Convergent genome streamlining accompanies independent miniaturization in the world's smallest fishes.

Miniaturization, the reduction of adult body size to an extreme degree, has evolved repeatedly across vertebrates. Yet its genomic underpinnings remain poorly understood. Cypriniformes, the most species-rich order of freshwater fishes, contains multiple miniaturized lineages that have evolved via contrasting developmental processes. Proportioned dwarfs are tiny-bodied but otherwise morphologically similar to larger relatives, while progenetic miniatures exhibit developmental truncation and retain larval-like anatomical features into adulthood. Using a new time-calibrated phylogeny of 309 cypriniform species and comparative genomic analyses of 33 high-quality genome assemblies, we investigated the evolutionary history and genomic correlates of miniaturization across this order. Ancestral state reconstruction revealed multiple independent origins of both miniature types, with reversals to the nonminiature state rarely recovered and miniature origins nonrandomly distributed across the phylogeny. The origins of the two types of miniatures differed in their timing. Progenetic miniatures arose predominantly in the Eocene whereas proportioned dwarfs arose mainly within the Miocene period. Genome size variation across Cypriniformes has been overwhelmingly driven by polyploidy. However, progenetic miniatures showed a directionally consistent, though nonsignificant, trend toward genome size reduction. Comparative genomic analyses revealed that all three independently evolved progenetic miniature lineages share convergent signatures of repeat loss alongside genome-wide intron shortening, patterns absent in proportioned dwarfs. Our study provides the broadest evidence to date that progenetic miniaturization, despite independent origins, is underpinned by predictable structural genomic changes, revealing a fundamental link between developmental truncation and genome architecture in vertebrates.