Evolutionary Origins and Adaptive Diversification of Toxin Resistance in Predatory Fish Consuming Cyanobacteria-Rich Prey in Nigerian Freshwater Systems

Cyanobacterial toxins represent potent cellular poisons that accumulate through freshwater food webs, and the mechanisms by which predatory fish evolve tolerance to cyanotoxin exposure represent a theoretically significant but empirically unexplored question in evolutionary toxicology. This research investigates the molecular evolutionary origins of microcystin and saxitoxin resistance in predatory fish from Nigerian lakes and reservoirs with contrasting cyanobacterial bloom histories. Population samples of Lates niloticus, Clarias gariepinus, and Synodontis schall were collected from five Nigerian reservoirs with a 20-year microcystin bloom history and five reference water bodies without bloom history. Whole-genome sequencing and comparative genomics were used to identify resistance-associated variants in candidate genes encoding protein phosphatase 2A, multidrug resistance transporters, and CYP450 detoxification enzymes. Functional validation of candidate resistance mutations was performed through heterologous expression in yeast systems and in vitro enzyme kinetic assays. Transcriptomic profiling characterised the gene expression response to experimental microcystin exposure in resistant and susceptible populations. Convergent amino acid substitutions in the protein phosphatase 2A catalytic subunit were identified in populations from three reservoirs with distinct bloom histories, suggesting repeated evolution of the same molecular resistance mechanism. Resistance variants in ABCB multidrug transporter genes showed signatures of positive selection in bloom-history populations. Functional assays confirmed that the protein phosphatase mutations reduce microcystin binding affinity by 3.7 to 6.2-fold. Transcriptomic analysis revealed pre-adaptive upregulation of glutathione-S-transferase pathways in fish from bloom-history populations. This research establishes the first molecular evolutionary framework for cyanotoxin resistance in African freshwater fish. Keywords: cyanotoxin resistance, microcystin, molecular evolution, freshwater fish, adaptive evolution.