Sexual Recruitment Research Articles

Assisted sexual coral recruits show high thermal tolerance to the 2023 Caribbean mass bleaching event.

Assisted sexual coral propagation, resulting in greater genet diversity via genetic recombination, has been hypothesized to lead to more adaptable and, hence, resilient restored populations compared to more common clonal techniques. Coral restoration efforts have resulted in substantial populations of 'Assisted sexual Recruits' (i.e., juvenile corals derived from assisted sexual reproduction; AR) of multiple species outplanted to reefs or held in in situ nurseries across many locations in the Caribbean. These AR populations provided context to evaluate their relative resilience compared to co-occurring coral populations during the 2023 marine heat wave of unprecedented duration and intensity that affected the entire Caribbean. Populations of six species of AR, most ranging in age from 1-4 years, were surveyed across five regions during the mass bleaching season in 2023 (Aug-Dec), alongside co-occurring groups of corals to compare prevalence of bleaching and related mortality. Comparison groups included conspecific adult colonies as available, but also the extant co-occurring coral assemblages in which conspecifics were rare or lacking, as well as small, propagated coral fragments. Assisted sexual recruits had significantly lower prevalence of bleaching impacts (overall pooled ~ 10%) than conspecific coral populations typically comprised of larger colonies (~ 60-100% depending on species). In addition, small corals derived from fragmentation (rather than sexual propagation) in two regions showed bleaching susceptibility intermediate between AR and wild adults. Overall, AR exhibited high bleaching resistance under heat stress exposure up to and exceeding Degree Heating Weeks of 20°C-weeks. As coral reefs throughout the globe are subject to increasingly frequent and intense marine heatwaves, restoration activities that include sexual reproduction and seeding can make an important contribution to sustain coral populations.

Livestock grazing modes induced the rapid differentiation of community recruitment in alpine meadow

Herbivores moderate grazing is a fundamental driver of plant diversity maintenance in grassland ecosystems. However, the effects of grazing with different herbivores on plant community composition and recruitment are still little known. Here, we conducted a field moderate grazing experiment to examine the effects of different grazing modes under moderate intensity in the cold season, including sheep grazing (SG), yak grazing (YG), and mixed grazing by sheep and yak (MG), on plant community composition and recruitment. Results showed that all grazing modes significantly decreased plant cover and average height of standing plant litter. Additionally, herbivores grazing significantly increased species richness but decreased community evenness (P < 0.05), with highest richness and evenness found in YG and SG plots, respectively. The trends of increasing aboveground biomass of common and rare species groups were most pronounced in the SG plots relative to the YG and MG plots. MG promoted the increasing of the common species group. From community recruitment, moderate grazing increased the sexual recruitment and decreased the asexual recruitment, and MG significantly inhibited the recruitment of asexual seedlings. Meanwhile, all grazing modes increased the species of poor palatability. These findings imply that the cold-season grazing with moderate intensity facilitated a rapid divergence succession of alpine meadow community by regulating the relative abundance of common and rare species, as well as the asexual seedlings recruitment induced by the removal of standing plant litter. Consequently, the cold-season grazing with different herbivores under moderate intensity can be used as a management tool to modulate grassland community structure and maintain species diversity in alpine grassland ecosystem.

Population and clonal structure of Acropora cf. hyacinthus to inform coral restoration practices on the Great Barrier Reef

