Genomic Basis of Freshwater Adaptation in the Palaemonid Prawn Genus Macrobrachium: Convergent Evolution Following Multiple Independent Colonization Events.

Adaptation to different salinity environments can enhance morphological and genomic divergence between related aquatic taxa. Species of prawns in the genus Macrobrachium naturally inhabit different osmotic niches and possess distinctive lifecycle traits associated with salinity tolerance. This study was conducted to investigate the patterns of adaptive genomic divergence during freshwater colonization in 34 Macrobrachium species collected from four continents; Australia, Asia, North and South America. Genotyping-by-sequencing (GBS) technique identified 5018 loci containing 82,636 single nucleotide polymorphisms (SNPs) that were used to reconstruct a phylogenomic tree. An additional phylogeny was reconstructed based on 43 candidate genes, previously identified as being potentially associated with freshwater adaptation. Comparison of the two phylogenetic trees revealed contrasting topologies. The GBS tree indicated multiple independent continent-specific invasions into freshwater by Macrobrachium lineages following common marine ancestry, as species with abbreviated larval development (ALD), i.e., species having a full freshwater life history, appeared reciprocally monophyletic within each continent. In contrast, the candidate gene tree showed convergent evolution for all ALD species worldwide, forming a single, well-supported clade. This latter pattern is likely the result of common evolutionary pressures selecting key mutations favored in continental freshwater habitats Results suggest that following multiple independent invasions into continental freshwaters at different evolutionary timescales, Macrobrachium taxa experienced adaptive genomic divergence, and in particular, convergence in the same genomic regions with parallel shifts in specific conserved phenotypic traits, such as evolution of larger eggs with abbreviated larval developmental.