamanders, and caecilians -- are the most threatened vertebrate class globally, with over 41% assessed as threatened on the IUCN Red List, yet their macroevolutionary history and the genomic basis of their extraordinary diversity in morphology, physiology, and life history remain incompletely resolved. Previous amphibian phylogenies have been limited by sparse taxon sampling, reliance on a small number of mitochondrial genes, and inability to resolve ancient nodes that represent the deepest evolutionary divergences within the class. This study presents the most comprehensive integrative phylogenomic analysis of amphibian diversity yet conducted, combining whole-genome sequencing of 247 species (representing all major lineages), ultra-conserved element capture sequencing of 847 additional species, and a time-calibrated Bayesian molecular clock analysis using 38 fossil calibration points. The resulting phylogeny of 1,094 amphibian species resolves all major amphibian orders and families with posterior probabilities > 0.95, including four nodes that were previously strongly contested. Divergence time estimates place the origin of modern amphibians (crown Lissamphibia) at 341 +- 18 million years ago (Carboniferous), with rapid diversification of all three orders within 20 million years of the Permian-Triassic mass extinction. Diversification rate analyses identify 24 independent origins of direct development (bypassing the free-living aquatic tadpole stage) as the most consistently associated genomic innovation with elevated speciation rates in frogs. Ancestral genome size reconstruction reveals that genome size reduction was a key evolutionary transition enabling terrestrial diversification in salamanders. Conservation implications are derived from the identification of 84 evolutionarily distinct and globally endangered (EDGE) amphibian lineages requiring immediate conservation priority.