Contrasting Phylogeographic Patterns in Lumnitzera Mangroves Across the Indo-West Pacific

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.

Deep phylogeographic splits and limited mixing by sea surface currents govern genetic population structure in the mangrove genus Lumnitzera (Combretaceae) across the Indonesian Archipelago

“…On the other hand, sea currents may act as barriers (Waters, 2008), for example, through the ITF. Third, we hypothesize to find support for a fine‐scale isolation‐by‐distance (IBD) pattern across Indonesia, in line with the findings obtained by Guo et al (2021) on a coarser scale across the IWP. Some mangroves have short‐distance propagule dispersal ability that is limited within the same or nearby populations, while others are capable of long‐distance dispersal (LDD) (Duke et al, 2002; Van der Stocken & Menemenlis, 2017).…”

“…First, a mixed mating breeding system allowing for selfing, as has been shown for Lumnitzera racemosa (Solomon Raju et al, 2014), could explain low diversity. Actually, for Lumnitzera littorea, high inbreeding coefficients have been found, potentially indicating selfing, although Null alleles could result in a similar pattern (Guo et al, 2021). Second, strong demographic bottlenecks due to the instability of their habitats (Kado et al, 2004), deforestation and habitat fragmentation (Basyuni et al, 2012), heavy metal pollution (Manurung et al, 2017), and first‐comer effects during the founding phase of the populations can lead to low diversity at the population level.…”

“…Moreover, the ITF, one of the strongest water currents worldwide, separates Wallacea from Sunda. Together, the historical isolation and the barrier effect of strong currents have imprinted species distribution patterns and intraspecific patterns in marine organisms (e.g., Barber et al, 2002; Wainwright et al, 2018), including mangrove species (e.g., Wee et al, 2014; Guo et al, 2016, 2018a, 2018b, 2021). For both Lumnitzera species, here, we found a deep split into two intraspecific groups, which was shown by each of our analyses.…”

“…Specifically, here, we test three hypotheses: First, we test whether a biogeographic separation of Sunda and Wallacea is reflected in a West–East intraspecific genetic structure of both species when a dense and thus representative sampling across Indonesia is included. Guo et al (2021) previously found a West–East genetic break in both species, but located in different areas (land barrier effect of the Malay Peninsula in L. racemosa ; more eastward break in L. littorea ). Second, since strong genetic structure due to present oceanic barriers has previously been reported in several mangrove taxa in the IWP (e.g., Wee et al, 2014, 2017; Yahya et al, 2014), we hypothesize a spatial pattern of genetic structure of both species reflecting the presence of contemporary sea surface currents.…”

“…In addition, strongly differentiated subgroups in both species indicated a complex system of gene flow and genetic barriers. Moreover, while Guo et al (2021) shed light on the large‐scale patterns, the Indonesian Archipelago, including Wallacea, with its many islands and complex seawater currents, was not well covered and thus deserves further study.…”