A key goal of coral restoration is to re-establish self-sustaining coral populations and ensure resilience to future stressors, which requires that genetic diversity is maximised. However, coral genetic and genotypic (clonal) diversity is variable across reef sites via success of sexual recruitment, and cryptic species diversity can complicate breeding efforts. Assessing genotypic and genetic diversity of colonies to be used in restoration is therefore critical to avoid founder, inbreeding or outbreeding effects. Considering recent efforts to upscale coral propagation on the Great Barrier Reef (GBR), we examined species, population and clonal structure of a commonly out-planted tabular Acropora species—Acropora hyacinthus (Dana, 1864). A total of 189 colonies were sampled from six reef systems throughout the northern-central GBR and genotyped using an Acropora-specific Affymetrix microarray, which resulted in 1387 variant sites that passed quality control. Cryptic species were readily resolved and all sampled A. hyacinthus colonies represented unique genotypes within sites at three reefs. At reefs that contained multi-ramet genets (clonal genotypes), the mean and maximum between-ramet distances were 0.68 and 1.99 m, respectively. Therefore, sampling colonies > 2 m apart increases the likelihood these colonies represent distinct genets. Such a sampling design therefore maximises genotypic diversity when sourcing colonies for propagation and out-planting. Based on these variant sites, we found no between-reef genetic divergence based on locality. Furthermore, through unintentional sampling of non-target tabular Acroporid species, we show how this genotyping method may be used for resolving taxonomic uncertainty as well as population dynamics.

Should I stay or should I go? Coral bleaching from the symbionts' perspective.

Coral bleaching, the stress-induced breakdown of coral-algal symbiosis, threatens reefs globally. Paradoxically, despite adverse fitness effects, corals bleach annually, even outside of abnormal temperatures. This generally occurs shortly after the once-per-year mass coral spawning. Here, we propose a hypothesis linking annual coral bleaching and the transmission of symbionts to the next generation of coral hosts. We developed a dynamic model with two symbiont growth strategies, and found that high sexual recruitment and low adult coral survivorship and growth favour bleaching susceptibility, while the reverse promotes bleaching resilience. Otherwise, unexplained trends in the Indo-Pacific align with our hypothesis, where reefs and coral taxa exhibiting higher recruitment are more bleaching susceptible. The results from our model caution against interpreting potential shifts towards more bleaching-resistant symbionts as evidence of climate adaptation-we predict such a shift could also occur in declining systems experiencing low recruitment rates, a common scenario on today's reefs.

Genetic insights into pepper-bark tree (Warburgia salutaris) reproduction in South Africa

Genetic diversity in over-exploited and threatened species has important, but often neglected, implications for conservation. In South Africa, the bark of the regionally endangered pepper-bark tree, Warburgia salutaris, is extensively harvested for subsistence medicine. We developed 11 polymorphic microsatellite markers to examine the genetic diversity and structure among 227 W. salutaris individuals sampled from across much of its range in South Africa’s Kruger National Park (KNP) and 31 trees growing in local nurseries. We also tested the cross-amplification success of these markers in two closely related species, Warburgia elongata (IUCN listed as Endangered) and W. stuhlmannii (IUCN listed as Vulnerable). Contrary to our expectations, we found evidence of clonality and for high genetic diversity in all wild KNP sub-populations of W. salutaris. We identified between 114 and 149 unique multi-locus genotypes in the 258 individuals included in our analyses. Bayesian assignment tests identified three distinct genetic clusters that are represented by the sampled KNP sub-population of W. salutaris, with minimal geographic structure and some admixture. Three genetic clusters were also identified among all sampled individuals, i.e., KNP and nursery trees. Nursery W. salutaris germplasm comprised unique genotypes with similar levels of observed heterozygosity to the wild population. High levels of clonality and reported self-incompatibility, along with short distance pollination, likely explain the lack of recent sexual recruitment (i.e., reduced fruit set and lack of seedlings) in the wild population. These findings will be useful to identify sub-populations where genets may be sampled to maximize genetic diversity within subpopulations for restoration throughout KNP.

Clones on the run: The genomics of a recently expanded partially clonal species.

