RESUMO
Berghia stephanieae (Nudibranchia, Cladobranchia) is a photosymbiotic sea slug that feeds exclusively on sea anemones from the genus Exaiptasia. It then specifically incorporates dinoflagellates belonging to the Symbiodiniaceae obtained from their prey. Here, we present the complete mitochondrial genome sequence of B. stephanieae combining Oxford Nanopore long read and Illumina short-read sequencing data. The mitochondrial genome has a total length of 14,786 bp, it contains the 13 protein-encoding genes, 23 tRNAs, and two rRNAs and is similar to other nudibranchs except for the presence of a duplicated tRNA-Ser 1.
RESUMO
Functional kleptoplasty is a photosymbiotic relationship, in which photosynthetically active chloroplasts serve as an intracellular symbiont for a heterotrophic host. Among Metazoa, functional kleptoplasty is only found in marine sea slugs belonging to the Sacoglossa and recently described in Rhabdocoela worms. Although functional kleptoplasty has been intensively studied in Sacoglossa, the fundamentals of the specific recognition of the chloroplasts and their subsequent incorporation are unknown. The key to ensure the initiation of any symbiosis is the ability to specifically recognize the symbiont and to differentiate a symbiont from a pathogen. For instance, in photosymbiotic cnidarians, several studies have shown that the host innate immune system, in particular scavenger receptors (SRs) and thrombospondin-type-1 repeat (TSR) protein superfamily, is playing a major role in the process of recognizing and differentiating symbionts from pathogens. In the present study, SRs and TSRs of three Sacoglossa sea slugs, Elysia cornigera, Elysia timida, and Elysia chlorotica, were identified by translating available transcriptomes into potential proteins and searching for receptor specific protein and/or transmembrane domains. Both receptors classes are highly diverse in the slugs, and many new domain arrangements for each receptor class were found. The analyses of the gene expression of these three species provided a set of species-specific candidate genes, that is, SR-Bs, SR-Es, C-type lectins, and TSRs, that are potentially relevant for the recognition of kleptoplasts. The results set the base for future experimental studies to understand if and how these candidate receptors are indeed involved in chloroplast recognition.
RESUMO
Mutualistic symbioses are common throughout the animal kingdom. Rather unusual is a form of symbiosis, photosymbiosis, where animals are symbiotic with photoautotrophic organisms. Photosymbiosis is found among sponges, cnidarians, flatworms, molluscs, ascidians and even some amphibians. Generally the animal host harbours a phototrophic partner, usually a cyanobacteria or a unicellular alga. An exception to this rule is found in some sea slugs, which only retain the chloroplasts of the algal food source and maintain them photosynthetically active in their own cytosol - a phenomenon called 'functional kleptoplasty'. Research has focused largely on the biodiversity of photosymbiotic species across a range of taxa. However, many questions with regard to the evolution of the ability to establish and maintain a photosymbiosis are still unanswered. To date, attempts to understand genome adaptations which could potentially lead to the evolution of photosymbioses have only been performed in cnidarians. This knowledge gap for other systems is mainly due to a lack of genetic information, both for non-symbiotic and symbiotic species. Considering non-photosymbiotic species is, however, important to understand the factors that make symbiotic species so unique. Herein we provide an overview of the diversity of photosymbioses across the animal kingdom and discuss potential scenarios for the evolution of this association in different lineages. We stress that the evolution of photosymbiosis is probably based on genome adaptations, which (i) lead to recognition of the symbiont to establish the symbiosis, and (ii) are needed to maintain the symbiosis. We hope to stimulate research involving sequencing the genomes of various key taxa to increase the genomic resources needed to understand the most fundamental question: how have animals evolved the ability to establish and maintain a photosymbiosis?
