How ancient forest fragmentation and riparian connectivity generate high levels of genetic diversity in a micro-endemic Malagasy tree

Abstract

Understanding landscape changes is central to predicting evolutionary trajectories and defining conservation practices. While human-driven deforestation is intense throughout Madagascar, exception in areas like the Loky-Manambato region (North) raises questions. This region also harbors a rich and endemic flora, whose evolutionary origin remains poorly understood.

We assessed the genetic diversity of an endangered micro-endemic Malagasy olive species (Noronhia spinifolia) to better understand the vegetation dynamic in the Loky-Manambato region and its influence on past evolutionary processes. We characterized 72 individuals sampled across eight forests through nuclear and mitochondrial restriction associated sequencing data (RADseq) and chloroplast microsatellites (cpSSR). Extremely high genetic diversity was revealed in two genomic compartments (chloroplast h = 0.99, and mitochondrial h = 0.85). Combined population and landscape genetics analyses indicate that N. spinifolia diversity is best explained by the current forest cover (R² = 0.90), highlighting a long-standing habitat mosaic in the region. Our results further suggest a predominant role of forest-dwelling organisms in mediating pollen and seed dispersals.

This sustains a major and long-term role of riparian corridors in maintaining connectivity across those antique mosaic-habitats, calling for the study of organismal interactions that promote gene flow.