A Research Review of Interventions to Increase the Persistence and Resilience of Coral Reefs (2019) / Chapter Skim
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2 Genetic and Reproductive Interventions
2 Genetic and Reproductive Interventions
Pages 41-76
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. Genetic and reproductive interventions provide an opportunity to increase genetic diversity within populations to allow them to adapt to a changing environment, or permit selection of traits that may improve the resilience of coral populations and species.
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MANAGED SELECTION What It Is Managed selection is the detection of corals with above average stress tolerance, and the use of them in subsequent interventions such as managed breeding, symbiont and microbiome isolation and manipulation, managed relocation, or genetic manipulation. Corals can thrive over a variety of environmental conditions, from the cooler waters of high-atitude reefs such as Bermuda, Hawaii, and Tonga to the warmer l waters of equatorial islands, power plant effluents, and shallow patch reefs (Coles et al., 2018; Keshavmurthy et al., 2012)
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at hundreds or thousands of gene loci. Benefit and Goals Managed selection takes advantage of the high level of genetic diversity found in many coral species, and the potential for natural selection to generate concentrations of adaptive alleles in habitats with high exposure to stressful conditions.
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As the raw material for subsequent use, corals chosen through managed selection are locally available, represent native coral genotypes, and can occur in large enough numbers to represent substantial genetic diversity for other traits. Because of these advantages, managed selection is currently a main target of operation for coral restoration before other more manipulative genetic interventions can be developed.
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. This information in turn can be used to inform actions such as managed breeding and assisted gene flow (Flanagan et al., 2018)
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, but individual phenotyping of corals after common gardening is currently feasible in marine laboratories with running seawater systems, corals transplanted into field conditions, or in coral husbandry businesses (e.g., Muller et al., 2018)
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The local availability of these corals would make their use in other interventions, including relocation and managed breeding, more feasible. Risks Broad scale tradeoffs are expected in the evolution of heat tolerance (Huey and Kingsolver, 1989)
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There are currently few ways of confidently predicting exactly where this asymptote will be for any coral species. In such cases, use of natural variants can be expected to extend the lifetime of current reefs, and provide raw genetic material to generate extreme stress tolerance in the future through breeding or manipulation.
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From page 49... ...
Managed breeding relies on the outcrossing of genetically distinct individuals, regardless of their taxonomic status. This chapter encompasses three approaches under this category that support varying goals in restoration, from increasing coral cover while preserving local genetic diversity to increasing cover by introducing individuals with novel genotypes and higher fitness.
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Supportive breeding within populations seeks to increase population sizes and local genetic diversity, thus improving long-term persistence. Therefore, this intervention supports recovery goals that aim to improve coral cover while maintaining genetic variation within native species.
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Supportive breeding within populations would rely on species that can be readily propagated using sexual reproduction, have a high
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Recent efforts in the Atlantic, Caribbean, and Pacific have demonstrated it is possible reintroduction success, and would significantly contribute to reef building. Fitness outcomes associated with supportive breeding and restoration have been extensively studied in many marine species (Bell et al., 2005; Blaxter, 2000; Hedgecock and Coykendall, 2007; Naish et al., 2008; Waples et al., 2012)
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, can increase coral survival and reproductive success by increasing the grazing on algal competitors (Idjadi et al., 2010) , though overgrazing may damage recruits (Davies et al., 2013)
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. Current Feasibility Supportive breeding within populations relies on the propagation of corals using sexual over asexual methods, and success is dependent on high survivorship after reintroduction and demonstrated recruitment following outplanting (see Box 2.1)
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The long-term success of supportive breeding programs over several generations has yet to be realized. The infrastructure and protocols for outcrosses between populations would rely on those developed for supportive breeding, and therefore are technically feasible.
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, and thus serve as candidates for the investigation of fitness in interspecific crosses between species representing a range of divergences. Few studies have directly investigated hybrid fitness relative to parental species.
