Fused embryos and pre-metamorphic conjoined larvae in a broadcast spawning reef coral

Frontloading of stress response genes enhances robustness to environmental change in chimeric corals

Background

Chimeras are genetically mixed entities resulting from the fusion of two or more conspecifics. This phenomenon is widely distributed in nature and documented in a variety of animal and plant phyla. In corals, chimerism initiates at early ontogenic states (larvae to young spat) and results from the fusion between two or more closely settled conspecifics. When compared to genetically homogenous colonies (non-chimeras), the literature has listed ecological and evolutionary benefits for traits at the chimeric state, further positioning coral chimerism as an evolutionary rescue instrument. However, the molecular mechanisms underlying this suggestion remain unknown.

Results

To address this question, we developed field monitoring and multi-omics approaches to compare the responses of chimeric and non-chimeric colonies acclimated for 1 year at 10-m depth or exposed to a stressful environmental change (translocation from 10- to 2-m depth for 48h). We showed that chimerism in the stony coral Stylophora pistillata is associated with higher survival over a 1-year period. Transcriptomic analyses showed that chimeras lose transcriptomic plasticity and constitutively express at higher level (frontload) genes responsive to stress. This frontloading may prepare the colony to face at any time environmental stresses which explain its higher robustness.

Conclusions

These results show that chimeras are environmentally robust entities with an enhanced ability to cope with environmental stress. Results further document the potential usefulness of chimeras as a novel reef restoration tool to enhance coral adaptability to environmental change, and confirm that coral chimerism can be an evolutionary rescue instrument.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01371-7.

Together stronger: Intracolonial genetic variability occurrence in Pocillopora corals suggests potential benefits

We investigated the occurrence of intracolonial genetic variability (IGV) in Pocillopora corals in the southwestern Indian Ocean. Ninety-six colonies were threefold-sampled from three sites in Reunion Island. Nubbins were genotyped using 13 microsatellite loci, and their multilocus genotypes compared. Over 50% of the colonies presented at least two different genotypes among their three nubbins, and IGV was found abundant in all sites (from 36.7% to 58.1%). To define the threshold distinguishing mosaicism from chimerism, we developed a new method based on different evolution models by computing the number of different alleles for the infinite allele model (IAM) and the Bruvo's distance for the stepwise mutation model (SMM). Colonies were considered as chimeras if their nubbins differed from more than four alleles and if the pairwise Bruvo's distance was higher than 0.12. Thus 80% of the IGV colonies were mosaics and 20% chimeras (representing almost 10% of the total sampling). IGV seems widespread in scleractinians and beyond the disabilities of this phenomenon reported in several studies, it should also bring benefits. Next steps are to identify these benefits and to understand processes leading to IGV, as well as factors influencing them.

Venturing in coral larval chimerism: a compact functional domain with fostered genotypic diversity

The globally distributed coral species Pocillopora damicornis is known to release either sexual or asexual derived planula-larvae in various reef locations. Using microsatellite loci as markers, we documented the release of asexually derived chimeric larvae (CL), originating from mosaicked maternal colonies that were also chimeras, at Thai and Philippines reefs. The CL, each presenting different combinations of maternal genotypic constituents, create genetically-complex sets of asexual propagules. This novel mode of inheritance in corals challenges classical postulations of sexual/asexual reproduction traits, as asexual derived CL represent an alliance between genotypes that significantly sways the recruits' absolute fitness. This type of inherited chimerism, while enhancing intra-entity genetic heterogeneity, is an evolutionary tactic used to increase genetic-heterogeneity, primarily in new areas colonized by a limited number of larvae. Chimerism may also facilitate combat global change impacts by exhibiting adjustable genomic combinations of within-chimera traits that could withstand alterable environmental pressures, helping Pocillopora become a successful cosmopolitan species.