Abstract
Mangroves are ecologically important forest communities in tropical and subtropical coasts, the effective management of which requires understanding of their phylogeographic patterns. However, these patterns often vary among different species, even among ecologically similar taxa or congeneric species. Here, we investigated the levels and patterns of genetic variation within Lumnitzera consisting of two species (L. racemosa and L. littorea) with nearly sympatric ranges across the Indo-West Pacific (IWP) region by sequencing three chloroplast DNA regions (for both species) and genotyping 11 nuclear microsatellite loci (for L. littorea). Consistent with findings in studies on other mangrove species, we found that both L. racemosa and L. littorea showed relatively high genetic variation among populations but low genetic variation within populations. Haplotype network and genetic clustering analyses indicated two well-differentiated clades in both L. racemosa and L. littorea. The relationship between geographic and genetic distances and divergence time estimates of the haplotypes indicated that limited dispersal ability of the propagules, emergence of land barriers during ancient sea-level changes, and contemporary oceanic circulation pattern in the IWP influenced the current population structure of the two species. However, the position of genetic break was found to vary between the two species: in L. racemosa, strong divergence was observed between populations from the Indian Ocean and the Pacific Ocean possibly due to land barrier effect of the Malay Peninsula; in L. littorea, the phylogeographic pattern was created by a more eastward genetic break along the biogeographic barrier identified as the Huxley’s line. Overall, our findings strongly supported previous hypothesis of mangrove species divergence and revealed that the two Lumnitzera species have different phylogeographic patterns despite their close genetic relationship and similar current geographic distribution. The findings also provided references for the management of Lumnitzera mangroves, especially for the threatened L. littorea.
Highlights
70 woody plant species belonging to about 30 genera in 20 families are generally recognized as being mangroves (Duke, 2017)
We aimed to address the following questions: (1) What is the pattern of genetic variation across the distribution of L. littorea? (2) Does the Malay Peninsula serve as a land barrier in L. littorea, as demonstrated in L. racemosa? (3) Are there any other cryptic barriers that could have affected the current distribution and patterns of genetic variation of L. littorea and L. racemosa? We start by inferring the period of divergence between Lumnitzera and its closest relative, and compare the genetic structures of its member species across their distributions using maternally inherited chloroplast DNA
The concatenated chloroplast DNA (cpDNA) sequences had a total length of 1,755 bp and 2,054 bp, with 14 variable sites and five variable sites in L. racemosa and L. littorea, respectively (Supplementary Table S3)
Summary
70 woody plant species belonging to about 30 genera in 20 families are generally recognized as being mangroves (Duke, 2017). Recent studies have shown that the Malay Peninsula and/or ocean currents in the IWP have played important roles in shaping patterns of genetic variation (Tomizawa et al, 2017), even among members within major mangrove genera such as Rhizophora (Ng et al, 2015; Wee et al, 2015) and Xylocarpus (Guo et al, 2018b) Such differences in phylogeographic patterns among closely related species have often been attributed to subtle differences in dispersal abilities and/or the evolutionary histories of individual species (Ng et al, 2015). Given the polyphyletic origin of mangroves (Duke, 1995), it is important to explore the genetic characteristics of mangrove species having different life histories
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