Host–Endosymbiont Genome Integration in a Deep-Sea Chemosymbiotic Clam

Endosymbiosis with chemosynthetic bacteria has enabled many deep-sea invertebrates to thrive at hydrothermal vents and cold seeps, but most previous studies on this mutualism have focused on the bacteria only. Vesicomyid clams dominate global deep-sea chemosynthesis-based ecosystems. They differ from most deep-sea symbiotic animals in passing their symbionts from parent to offspring, enabling intricate coevolution between the host and the symbiont. Here, we sequenced the genomes of the clam Archivesica marissinica (Bivalvia: Vesicomyidae) and its bacterial symbiont to understand the genomic/metabolic integration behind this symbiosis. At 1.52 Gb, the clam genome encodes 28 genes horizontally transferred from bacteria, a large number of pseudogenes and transposable elements whose massive expansion corresponded to the timing of the rise and subsequent divergence of symbiont-bearing vesicomyids. The genome exhibits gene family expansion in cellular processes that likely facilitate chemoautotrophy, including gas delivery to support energy and carbon production, metabolite exchange with the symbiont, and regulation of the bacteriocyte population. Contraction in cellulase genes is likely adaptive to the shift from phytoplankton-derived to bacteria-based food. It also shows contraction in bacterial recognition gene families, indicative of suppressed immune response to the endosymbiont. The gammaproteobacterium endosymbiont has a reduced genome of 1.03 Mb but retains complete pathways for sulfur oxidation, carbon fixation, and biosynthesis of 20 common amino acids, indicating the host’s high dependence on the symbiont for nutrition. Overall, the host–symbiont genomes show not only tight metabolic complementarity but also distinct signatures of coevolution allowing the vesicomyids to thrive in chemosynthesis-based ecosystems.

METABOLIC INTEGRATION: BEYOND THE BUILDING BLOCKS

The theme "metabolic Integration" discussed during Biochemistry classes is considered by many students a complex issue. It could be due to their difficulty in understanding that the metabolic pathways are not isolated reactions, but a completely interdependent system finely regulated. Given this reality, a didactic game was developed. The main objective was to challenge students to understand the metabolism integration, through a playful, interactive and dynamic way. The class was divided into groups and to each group was given a set of parts that represented an important pathway of energetic metabolism. The aim of each group was to complete the metabolic process assigned to them. However, during the assembly, they realized that was always lacking some part of the puzzle and that the game only succeeds if all the groups exchange parts with each other. After that, the pieces came together in order to assemble all the processes in an integrated way. The game was organized into two situations: metabolic reactions that occur in the fasted state and reactions of the fed state. When the groups realized they needed to join themselves to complete the processes, they also had to get into a consensus that the "body" in which the reactions were happening, was in a fasted state or not, because the pieces didn’t match each other if both metabolic states were being assembled at the same time. It is not suitable to the organism performs the reactions of antagonistic states at the same time/or at the same velocity. Along the schema assembly, key points were didactically marked in some pieces with colors and warnings. The proposal was to open a discussion after assembling of the parts.The game was applied to students at the first year of medicine school and had a great acceptance.Key words: metabolism, integration, game.