Mangrove Phylogeography Reveals Distinct Colonisation Events of Evolutionary Significant Units Across the Western Indian Ocean ( Bruguiera gymnorhiza

ABSTRACT

Mangrove tree populations of Rhizophora mucronata and Avicennia marina in the Western Indian Ocean (WIO) comprise distinct evolutionary significant units (ESUs) with contrasting dispersal histories between remote islands and the eastern African coastline. Because earlier genetic surveys across the Indo‐West Pacific revealed strong population differentiation in Bruguiera gymnorhiza , we hypothesised the presence of comparable ESUs within the WIO mangroves. Therefore, we investigated the phylogeography of B. gymnorhiza using complete chloroplast genomes and nuclear ribosomal DNA sequences from 14 provenances spanning the granitic Seychelles, Aldabra Atoll, Madagascar and the African coast. Maximum‐likelihood phylogenies and haplotype network analyses identified three ESUs corresponding to the Seychelles, Aldabra and Africa/Southwest Madagascar, highlighting a within‐species conservation issue. WIO populations were more differentiated from Asian B. gymnorhiza than B ruguiera cylindrica is from B ruguiera hainesii . Approximate Bayesian computation suggested that remote island populations diverged from African lineages approximately 26,000–31,000 generations ago and from each other about 6000 generations ago, corresponding to Mid‐ and Late‐Pleistocene periods, respectively. These divergence times are consistent with independent colonisation events inferred for other mangrove taxa. Chloroplast genomes of extant B. gymnorhiza lineages were more differentiated than those of R. mucronata or A. marina , suggesting an earlier arrival and longer time persistence of Bruguiera lineages in the WIO mangrove ecosystems. Our results demonstrate deep historical structure among B. gymnorhiza populations across the WIO region. A geographic concordance of genetic barriers with other mangrove species highlights the importance of conserving isolated island populations as unique evolutionary units of mangrove ecosystems.