The Large-Scale Phylogenomic and Molecular Clock Analysis Provide Timescale Colonization of Mollusks for Freshwater

Abstract

As the phylum with the second largest biodiversity, mollusks distribute in almost all habits on the earth, especially being abundant in marine and freshwater. Throughout geological time, several molluscan lineages independently colonized freshwater, shaping the freshwater ecosystem and providing an adequate environment for successive animal colonization of freshwater. However, the timing of their colonization for freshwater was still unclear. Herein, based on Benchmarking Universal Single-Copy Orthologs (BUSCO), we obtained an elaborate data set including 618 genes with a total length of 2,435,852 amino acid sites across 112 mollusks to reconstruct the phylogeny of Mollusca. The phylogenetic trees recovered by RAxML and MrBayes show basically the same topology with well-supported. By the divergence times estimation with fossil calibration, we presented a comprehensive timescale of molluscan evolution and revealed that seven classes converged on a Cambrian diversification (539.86-493.45 Ma). What’s more, molecular clock analysis revealed the times of three major independent freshwater colonization. The order Unionidae (Bivalvia) first colonized freshwater in the Ordovician (about 472.84 Ma) to serve as the first herbivores in the freshwater ecosystem, the appearance of the first freshwater plants (about 480 Ma) was considered associated with this event. The second colonization event occurred in the Carboniferous (217.78 Ma), Ampullariidae (Gastropoda) colonized freshwater and used lungs to adapt to the high temperature and low oxygen at the time. The third colonization (137.23 Ma) occurred after the Triassic-Jurassic mass extinction (about 201 Ma), and the large niche vacancies in freshwater caused by this extinction may provide the opportunities for the families Physidae, Planorbidae and Lymnaeidae (Gastropoda) into freshwater. Our results revealed the formation and colonization of freshwater mollusks that may be related to the evolution of freshwater plants and ecological events and could offer resolution and novel insights into the colonization of mollusks for freshwater.

Keywords: Mollusca, phylogeny, Phylogenomics, freshwater, marine, molecular clock.