Why species that in their core areas mainly reproduce sexually become enriched with clones in marginal populations ("geographic parthenogenesis") remains unclear. Earlier hypotheses have emphasized that selection might promote clonality because it protects locally adapted genotypes. On the other hand, it also hampers recombination and adaptation to changing conditions. The aim of the present study was to investigate the early stages of range expansion in a partially clonal species and what drives an increase in cloning during such expansion. We used genome-wide sequencing to investigate the origin and evolution of large clones formed in a macroalgal species (Fucus vesiculosus) during a recent expansion into the postglacial Baltic Sea. We found low but persistent clonality in core populations, while at range margins, large dominant clonal lineages had evolved repeatedly from different sexual populations. A range expansion model showed that even when asexual recruitment is less favourable than sexual recruitment in core populations, repeated bottlenecks at the expansion front can establish a genetically eroded clonal wave that spreads ahead of a sexual wave into the new area. Genetic variation decreases by drift following repeated bottlenecks at the expansion front. This results in the emerging clones having low expected heterozygosity, which corroborated our empirical observations. We conclude that Baker's Law (clones being favoured by uniparental reproductive assurance in new areas) can play an important role during range expansion in partially clonal species, resulting in a complex spatiotemporal mosaic of clonal and sexual lineages that might persist during thousands of generations.

Enhancing survival of ex-situ reared sexual recruits of Acropora palmata for reef rehabilitation

Acropora palmata is one of the major reef-building coral species in the Caribbean. The species has suffered drastic declines in abundance and sexual recruitment over the past decades. One method for active rehabilitation of A. palmata reefs is by assisting the production of sexual recruits under controlled lab conditions. Within this study, the effect of different aquaculture regimes and culturing periods on the survival rates of these recruits was investigated. In August 2016, coral spawn was collected on a reef nearshore New Providence, Bahamas, cross-fertilized, and reared in mobile laboratory facilities from Van Oord Dredging and Marine Contractors. Larvae were settled on pre-conditioned aragonite plugs.Sexual recruits were cultured under four different aquaculture conditions: ambient vs. high Total Alkalinity (TA) (∼2.8 mEq L−1 vs. 4.8 mEq L−1) and with vs. without feeding Artemia nauplii. Recruit size was monitored by tracking living tissue area and the number of polyps of a subset of recruits. Plates with recruits were outplanted to a nursery on the reef after 4, 9 and 14 weeks of aquaculture. Survival was determined during the aquaculture phase (at 4, 9 and 14 weeks after settlement), and after outplanting (at 27 and 44 weeks after settlement).During the aquaculture phase, survival was significantly lower in seawater with increased TA compared to ambient seawater conditions. The average number of polyps per recruit was significantly higher in the treatments with feeding. After outplanting to the reef, both survival and recruit size were highest in the feeding treatments. The most successful aquaculture treatment in this study was a combination of increased TA and feeding during 9 weeks of aquaculture, which resulted in a doubling of survival and recruit size at 10 months after settlement compared to ambient conditions. Ambient conditions did not enhance survivorship nor recruit size at 10 months after settlement, as compared to the other aquaculture treatments. Nevertheless, the success of ambient aquaculture conditions exceeded natural conditions, as no natural recruitment of A. palmata was observed in this study. We conclude that feeding during and ex-situ culture period enhances ex-situ growth rates and in situ recruit survival of A. palmata juveniles. No positive effects of the aquaculture treatment with only increased TA were found. Building on these results, recommendations are provided for future reef rehabilitation efforts using ex-situ rearing of sexually reproduced A. palmata recruits.

Clonal diversity and recruitment strategy of the two dominant seagrass species Cymodocea rotundata and Enhalus acoroides in the southern Philippines

The studied fifteen populations of Cymodocea rotundata and Enhalus acoroides in Mindanao, southern Philippines revealed a remarkable strategy for reproduction and dispersal. E. acoroides populations exhibits high rates of sexual recruitment with high clonal richness ranged from 0.9 to 1.0, while C. rotundata exhibited monoclonal to high clonal richness ranged from 0 to 1.0. Overall, clonal richness was high indicating sexual reproduction is prevalent, but varied for C. rotundata. The largest genet found in the study was at Rizal (RIZ), northern Mindanao wherein throughout the sampling area only one genet was identified, maybe influenced by environmental condition and hydrodynamic of the water currents and tidal fluctuation in this range-edge population. Significant pairwise population genetic differentiation was found among many sites. Isolation by distance (IBD) was detected in C. rotundata (P < 0.05) but not in E. acoroides (P = 0.253). IBD manifested among populations maybe influenced by the ocean currents in Mindanao, but it has to be confirmed yet in the future study. The results have implications for understanding on how the environmental conditions can influenced the dispersal strategy of these species in natural settings. These species manifested high plasticity most likely to adopt to environmental conditions favoring natural selection for long term demographic stability and fitness. The genetic diversity, structure, reproduction and dispersal strategies observed in this study may prove useful for eventual genetic conservation plans of these two foundational seagrass species.

