Abstract
Rhizophora species are the most widely distributed mangrove trees in the Indo-West Pacific (IWP) region. Comparative studies of these species with shared life history traits can help identify evolutionary factors that have played most important roles in determining genetic diversity within and between populations in ocean-current dispersed mangrove tree species. We sampled 935 individuals from 54 natural populations for genotyping with 13 microsatellite markers to investigate the level of genetic variation, population structure, and gene flow on a broad geographic scale in Rhizophora apiculata, Rhizophora mucronata, and Rhizophora stylosa across the IWP region. In contrast to the pattern expected of long-lived woody plants with predominant wind-pollination, water-dispersed seeds and wide geographic range, genetic variation within populations was generally low in all the three species, especially in those peripheral populations from geographic range limits. Although the large water-buoyant propagules of Rhizophora have capacity for long distance dispersal, such events might be rare in reality, as reflected by the low level of gene flow and high genetic differentiation between most of population pairs within each species. Phylogeographic separation of Australian and Pacific island populations from SE Asian lineages previously revealed with DNA sequence data was still detectable in R. apiculata based on genetic distances, but this pattern of disjunction was not always evident in R. mucronata and R. stylosa, suggesting that fast-evolving molecular markers could be more suitable for detecting contemporary genetic structure but not deep evolutionary divergence caused by historical vicariance. Given that mangrove species generally have small effective population sizes, we conclude that genetic drift coupled with limited gene flow have played a dominant role in producing the current pattern of population genetic diversity in the IWP Rhizophora species, overshadowing the effects of their life history traits. Recent population fragmentation and disturbances arising from human activities could further endanger genetic diversity in mangrove trees.
Highlights
Mangroves consist of a diverse group of species of trees, shrubs, palms, vines, and ferns from 20 families (Tomlinson, 1986; Duke et al, 1998)
We address the following questions: What is the level of genetic diversity in Indo-West Pacific (IWP) Rhizophora species? Is the pattern of population structure the same among these species? Do populations from the species’ distributional margins possess less genetic diversity than those central populations? Is the morphological and phylogeographic discontinuity still detectable with microsatellites in R. apiculata as well as in R. mucronata and R. stylosa? We discuss the effects of life history traits and major evolutionary processes that might have played dominant roles in shaping the pattern of genetic diversity, as well as the significance of historical sea level fluctuations and physical barriers to dispersal on the genetic structuring of mangrove populations
All the estimates of genetic diversity were relatively low at the population level in all three IWP Rhizophora species (Table 1), compared to the population mean values measured with microsatellite markers for other plant species
Summary
Mangroves consist of a diverse group of species of trees, shrubs, palms, vines, and ferns from 20 families (Tomlinson, 1986; Duke et al, 1998). These species can be found in 128 countries or territories, but occur most commonly in tropical and subtropical coastlines, covering an estimated total area of 152,000 square kilometers globally (Spalding et al, 2010; Duke, 2013, 2014). Mangrove forests are among the most productive ecosystems in the world with important ecological and economic values. Much of the vast biodiversity of the mangrove ecosystem depends on a few key tree species that shape the system, such as species of Rhizophora, the most representative genus in the mangrove family Rhizophoraceae with broad distribution in both Indo-West Pacific (IWP) and Atlantic-East Pacific (AEP) regions
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