The complete mitochondrial genome of the 'solar-powered' sea slug Plakobranchus cf. ocellatus (Heterobranchia: Panpulmonata: Sacoglossa)

Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus

Some sea slugs sequester chloroplasts from algal food in their intestinal cells and photosynthesize for months. This phenomenon, kleptoplasty, poses a question of how the chloroplast retains its activity without the algal nucleus. There have been debates on the horizontal transfer of algal genes to the animal nucleus. To settle the arguments, this study reported the genome of a kleptoplastic sea slug, Plakobranchus ocellatus, and found no evidence of photosynthetic genes encoded on the nucleus. Nevertheless, it was confirmed that light illumination prolongs the life of mollusk under starvation. These data presented a paradigm that a complex adaptive trait, as typified by photosynthesis, can be transferred between eukaryotic kingdoms by a unique organelle transmission without nuclear gene transfer. Our phylogenomic analysis showed that genes for proteolysis and immunity undergo gene expansion and are up-regulated in chloroplast-enriched tissue, suggesting that these molluskan genes are involved in the phenotype acquisition without horizontal gene transfer.

Sequence comparison of the mitochondrial genomes in two species of the genus Nerita (Gastropoda: Neritimorpha: Neritidae): phylogenetic implications and divergence time estimation for Neritimorpha

Many Nerita species live in warm-water environments, and they are some of the few organisms from the intertidal zone that can live in both freshwater and seawater. Previous comparative studies of the mitogenomes of Nerita species suggest that the genome rearrangements are very conservative. Generally, the species possess a set of similar mitochondrial gene arrangements, but nucleotide sequences can be used to elucidate phylogenetic relationships at various levels of divergence. Here, the mitogenomes of Nerita undata and Nerita balteata were sequenced and found to be 15,583 bp and 15,571 bp, respectively. The mitogenomes of both species contain 13 protein-coding genes, 22 tRNA genes, and two rRNA genes. The nucleotides of the two mitogenomes are highly similar, with the same gene composition and genomic organization as those present in other Nerita species. The tRNA secondary structures were different from those of other gastropods: trnS2 is not folded into typical secondary structures, and the dihydrouridine (DHU) arm simply forms a loop. The phylogenetic analysis showed that Neritimorpha is a sister group of Vetigastropoda and Caenogastropoda. Nerita balteata is a sister group of Nerita versicolor and Nerita undata, and all three species belong to Neritimorpha. This study contributes towards the comparative mitogenomic analysis of Neritidae and phylogenetic considerations among Neritimorpha species. The estimation of divergence time revealed that the two Nerita species were differentiated in the late Paleogene of the Cenozoic Era, and their evolution may be related to environmental changes.

Mitochondrial Genome Assemblies of Elysia timida and Elysia cornigera and the Response of Mitochondrion-Associated Metabolism during Starvation

Some sacoglossan sea slugs sequester functional plastids (kleptoplasts) from their food, which continue to fix CO2 in a light dependent manner inside the animals. In plants and algae, plastid and mitochondrial metabolism are linked in ways that reach beyond the provision of energy-rich carbon compounds through photosynthesis, but how slug mitochondria respond to starvation or alterations in plastid biochemistry has not been explored. We assembled the mitochondrial genomes of the plastid-sequestering sea slugs Elysia timida and Elysia cornigera from RNA-Seq data that was complemented with standard sequencing of mitochondrial DNA through primer walking. Our data confirm the sister species relationship of the two Sacoglossa and from the analysis of changes in mitochondrial-associated metabolism during starvation we speculate that kleptoplasts might aid in the rerouting or recycling of reducing power independent of, yet maybe improved by, photosynthesis.