Perfect poopers; passerine birds facilitate sexual reproduction in clonal keystone plants of the boreal forest through directed endozoochory towards dead wood

Berry-producing Vaccinium shrubs are keystone species in boreal forest ecosystems and their berries provide an essential food source for many vertebrates. In Fennoscandia, both the abundance and cover of Vaccinium have decreased in forests, with intensive forestry practice as a main driver. Vaccinium seedling recruitment is constricted to recruitment windows of opportunity and appears to be infrequent, mainly due to seed dispersal limitation. Passerine birds are key players in seed dispersal, which can be directed towards perching points such as coarse woody debris.In this study, we investigated the potential for directed endozoochory by passerine birds towards cut stumps and if such stumps were viable recruitment windows for Vaccinium species, in an intensively managed boreal forest landscape of central Norway. We surveyed stump and paired forest floor microsites for passerine scat (i.e. potential seed rain) and Vaccinium seedlings. We collected passerine scat and conducted germination experiments to assess if they contained viable seed. We tested the microsite effect (i.e. forest floor/stump) on scat deposition and seedling establishment and then used modified ensemble classifiers to identify important environmental factors affecting scat deposition and seedling establishment patterns.We found that passerine scat was disproportionally deposited on stumps, and that the vast majority of scat contained viable Vaccinium seed. Stumps were also suitable recruitment windows for Vaccinium species, as a higher probability of seedling establishment occurred at stumps compared with the forest floor. However, scat deposition and seedling establishment at stumps were rather contextual and determined by environmental variables. The probability of scat deposition increased with higher complexity of the vertical forest structure and lower canopy cover, whereas seedling establishment required bryophyte cover and larger stumps, or alternatively smaller stumps with competition-free spaces.Our results highlight a pathway to successful sexual reproduction for Vaccinium species in managed forests: passerine birds direct endozoochorous seed dispersal towards tree stumps, which can offer suitable conditions for seedling establishment. However, the spatiotemporal variability both in forest stand structure and in stump conditions have a strong influence on the success of sexual recruitment via this pathway.

Effects of highland environments on clonal diversity in aquatic plants: An interspecific comparison study on the Qinghai-Tibetan Plateau.

Clonal reproduction is one of the most distinctive characteristics of plants and is common and diverse in aquatic macrophytes. The balance between sexual and asexual reproduction is affected by various conditions, especially adverse environments. However, we know little about clonal diversity of aquatic plants under suboptimal conditions, such as at high altitudes, and having this information would help us understand how environmental gradients influence patterns of clonal and genetic variation in freshwater species. The microsatellite data of four aquatic taxa in our previous studies were revisited to estimate clonal and genetic diversity on the Qinghai-Tibetan Plateau. Clonal diversity among different genetic groups was compared. Local environmental features were surveyed. Beta regressions were used to identify the environmental factors that significantly explained clonal diversity for relative taxon. The level of clonal diversity from high to low was Stuckenia filiformis > Hippuris vulgaris > Myriophyllum species > Ranunculus section Batrachium species. A positive correlation between clonal and genetic diversity was identified for all taxa, except H. vulgaris. Clonal diversity was affected by climate in S. filiformis and by the local environment in H. vulgaris. For Myriophyllum spp., low elevation and high sediment nutrition were significant for sexual recruitment. The environmental effects on clonal diversity were not significant in R. sect. Batrachium spp. Clonal diversity of aquatic plants is moderate to high and varies greatly in highlands. The effects of breeding systems and environmental factors on the patterns of clonal variation were identified. Elevational gradients, climates and local conditions play different roles in clonal diversity among relative taxon. Our results highlight the importance of sexual recruitment in alpine aquatic plant populations and the influence of environmental factors on the genetic patterns in freshwater species at local and regional scales.

