Land masses and oceanic currents drive population structure of Heritiera littoralis, a widespread mangrove in the Indo-West Pacific

Abstract

Phylogeographic forces driving evolution of sea-dispersed plants are often influenced by regional and species characteristics, although not yet deciphered at a large spatial scale for many taxa like the mangrove species Heritiera littoralis. This study aimed to assess geographic distribution of genetic variation of this widespread mangrove in the Indo-West Pacific region and identify the phylogeographic factors influencing its present-day distribution. Analysis of five chloroplast DNA fragments' sequences from 37 populations revealed low genetic diversity at the population level and strong genetic structure of H. littoralis in this region. The estimated divergence times between the major genetic lineages indicated that glacial level changes during the Pleistocene epoch induced strong genetic differentiation across the Indian and Pacific Oceans. In comparison to the strong genetic break imposed by the Sunda Shelf toward splitting the lineages of the Indian and Pacific Oceans, the genetic differentiation between Indo-Malesia and Australasia was not so prominent. Long-distance dispersal ability of H. littoralis propagules helped the species to attain transoceanic distribution not only across South East Asia and Australia, but also across the Indian Ocean to East Africa. However, oceanic circulation pattern in the South China Sea was found to act as a barrier creating further intraoceanic genetic differentiation. Overall, phylogeographic analysis in this study revealed that glacial vicariance had profound influence on population differentiation in H. littoralis and caused low genetic diversity except for the refugia populations near the equator which might have persisted through glacial maxima. With increasing loss of suitable habitats due to anthropogenic activities, these findings therefore emphasize the urgent need for conservation actions for all populations throughout the distribution range of H. littoralis.

Keywords: Pleistocene; chloroplast DNA; divergence; glacial refugia; phylogeography; vicariance.

Figure 1

Distribution of Heritiera littoralis in the Indo‐West Pacific (IWP) based on—(a) occurrence records in the GBIF, and (b) actual sampling locations of this study. Abbreviations of the sampling sites have been given in Appendix S1. The locations of the Sunda and Sahul shelves, and major oceanic currents of the Indonesian Throughflow have been adopted from Lohman et al. (2011) and Hall (2009)

Figure 2

The seven scenarios tested for each of the two approximate Bayesian computation (ABC) models implemented in the software DIYABC ver.2.0. The population groups considered in each approach have been provided. In all scenarios, t# represents time scale measured in number of generations and N# represents effective population size of the corresponding populations during the relevant time period (e.g., 0–t1, t1–t2). Abbreviations of the sampling sites have been given in Appendix S1

Figure 3

Relationship and distribution of 13 cpDNA haplotypes in Heritiera littoralis—(a) phylogenetic relationships of the haplotypes resolved through Maximum‐Parsimony method with numbers on branches showing the supporting ratio obtained from bootstrapping with 1,000 replicates; (b) median‐joining network for the haplotypes in which the size of the circle is proportional to the frequency of each sampled haplotype and the black dots on the branches indicate the number of steps separating adjacent haplotypes. Three hypothetical haplotype groups are indicated as group 1, 2, and 3; (c) geographical distribution of the haplotypes. Abbreviations of the sampling sites have been given in Appendix S1

Figure 4

Relationship between geography and genetic differentiation across sampling locations of Heritiera littoralis in the IWP—(a) Principal coordinate analysis (PCoA) grouping 37 sampling sites into four groups, generally consistent with the population clusters identified by SAMOVA, (b) Scatterplot of Mantel test showing relationship between pairwise genetic and geographic distances, and c) Potential gene flow barriers, represented with red lines, between the sampling sites around each of which the blue polygons depict the Voronoï tessellation. Abbreviations of the sampling sites have been given in Appendix S1