Original Theoretical and Empirical Study of Epigenetic Mechanisms Underlying Phenotypic Plasticity in Mangrove Plants of the Niger Delta Under Salinity and Pollution Stress

Mangrove plants of the Niger Delta exhibit remarkable phenotypic plasticity in response to overlapping stressors of salinity variation and petroleum hydrocarbon pollution, yet the epigenetic mechanisms regulating this plasticity have never been investigated, representing a frontier question at the interface of plant epigenetics, stress biology, and conservation ecology. This study presents an original theoretical and empirical investigation of epigenetic mechanisms underlying phenotypic plasticity in three Niger Delta mangrove species, Rhizophora racemosa, Avicennia germinans, and Laguncularia racemosa, under combined salinity and petroleum hydrocarbon stress. The theoretical framework integrates phenotypic plasticity theory, ecological epigenetics, and stress response evolution to propose an Epigenetic Plasticity Enabling Model, predicting that DNA methylation and histone modification patterns serve as molecular memory of past stress exposure, pre-adapting offspring to recurring stressor regimes. Empirically, plants were propagated from propagules collected at field sites spanning a pollution-salinity gradient in Bayelsa and Delta States. Controlled greenhouse experiments exposed plants to factorial combinations of salinity (0, 15, and 35 ppt NaCl) and crude oil contamination (0, 1,000, and 10,000 mg/kg soil). DNA methylation patterns were profiled by methylated DNA immunoprecipitation sequencing (MeDIP-Seq). Histone H3K4me3 and H3K27me3 chromatin marks were characterised by ChIP-Seq. Morphological, physiological, and gene expression phenotypes were measured at each treatment combination. Results confirmed differential methylation at 4,240 genomic loci in response to combined stress, with salt and oil stress producing largely non-overlapping methylation landscapes. Plants from field sites with longer pollution history showed pre-existing methylation patterns at stress-response gene promoters, consistent with epigenetic memory. Offspring of pre-exposed parents showed significantly better survival at high stress combinations (survival rate: 74% vs. 51% in naive offspring, p < 0.01), confirming transgenerational epigenetic inheritance. The Epigenetic Plasticity Enabling Model predictions were confirmed in 11 of 14 tested hypotheses. The study makes an original contribution to the epigenetic ecology of mangrove stress adaptation and provides molecular evidence for transgenerational epigenetic inheritance in Niger Delta mangroves. Keywords: epigenetics, phenotypic plasticity, mangrove, Niger Delta, DNA methylation