Eco-evolutionary Dynamics of Host-Parasite Coevolution in Cichlid Fishes of the Niger Delta River Systems, South South Nigeria

Host-parasite coevolution generates some of the most rapid and consequential eco-evolutionary dynamics in natural populations, yet these processes have been almost entirely unstudied in African freshwater fish systems. This study investigates eco-evolutionary dynamics of host-parasite interactions between cichlid fish hosts and their monogenean ectoparasites across a network of ecologically and anthropogenically varying river habitats in the Niger Delta. Fourteen cichlid species and their associated Dactylogyrus and Gyrodactylus parasite communities were sampled from twenty river sites spanning a gradient of environmental quality and isolation. Host immune gene diversity at the major histocompatibility complex was characterised through deep amplicon sequencing. Parasite community composition, infection intensity, and local adaptation were assessed through reciprocal infection experiments. Population genomic analyses examined signatures of parasite-driven balancing selection at major histocompatibility complex loci and directional selection at immune pathway genes. Ecological modelling examined how environmental degradation modulates the coevolutionary trajectory of host-parasite interactions. Results reveal strong signatures of balancing selection at major histocompatibility complex loci consistent with parasite-driven frequency-dependent selection in host populations from high-diversity, environmentally intact habitats. Environmental degradation disrupted coevolutionary dynamics by homogenising parasite communities, reducing selection intensity, and allowing genetic drift to erode major histocompatibility complex diversity in isolated, degraded populations. Reciprocal infection experiments confirmed local adaptation of parasites to sympatric hosts at six of twelve site pairs. The study makes a significant contribution to eco-evolutionary theory by demonstrating that anthropogenic environmental change can disrupt coevolutionary dynamics in ways that reduce host adaptive capacity. Keywords: eco-evolutionary dynamics, host-parasite coevolution, cichlid, Niger Delta, major histocompatibility complex.