Controls of Aurelia coerulea and Nemopilema nomurai (Cnidaria: Scyphozoa) blooms in the coastal sea of China: Strategies and measures

Outbreaks of scyphozoan Aurelia coerulea and Nemopilema nomurai in the coastal sea of China are managed in recent years because they have severely jeopardized local socioeconomic development and ecological health. In this study, we propose specific strategies to control these blooms based on the different physio-ecological characteristics of their polyps, which can produce medusae by strobilation. High densities of A. coerulea polyps can survive chronically on the surfaces of some artificial constructions submerged in harbors or bays, China. Through buddings, they can resist the invasion of biofouling organisms and proliferate on the surfaces of some fouling organisms (e.g., ascidians, and bryozoans). However, N. nomurai polyps have not been recorded in natural environment. The in situ experiments found that polyps on settling plates fail to survive via podocysts due to severe biofouling invasion and post-strobilated degeneration in late spring and summer. As a result, the population size following is strongly dependent on the sexual recruitment of medusae during late summer and autumn. Therefore, we suggest that the reasonable governance strategy is to manage polyp populations together with biofouling organisms for A. coerulea blooms, however, with a focus on the medusa stage (particularly young medusae) to decrease the sexual reproduction in N. nomurai blooms. Accordingly, massive occurrences of A. coerulea in Qingdao Middle Port, China were alleviated by eliminating polyps and biofouling organisms on the undersurfaces of floating docks and then brushing the surfaces with modified alloprene paints. Some applicable control measures, including resource utilization of N. nomurai medusae and more severe and earlier summer fishing moratoriums, were used to possibly help restrain outbreaks of N. nomurai in Chinese coastal waters.

Pollinator movement activity influences genetic diversity and differentiation of spatially isolated populations of clonal forest herbs

In agricultural landscapes, forest herbs live in small, spatially isolated forest patches. For their long-term survival, their populations depend on animals as genetic linkers that provide pollen- or seed-mediated gene flow among different forest patches. However, whether insect pollinators serve as genetic linkers among spatially isolated forest herb populations in agricultural landscapes remains to be shown. Here, we used population genetic methods to analyze: (A) the genetic diversity and genetic differentiation of populations of two common, slow-colonizing temperate forest herb species [Polygonatum multiflorum (L.) All. and Anemone nemorosa L.] in spatially isolated populations within three agricultural landscapes in Germany and Sweden and (B) the movement activity of their most relevant associated pollinator species, i.e., the bumblebee Bombus pascuorum (Scopoli, 1,763) and the hoverfly Melanostoma scalare (Fabricus, 1,794), respectively, which differ in their mobility. We tested whether the indicated pollinator movement activity affected the genetic diversity and genetic differentiation of the forest herb populations. Bumblebee movement indicators that solely indicated movement activity between the forest patches affected both genetic diversity and genetic differentiation of the associated forest herb P. multiflorum in a way that can be explained by pollen-mediated gene flow among the forest herb populations. In contrast, movement indicators reflecting the total movement activity at a forest patch (including within-forest patch movement activity) showed unexpected effects for both plant-pollinator pairs that might be explained by accelerated genetic drift due to enhanced sexual reproduction. Our integrated approach revealed that bumblebees serve as genetic linkers of associated forest herb populations, even if they are more than 2 km apart from each other. No such evidence was found for the forest associated hoverfly species which showed significant genetic differentiation among forest patches itself. Our approach also indicated that a higher within-forest patch movement activity of both pollinator species might enhance sexual recruitment and thus diminishes the temporal buffer that clonal growth provides against habitat fragmentation effects.

