Dispersal allows species to shift their distributions in response to changing climate conditions. As a result, dispersal is considered a key process contributing to a species' long-term persistence. For many passive dispersers, fluid dynamics of wind and water fuel these movements and different species have developed remarkable adaptations for utilizing this energy to reach and colonize suitable habitats. The seafaring propagules (fruits and seeds) of mangroves represent an excellent example of such passive dispersal. Mangroves are halophytic woody plants that grow in the intertidal zones along tropical and subtropical shorelines and produce hydrochorous propagules with high dispersal potential. This results in exceptionally large coastal ranges across vast expanses of ocean and allows species to shift geographically and track the conditions to which they are adapted. This is particularly relevant given the challenges presented by rapid sea-level rise, higher frequency and intensity of storms, and changes in regional precipitation and temperature regimes. However, despite its importance, the underlying drivers of mangrove dispersal have typically been studied in isolation, and a conceptual synthesis of mangrove oceanic dispersal across spatial scales is lacking. Here, we review current knowledge on mangrove propagule dispersal across the various stages of the dispersal process. Using a general framework, we outline the mechanisms and ecological processes that are known to modulate the spatial patterns of mangrove dispersal. We show that important dispersal factors remain understudied and that adequate empirical data on the determinants of dispersal are missing for most mangrove species. This review particularly aims to provide a baseline for developing future research agendas and field campaigns, filling current knowledge gaps and increasing our understanding of the processes that shape global mangrove distributions.

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A general framework for propagule dispersal in mangroves.

Habitat fragmentation is known to generally reduce the size of plant populations and increase their isolation, leading to genetic erosion and increased between-population genetic differentiation. In Flanders (northern Belgium) Primula vulgaris is very rare and declining. Populations have incurred strong fragmentation for the last decades and are now restricted to a few highly fragmented areas in an intensively used agricultural landscape. Previous studies showed that small populations of this long-lived perennial herb still maintained high levels of genetic variation and low genetic differentiation. This pattern can either indicate recent gene flow or represent historical variation. Therefore, we used polymorphic microsatellite loci to investigate genetic variation and structure in adult (which may still reflect historical variation) and seedling (recent generation, thus affected by current processes) life stages. The recent generation (seedlings) showed a significant loss of observed heterozygosity (H
 o) together with lower expected heterozygosity (H
 e), a trend for higher inbreeding levels (F
 IS) and higher differentiation (F
 ST) between populations compared to the adult generation. This might result from (1) a reduction in effective population size, (2) higher inbreeding levels in the seedlings, (3) a higher survival of heterozygotes over time due to a higher fitness of heterozygotes (heterosis) and/or a lower fitness of homozygotes (inbreeding depression), (4) overlapping generations in the adult life stage, or (5) a lack of establishment of new (inbred) adults from seedlings due to degraded habitat conditions. Combining restoration of both habitat quality and gene flow between populations may be indispensable to ensure a sustainable conservation of fragmented populations.

Genetic diversity in adult and seedling populations of Primula vulgaris in a fragmented agricultural landscape

Primula species are important early spring garden plants with a centre of diversity and speciation in the East Himalaya-Hengduan Mountains in Western China. Studies on population genetics, speciation and phylogeny of Primula have been impeded by a lack of genomic resources. In the present study, we sequenced the transcriptomes of two closely related primrose species, Primula poissonii and Primula wilsonii, using short reads on the Illumina Genome Analyzer platform. We obtained 55,284 and 55,011 contigs with N50 values of 938 and 1,085 for P. poissonii and P. wilsonii, respectively, and 6,654 pairs of putative orthologs were identified between the two species. Estimations of non-synonymous/synonymous substitution rate ratios for these orthologs indicated that 877 of the pairs may be under positive selection (Ka/Ks > 0.5), and functional enrichment analysis revealed that significant proportions of the orthologs were in the categories DNA repair, stress resistance, which may provide some hints as to how the two closely related Primula species adapted differentially to extreme environments, such as habitats characterized by aridity, high altitude and high levels of ionizing radiation. It was possible for the first time to estimate the divergence time between the radiated species pair, P. poissonii and P. wilsonii; this was found to be approximately 0.90 ± 0.57 Mya, which falls between the Donau and Gunz glaciation in the Middle Pleistocene. Primers based on 54 pairs of orthologous SSR-containing sequences between the two Primula species were designed and verified. About half of these pairs successfully amplified for both species. Of the 959 single copy nuclear genes shared by four model plants (known as APVO genes), 111 single copy nuclear genes were verified as being present in both Primula species and exon-anchored and intron-spanned primers were designed for use. We characterized the transcriptomes for the two Primula species, and produced an unprecedented amount of genomic resources for these important garden plants. Evolutionary analysis of these two Primula species not only revealed a more precise divergence time, but also provided some novel insights into how differential adaptations occurred in extreme habitats. Furthermore, we developed two sets of genetic markers, single copy nuclear genes and nuclear microsatellites (EST-SSR). Both these sets of markers will facilitate studies on the genetic improvement, population genetics and phylogenetics of this rapidly adapting taxon.

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Comparative transcriptome analysis and marker development of two closely related Primrose species (Primula poissonii and Primula wilsonii).

