Group I introns and homing endonuclease genes (HEGs) are mobile genetic elements, capable of invading target sequences in intron-less genomes. LAGLIDADG HEGs are the largest family of endonucleases, playing a key role in the mobility of group I introns in a process known as ‘homing’. Group I introns and HEGs are rare in metazoans, and can be mainly found inserted in the COXI gene of some sponges and cnidarians, including stony corals (Scleractinia) and mushroom corals (Corallimorpharia). Vertical and horizontal intron transfer mechanisms have been proposed as explanations for intron occurrence in cnidarians. However, the central role of LAGLIDADG motifs in intron mobility mechanisms remains poorly understood. To resolve questions regarding the evolutionary origin and distribution of group I introns and HEGs in Scleractinia and Corallimorpharia, we examined intron/HEGs sequences within a comprehensive phylogenetic framework. Analyses of LAGLIDADG motif conservation showed a high degree of degradation in complex Scleractinia and Corallimorpharia. Moreover, the two motifs lack the respective acidic residues necessary for metal-ion binding and catalysis, potentially impairing horizontal intron mobility. In contrast, both motifs are highly conserved within robust Scleractinia, indicating a fully functional endonuclease capable of promoting horizontal intron transference. A higher rate of non-synonymous substitutions (Ka) detected in the HEGs of complex Scleractinia and Corallimorpharia suggests degradation of the HEG, whereas lower Ka rates in robust Scleractinia are consistent with a scenario of purifying selection. Molecular-clock analyses and ancestral inference of intron type indicated an earlier intron insertion in complex Scleractinia and Corallimorpharia in comparison to robust Scleractinia. These findings suggest that the lack of horizontal intron transfers in the former two groups is related to an age-dependent degradation of the endonuclease activity. Moreover, they also explain the peculiar geographical patterns of introns in stony and mushroom corals.

BackgroundMany cnidarians, including stony corals, engage in complex symbiotic associations, comprising the eukaryotic host, photosynthetic algae, and highly diverse microbial communities—together referred to as holobiont. This taxonomic complexity makes sequencing and assembling coral host genomes extremely challenging. Therefore, previous cnidarian genomic projects were based on symbiont-free tissue samples. However, this approach may not be applicable to the majority of cnidarian species for ecological reasons. We therefore evaluated the performance of an alternative method based on sequence binning for reconstructing the genome of the stony coral Porites rus from a hologenomic sample and compared it to traditional approaches.ResultsOur results demonstrate that binning performs well for hologenomic data, producing sufficient reads for assembling the draft genome of P. rus. An assembly evaluation based on operational criteria showed results that were comparable to symbiont-free approaches in terms of completeness and usefulness, despite a high degree of fragmentation in our assembly. In addition, we found that binning provides sufficient data for exploratory k-mer estimation of genomic features, such as genome size and heterozygosity.ConclusionsBinning constitutes a powerful approach for disentangling taxonomically complex coral hologenomes. Considering the recent decline of coral reefs on the one hand and previous limitations to coral genome sequencing on the other hand, binning may facilitate rapid and reliable genome assembly. This study also provides an important milestone in advancing binning from the metagenomic to the hologenomic and from the prokaryotic to the eukaryotic level.

Ciguatera fih poisoning is a seafood-borne illness caused by the consumption of fih that have accumulated lipid-soluble ciguatoxins, produced by dinoflgellates of the genera Gambierdiscus, Ostreopsis, Coolia and Prorocentrum. The true extent of the disease and its impact on tourism and public health on the Caribbean islands is poorly understood. For this reason, we analyze the incidence of ciguatera in the Caribbean states (CS) and San Andrés Island (SAI), seeking spatial and temporal trends. Through epidemiological reports obtained from the Caribbean Epidemiology Centre (CAREC) and the Departmental Health Secretariat of SAI, the per capita incidence of ciguatera has been calculated for the periods 1980 to 2010 and 2007 to 2011, respectively. Subsequently the data were analyzed using t tests, incidence rate ratio and rank correlation. The data show that, over the period 1980-2010, there were 10 710 cases reported from 18 CAREC countries, with an average annual incidence of 42/100 000. There was an increase between the periods 1980-1990 and 2000-2010 with annual averages calculated from reported cases of 34.2 and 45.2 / 100 000 respectively. The island of Montserrat presented the highest incidence in the region, 350 / 100 000 while SAI showed an incidence of 25/100 000 inhabitants, occupying the eighth position among the analyzed islands. The rate ratio (average annual incidence 2000-2010 / average annual incidence 1980-1990) was 1.36, so there was a 32% increase in the average annual incidence among CAREC countries and almost 300% between the two time periods. The ciguatera incidence level in the Caribbean has increased over the last 31 years, mainly in the Eastern Caribbean. The Bahamas, Antigua and Barbuda contribute greatly to the reported increase. Considering that the development model of much of the region is based on the tourism industry and that fih is a major protein source for Caribbean communities, it can be concluded that ciguatera is a growing problem which is expected to increase in parallel with environmental change.

Scleractinian corals of the genus Porites are found across a wide geographical range along the tropical seas. Some species of the genus such as Porites rus are important reef builders in coral reef ecosystems and display a remarkable stress tolerance. Despite their physiological particularities and ecological importance, there is a scarcity of molecular data for members of this genus. Here, we report the first complete mitochondrial genome of Porites rus (Genbank accession number LN864762) which is 18 647 bp in size. It has the typical coral mitochondrial gene arrangement, consisting of 14 protein-coding genes, with a GC content of 36.2%, 12 tRNAs and two rRNAs. The P. rus mitochondrial genome displays two groups: 1 intron in cox1 and nad5 genes. Phylogenetic analyses support the monophyly of the genus Porites. The complete mitochondrial genome will be helpful for addressing questions regarding mitochondrial gene evolution.