Development of highly polymorphic microsatellite markers for Campanula glomerata L. (Campanulaceae).

Campanula glomerata L. (Campanulaceae) is a self-incompatible, insect-pollinated herb occurring in calcareous grasslands, and is declining and (critically) endangered in many parts of its European distribution range. It often exists as small and isolated populations. A recovery plan of C. glomerata has been implemented in southern Belgium, involving plant translocations. We developed microsatellite markers using an enriched genomic library and characterized 16 loci in 111 individuals from eight populations. These 16 loci were highly polymorphic, with 11 to 31 alleles per locus for a total of 329 alleles, and expected heterozygosity (He) ranging from 0.470 to 0.938. These highly polymorphic loci constitute a promising tool for detailed genetic analyses: assigning individuals to distinct multilocus genotypes will allow quantifying pollen dispersal, clonal propagation and sexual recruitment and identifying admixed seed progeny and their pollen donors. Evaluating the genetic status of existing populations and a genetic monitoring of the translocated populations will contribute to optimize success in restoring viable and evolutionary resilient populations.

Abiotic and Biotic Interactions Control Ruppia maritima Life History Development Within a Heterogeneous Coastal Landscape

Habitat-forming submerged aquatic vegetation (SAV) in estuaries increasingly depend on sexual recruitment from propagules for population sustainability under varying conditions from climate and human-induced changes. Seed recruitment requires successful transitions through multiple life history stages, but few studies have examined abiotic and interspecies interactions on multiple life stages in SAV. A series of field experiments in Florida Bay quantified impacts of abiotic (salinity, temperature, light, sediment nutrients) and biotic (interspecific SAV interactions) factors on seedlings, adult individuals (genets), and shoots (ramets) of the SAV Ruppia maritima. Five study sites were established at the Everglades-Florida Bay ecotone, where seed recruitment sustains R. maritima. Seedling survival was low (≤ 16%), and further reduced under low light, P-limitation and competition with the macroalga Chara hornemannii. Significantly high genet survival and vegetative and sexually reproductive shoot production (4400 m−2 and 300 m−2, respectively) were found at a site where nutrients (P) were sufficient, when C. hornemannii was absent and light levels were three-times higher during the dry season (~ 14000 lumens m−2; December–March) than any other time of year. At P-limited sites, vegetative shoot production occurred with adequate light (> 20,000 lumens m−2) at stable salinities (CV 0.5) where 30–45-day averages remained < 15. However, variable or elevated salinity delayed vegetative shoot production ~ 60 days, suggesting salinity stress limits asexual reproduction. Understanding how light and P-limitation, salinity stress, and competition controls R. maritima recruitment in the Everglades is essential to its management for restoration and climate change.

Spatiotemporal variation in coral recruitment and its association with seawater temperature

AbstractThe supply of propagules mediates recruitment and population dynamics, thereby driving community resilience following disturbances. These relationships are of interest on tropical reefs, where coral populations have drastically declined in abundance and sexual recruitment is the only means by which they will recover. To better understand the causes and implications of variation in this vital rate (i.e., recruitment), coral recruitment was measured in Mo'orea, French Polynesia, and St. John, U.S. Virgin Islands, using settlement tiles deployed from 2005 to 2019. The results were used to test two hypotheses: (1) annual variation in recruitment is a weak predictor of long‐term variation in recruitment, but (2) it is associated with seawater temperature. Coral recruitment varied over space and time, so that differences in recruitment between consecutive years were uninformative of long‐term trends. Recruitment varied among years in an apparently chaotic manner, but the variation reflected linear and quadratic associations with mean annual temperature and the daily variation in temperature. These associations are consistent with theory addressing the mechanisms by which temperature affects coral larvae and recruitment. Comprehension of these mechanisms is required to accurately interpret evidence of coral recruitment collapse, and to elucidate the conditions favoring recovery of coral communities through recruitment.

