Mid-Miocene Formation of the Yangtze River-Lake System Revealed by Ancestral Spawning Reconstruction of Endemic Cyprinids

The Yangtze River is cross-linked with numerous lakes within its floodplain and is a worldwide biodiversity hotspot. There is no evidence indicating when this unique river-lake system developed. The endemic East Asian cyprinid clade has evolved diverse spawning adaptations to different flow conditions. Our ancestral egg-type reconstruction showed an ancestral state of adhesive eggs and later demersal eggs origination (both stream adaptations). Semi-buoyant eggs emerged ~18 Mya as a fast-flowing river adaptation, with increased hydration via three yolk protein degradation pathways, ion transport pathways and egg envelope permeability transition pores. Adhesive eggs evolved secondarily ~14 Mya with the egg envelope increasing to four layers and an adhesive layer, along with an increase in adhesiveness via microfilament/adhesive-related protein crosslinking and enhanced glycosaminoglycan biosynthesis, improving adherence to submerged lake plants, indicating that the cross-linked river-lake system formed in the mid-Miocene. This study provides a unique biological evidence for large-scale water system evolution.