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Currently, supportive breeding programs based on larval releases or juvenile outplanting have been conducted as single experimental events. Ideally, populations should become self-sustaining after a limited-time captive breeding and release program; however, in other taxa, population
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. Persistent gene flow between evolutionary divergent populations can result in loss of population structure and locally adaptive traits if hybrids are less fit, which could degrade the total genetic variation across the entire metapopulation (Spichtig and Kawecki, 2004)
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In all cases, risks should be evaluated with the development of genetic management plans that include clear performance indicators and monitoring plans. The goals of supportive breeding within populations are usually aimed at increasing population sizes and recruitment rates while maintaining or restoring the genetic diversity and fitness of a target wild population (although evidence of erosion of genetic diversity in coral populations is currently minimal; van Oppen et al., 2015a)
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. The risks associated with captive rearing are similar to those of supportive breeding.
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Outplanted corals settled on substrates may be readily tracked over time. Supportive breeding within populations relies on sexual reproduction, but asexual reproduction is known to be dominant on many reefs in the Caribbean (Miller et al., 2018)
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GAMETE AND LARVAL CAPTURE AND SEEDING What It Is Gamete and larval capture and seeding is a specific way to enhance the natural sexual reproductive processes of corals by using natural spawning events to supply gametes for future use or larvae for settlement and laboratory growth. Corals reproduce primarily through sexual processes that result in coral planula larvae.
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Providing the material for controlled crosses of gametes to select for resistance attributes; 4. Producing larvae for manipulations, including chimeric coral col onies and hybrids (described in the Managed Breeding section on hybridization between species)
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Following development, fully competent coral larvae can be seeded directly onto reefs using mesh enclosures, they can be transferred to reefs on settlement plates from controlled recruitment efforts, or released onto appropriate substrata en masse, particularly in cavities, cracks, and crevices where the larvae are likely to be retained and recruited (dela Cruz and Harrison, 2017)
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If gamete and larval capture and seeding is used to support other interventions, the risks of those interventions apply. The risks for selecting resistant genotypes or species for supportive breeding and hybridization are discussed in the Managed Breeding section in this chapter.
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. Algal symbionts from three coral species have also been cryopreserved (Hagedorn et al., 2015)
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. However, cryopreserved material can also be used for assisted gene flow and for research purposes, and thus should not be viewed simply as a last-ditch effort to prevent extinction (Hagedorn et al., 2017)
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Genetic manipulation is the direct alteration of the genome of an individual organism, which might be the coral or its algal symbiont. Modern laboratory-based approaches to genetically modify an organism involve genome editing through zinc finger nucleases, transcription
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. An additional goal is to use genetic manipulation to experimentally test hypotheses about the susceptibility of corals to stress and to identify the genetic causes of individual- or species-level variation in stress tolerance.
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. CRISPR/Cas9 in Corals A single paper on CRISPR/Cas9-based genetic manipulation in corals has been published (Cleves et al., 2018)
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. Current Feasibility The basic mechanism of genetic manipulation with CRISPR/Cas9 has been demonstrated in corals (Cleves et al., 2018)
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CRISPR/Cas9 could be used as a technique to test this hypothesis as a starting point in further development. A potential avenue for identifying target genes for genetic manipulation is through the use of gene co-expression networks (an analysis to identify genes with similar expression pattern)
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If a symbiont with broad ability to colonize many coral species were to be conferred with a gene for heat tolerance, it might be able to be added to the holobiome of many different adult coral colonies or multiple species quickly if the original heat-sensitive symbiont could be replaced, because 85% of corals recruit the symbiont algae each generation. Risk The application of gene drives in field conditions is still controversial.
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. Resistance can arise from standing genetic variation at the drive locus or because the drive mechanism is not perfectly efficient and is predicted to prevent drive fixation in wild populations.
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Infrastructure Research infrastructure Marine laboratories with access to test corals, husbandry facilities, long-term growout laboratories that can be used to generate sexually mature colonies, good molecular biology laboratories, microscopy, symbiont culture facilities, and other facilities would need to be available for each species and each ocean region. Bioinformatics infrastructure Genetic manipulation requires a wellknown genome sequence for each species used, the ability to probe genome changes, and the ability to follow phenotypic, genotypic, epigenetic, and genetic changes across multiple generations.
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From page 76... ...
A parallel, coordinated effort can lead to the creation of a model coral to establish fundamental knowledge about its biology and genetics that can be used to widen analyses to a plethora of other important species. The value of having a well-annotated, manually curated, and high-quality genomic and genetic resource for a coral species cannot be overstated in terms of supporting all downstream genetic and ecological manipulations.
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