Sonar and in situ surveys of eelgrass distribution, reproductive effort, and sexual recruitment contribution in a eutrophic bay with intensive human activities: Implication for seagrass conservation

Seagrass beds are recognized as pivotal and among the most vulnerable coastal marine ecosystems globally. The eelgrass Zostera marina L. is the most widely distributed seagrass species and dominates the temperate northern hemisphere. However, an alarming decline in seagrass has been occurring worldwide due to multiple stressors. Seagrass meadow degradation is particularly serious in the Bohai Sea, in temperate China; however, large areas (> 500 ha) of seagrass meadows and population recruitment have rarely been reported in this area. In the present study, we report on a large eelgrass bed in a eutrophic bay of the Bohai Sea. Sonar and field survey methods were used to investigate the distribution of seagrass and its population recruitment. We also analyzed the major threats to this large seagrass bed. Results showed that a large Z. marina bed with an area of 694.36 ha occurred in this area of the Bohai Sea, with a peripheral area of ~25 km2. Seagrass canopy height and plant coverage had a significant correlation with water depth. Asexual reproduction principally occurred in autumn and played a dominant role in population recruitment in vegetated areas, where no seedlings successfully colonized. In contrast, a considerable number of seedlings survived in the seagrass meadow gaps, and thus played a critical role in the recruitment in these areas. The maximum reproductive shoot densities were about 100 and 70 shoots m−2 at sampling site (S)-1 and S-2 in 2018, respectively, which was about two times more than in 2019 (50 and 20 reproductive shoots m−2 at S-1 and S-2, respectively). The potential seed output per unit area in 2019 was about 1020 seeds m−2 at S-1 and 830 seeds m−2 at S-2, and the seed output in the study area was at a low level compared with global values. Overall, high spring and summer water temperature appeared to induce sexual reproduction of Z. marina in the study area, including reproductive effort, reproductive investment, and seedling development. Furthermore, eelgrass height, aboveground biomass, and density were significantly related to water temperature. Among the potential threatening factors to seagrass in this area, the activities of clam harvesting were intense with daily clam catches >2000 kg, leading to patchy seagrass meadows, especially in the fringe areas. The seagrass bed was also threatened by marine pollution (nutrient loading) and land reclamation. Therefore, the protection and restoration of this seagrass bed are strongly recommended. Our study will provide fundamental information for the conservation and management strategies of large eelgrass beds in the Bohai Sea.

CONSERVATION GENOMICS OF WYETHIA RETICULATA (ASTERACEAE)

Wyethia reticulata Greene is a rare perennial herb found only on gabbro-derived soils of the Pine Hill formation in the Sierra Nevada foothills of El Dorado County, CA. Wyethia reticulata is capable of both vegetative (clonal) and sexual reproduction, although the former is thought to be more common than the latter, with sexual reproduction tied to fire. The potential dominance of vegetative reproduction has conservation implications for W. reticulata, as populations could appear healthy in terms of stand density, while losing genetic diversity due to a lack of sexual recruitment. Past genetic assessments of W. reticulata relied on low-resolution markers from a small number of populations and did not directly address conservation issues. Here, double-digest random amplified DNA (ddRAD) sequencing was used to develop a high-resolution genomic dataset for a sample of 168 W. reticulata samples from seven locations that represent every known population center of the species. The data was used to 1) calculate the frequency of asexual reproduction, 2) determine which populations contain unique genetic variation, and 3) test whether there are any ecological correlates of genetic patterns. In contrast to past genetic research, the results presented here indicate that clonality is not common; less than 2% of plants were identified as being derived from asexual reproduction. Nevertheless, some locations seem to be more affected by asexuality than others; the index of association varied from 4.4 to 19.4 (M̄ = 10.9). Genetic differentiation among locations was low (Fst = 0.03-0.04). However, clustering identified three distinct population segments that were not previously recognized. Elevation was the only environmental variable significantly associated with genetic differentiation. Based on these results, it is recommended that conservation effort be focused on genetically isolated population segments.