In flowering plants, pollen dispersal is often the major contributing component to gene flow, hence a key parameter in conservation genetics and population biology A cost-effective method to assess pollen dispersal consists of monitoring the dispersal of fluorescent dyes used as pollen analogues However, few comparisons between dye dispersal and realized pollen dispersal have been performed to validate the method We investigated pollen dispersal in two small populations of the insect-pollinated herb Primula elatior from urban forest fragments using direct (paternity analyses based on microsatellite DNA markers) and indirect (fluorescent dyes) methods We compared these methods using two approaches, testing for the difference between the distance distributions of observed dispersal events and estimating parameters of a dispersal model, and related these results to dye dispersal patterns in three large populations Dye and realized (based on paternity inference) pollen dispersal showed exponential decay distributions, with 742–948% of the depositions occurring at <50 m and a few longer distance dispersal events (up to 151 m) No significant difference in curve shape was found between dye and realized pollen dispersal distributions The best-fitting parameters characterizing the dye dispersal model were consistent with those obtained for realized pollen dispersal Hence, the fluorescent dye method may be considered as reliable to infer realized pollen dispersal for forest herbs such as P elatior However, our simulations reveal that large sample sizes are needed to detect moderate differences between dye and realized pollen dispersal patterns because the estimation of dispersal parameters suffers low precision

Fluorescent dye particles as pollen analogues for measuring pollen dispersal in an insect-pollinated forest herb

Mangroves are seafaring taxa through their hydrochorous propagules that have the potential to disperse over long distances. Therefore, investigating their patterns of gene flow provides insights on the processes involved in the spatial genetic structuring of populations. The coastline of Cameroon has a particular geomorphological history and coastal hydrology with complex contemporary patterns of ocean currents, which we hypothesize to have effects on the spatial configuration and composition of present-day mangroves within its spans. A total of 982 trees were sampled from 33 transects (11 sites) in 4 estuaries. Using 11 polymorphic SSR markers, we investigated genetic diversity and structure of Rhizophora racemosa, a widespread species in the region. Genetic diversity was low to moderate and genetic differentiation between nearly all population pairs was significant. Bayesian clustering analysis, PCoA, estimates of contemporary migration rates and identification of barriers to gene flow were used and complemented with estimated dispersal trajectories of hourly released virtual propagules, using high-resolution surface current from a mesoscale and tide-resolving ocean simulation. These indicate that the Cameroon Volcanic Line (CVL) is not a present-day barrier to gene flow. Rather, the Inter-Bioko-Cameroon (IBC) corridor, formed due to sea level rise, allows for connectivity between two mangrove areas that were isolated during glacial times by the CVL. Genetic data and numerical ocean simulations indicated that an oceanic convergence zone near the Cameroon Estuary complex (CEC) presents a strong barrier to gene flow, resulting in genetic discontinuities between the mangrove areas on either side. This convergence did not result in higher genetic diversity at the CEC as we had hypothesized. In conclusion, the genetic structure of Rhizophora racemosa is maintained by the contrasting effects of the contemporary oceanic convergence and historical climate change-induced sea level rise.

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Contrasting Effects of Historical Sea Level Rise and Contemporary Ocean Currents on Regional Gene Flow of Rhizophora racemosa in Eastern Atlantic Mangroves

Is the genetic structure of Mediterranean Ruppia shaped by bird-mediated dispersal or sea currents?

Understanding how landscape structure shapes the genetic structure of populations of keystone species is important for their long-term management. We tested the unidirectional dispersal hypothesis on the linear river landscape of the Wouri River and the one catchment-one gene pool hypothesis on red mangrove (Rhizophora racemosa) populations of the Cameroon Estuary complex. Therefore, we conducted release–recapture experiments in the field, and sampled 649 adult trees for plant material for genetic analyses. This allowed for estimating genetic diversity and structure, as well as dispersal directionality. Genetic diversity in populations downstream did not differ significantly from upstream populations and the molecular variance of populations did not correlate with their position on the linear landscape. Contemporary and historical migration estimates indicated bidirectional dispersal, i.e. in both the downstream and the upstream direction along the Wouri River. This was confirmed by the propagule dispersal directions derived from our field experiments. Bayesian clustering analysis assigned all individuals of this estuary into one cluster, suggesting high inter-catchment connectivity. River flow currents, tides, and wind plausibly operate together to ensure the high genetic connectivity within this complex estuary.

Bidirectional gene flow on a mangrove river landscape and between-catchment dispersal of Rhizophora racemosa (Rhizophoraceae)

Papyrus in tropical swamps regenerate vegetatively and through seeds. The study investigated clonal diversity of Cyperus papyrus bordering Lake Naivasha (Kenya). Three regions within the lake were investigated aiming to detect fine-scaled structure in a lakeward transect, including two sites subject to recent dynamic changes, namely a seedling zone and a harvested zone. Using microsatellites, out of 279 ramets, 271 were genets, with most allelic variation observed at individual level but little at site or region level, resulting in very high gene flow levels. At lake level a PCoA grouped lakeward samples distinctly from the landward sites. Somewhat higher inbreeding levels were observed in landward zones but only the harvested site had all loci significantly deviating from the equilibrium implicating the impact of a 6-monthly harvesting regime. A 1,500 m lakeward transect revealed a spatial genetic structure at short distance whereas a very small isolation-by-distance supported a continuous distribution model. The seedling zone contained the highest allelic diversity and individual seedlings were assigned to the nearest shoreline transect, suggesting localized seed rains. Highly diverse C. papyrus swamps of Lake Naivasha are hypothesized to originate mainly from seedling recruitment at short distances, such mature stands can maintain high clonal diversity.

Timothy Sierens

Papers

Is the genetic structure of Mediterranean Ruppia shaped by bird-mediated dispersal or sea currents?

Bidirectional gene flow on a mangrove river landscape and between-catchment dispersal of Rhizophora racemosa (Rhizophoraceae)

Diversity and fine-scale spatial genetic structure of Cyperus papyrus populations in Lake Naivasha (Kenya) using microsatellite markers.

Isolation and characterization of microsatellite loci in primrose (Primula vulgaris)

Lagoons and saltwater wetlands getting more diversity: A molecular approach reveals cryptic lineages of a euryhaline submerged macrophyte (Ruppia)