Seed Reserve Hot Spots for the Sub-Tropical Seagrass Halodule wrightii (Shoal Grass) in the Northern Gulf of Mexico

Halodule wrightii (shoal grass) is a dioecious seagrass with a widespread tropical and subtropical distribution. Like all seagrass species, H. wrightii has the ability to expand asexually through rhizome elongation and to reproduce sexually through seed. To better understand H. wrightii sexual recruitment dynamics in the northern Gulf of Mexico, we investigated seed bank densities at 815 sites from south Texas to the Florida Panhandle. H. wrightii seed reserves were spatially variable across the region, with seed densities ranging from 0 to 5290 seeds m−2. Spatial analysis revealed clusters of high seed densities (“hot spots”) in Upper Laguna Madre, TX, and Santa Rosa Sound, FL, and clusters of low seed densities (“cold spots”) in Lower Laguna Madre and Aransas Bay, TX. Hot spots were dominated by H. wrightii, whereas cold spots were dominated by Thalassia testudinum (turtle grass). We frequently found intact seed coat halves, suggesting germination; however, we also encountered broken seed coat pieces, characteristic of seed predation. Genotypic surveys within and adjacent to seed hot spots revealed genetically diverse adult populations 6 years post seed bank sampling. Our data show that H. wrightii seed reserves are heterogeneous across the northern Gulf of Mexico and that the factors driving variation in seed bank density, viability, and germination remain poorly understood. Information on the spatial heterogeneity of H. wrightii seed densities has relevance for seagrass management, including targeting meadows with high levels of reproductive effort for protection or designation as marine reserves.

Spatial genetic structuring in a widespread wetland plant on a plateau: Effects of elevation‐driven geographic isolation and environmental heterogeneity

Abstract Highlands are ideal research areas for improving our understanding of the influence of ecological factors on the diversity and spatial patterns of natural species. Elevation‐driven physical and environmental isolation greatly affect the evolution of plants. The mechanisms and essential drivers underlying these processes may differ among research scales, habitats and landscapes. Wetlands are important elements of the Qinghai–Tibetan Plateau, which is the highest plateau in the world, and these habitats harbour high aquatic organismal diversity. However, how the environments shape the genetic variation and structure of hydrophilous plants is poorly understood. Using microsatellite markers and a chloroplast fragment, we quantified the genetic diversity and spatial genetic pattern of Stuckenia filiformis, one of the most widespread aquatic plants on the plateau. The relative contributions of geography, climate and local conditions to intra‐ and interpopulation variation were estimated. The results showed that intrapopulation genetic variation of the plant is moderate to high and not constrained by high‐altitude environments. Topographical isolation mainly contributes to the genetic structure of S. filiformis, as inferred by simple sequence repeats and chloroplast DNA data. Significant effects of environmental variables on the spatial genetic patterns of this freshwater species were also suggested by landscape genetic analysis. Infrequent long‐distance dispersal, sexual recruitment and annual growth are probably important for the maintenance and distribution of this variation. Our findings imply a combined effect of geography and elevation‐driven environmental heterogeneity on the evolution of aquatic organisms in highlands.

Heat wave intensity can vary the cumulative effects of multiple environmental stressors on Posidonia oceanica seedlings

Climate change is introducing new stressors into already stressed ecosystems. Among these, extreme events such as heat waves play a crucial role in determining the structure of ecosystems. We tested single and combined effects of overgrazing, burial and heat waves on the seedlings of the habitat-forming species Posidonia oceanica. At current heat wave temperatures, overgrazing in isolation had more deleterious effects than seed burial, and effects were synergistic and additive when both factors co-occurred. The combined effect of overgrazing and seed burial with current heat waves could hamper P. oceanica seedling development, with similar or even higher levels than the sole effect of heat waves in the near future (29 °C). The effects of overgrazing and seed burial are expected to be overridden if heat waves temperatures exceed 29 °C. These results suggest that co-occurring environmental stressors, in combination with current heat waves, could compromise the sexual recruitment of this seagrass.