4:10 PM - 4:30 PM
[BBG01-08] Plakobranchus ocellatus, a shell-less gastropods, use food algal chloroplasts without horizontal gene transfer
★Invited Papers
Keywords:Kleptoplasty, Mollusc, Horizontal gene transfer, Photosynthesis
Some sea slug species of order Sacoglossa (Mollusc, Gastropod) sequester food algal chloroplasts into their cells for photosynthesis. This phenomenon is called kleptoplasty ("stolen chloroplast"). In general, the algal nucleus encodes most photosynthetic genes, and many of these genes encode fragile enzymes. Experimentally isolated chloroplast from algae loses their photosynthetic activity within a few hours or days. However, the sea slug retains the photosynthetic activity of sequestered chloroplasts (Kleptoplast) for several days to 10 months. The sea slug cells never sequester other algal organelles (e.g., algal nucleus). Although many researchers have focused on these non-self organelles' maintenance, the mechanism is still unknown. Our team has revealed the genomes of two sacoglossan species and has shown that the photosynthetic activity is maintained without horizontal gene transfer (HGT). This result indicates that complex adaptive traits, typified by photosynthesis, can be transferred across species without HGT. In this presentation, I will overview the research on kleptoplasty including our results.
Since the discovery of this phenomenon in 1965, many studies have verified the uptake of inorganics (carbon and nitrogen) and the retantion of the photosynthetic machinery activity. It has been revealed that the photosynthetic activity is maintained for 6-10 months in the longest case, although the longevity of photosynthetic activity varies depending on the sea slug and algal species. In many of the species having photosynthetic activities, the photosynthetic products are used as nutrition.
HGT had been the most promising hypothesis of the molecular mechanism of kleptoplasty. This hypothesis considered that algal-nuclear-derived photosynthetic genes are transferred to the sea slug nuclei in evolutionary time scale, and the sea slug produces photosynthetic proteins from these algae-derived genes to replace damaged algal protein. This hypothesis was based on the detection of algal genes from an Atlantic sea slug species, Elysia chlorotica, using PCR, Southern- and northern-blotting methods. In the 2010s, the genome sequencing of E. chlorotica was attempted to find evidence of HGT. However, the obtained genome was fragmented and provided no reliable evidence of HGT. The difficulty of DNA extraction and field collection has made E. chlorotica research difficult.
To settle the argument of HGT, we here sequenced the genome of Plakobranchus ocellatus, living in the tropical Indo-Pacific ocean. As a result, we found no algae-derived genes by the general homology search. However, these results were insufficient to prove the negative fact that "no algal-derived HGT is occurring".
We then acquired additional data and improved in silico analysis. First, we used de novo RNA-Seq analysis to obtain the food algal nuclear-encoded genes. Second, to ensure the sensitivity of our in silico analysis, we adopted the same analysis on the algal genome (Caulerpa lentillifera) in which photosynthetic genes are clearly present. As a result, no algae-derived HGT was found in any of the analyses from P. ocellatus data. However, the same analysis found 73-93% of the photosynthesis-related genes from C. lentillifera data, suggesting enough sensitivity of our method. Our physiochemical analysis, moreover, showed that P. ocellatus type Black has a light-dependent ability to generate greater amounts of oxygen than their respiration, and that its fasting tolerance is enhanced in light environments. We, hence, concluded that P. ocellatus maintains photosynthetic activity for several months without algae-derived HGT. Comparative genomic analysis and cross-tissue RNA-Seq of P. ocellatus and E. marginata found multiple candidates for kleptoplasty-relating genes on the molluscan side.
Since the discovery of this phenomenon in 1965, many studies have verified the uptake of inorganics (carbon and nitrogen) and the retantion of the photosynthetic machinery activity. It has been revealed that the photosynthetic activity is maintained for 6-10 months in the longest case, although the longevity of photosynthetic activity varies depending on the sea slug and algal species. In many of the species having photosynthetic activities, the photosynthetic products are used as nutrition.
HGT had been the most promising hypothesis of the molecular mechanism of kleptoplasty. This hypothesis considered that algal-nuclear-derived photosynthetic genes are transferred to the sea slug nuclei in evolutionary time scale, and the sea slug produces photosynthetic proteins from these algae-derived genes to replace damaged algal protein. This hypothesis was based on the detection of algal genes from an Atlantic sea slug species, Elysia chlorotica, using PCR, Southern- and northern-blotting methods. In the 2010s, the genome sequencing of E. chlorotica was attempted to find evidence of HGT. However, the obtained genome was fragmented and provided no reliable evidence of HGT. The difficulty of DNA extraction and field collection has made E. chlorotica research difficult.
To settle the argument of HGT, we here sequenced the genome of Plakobranchus ocellatus, living in the tropical Indo-Pacific ocean. As a result, we found no algae-derived genes by the general homology search. However, these results were insufficient to prove the negative fact that "no algal-derived HGT is occurring".
We then acquired additional data and improved in silico analysis. First, we used de novo RNA-Seq analysis to obtain the food algal nuclear-encoded genes. Second, to ensure the sensitivity of our in silico analysis, we adopted the same analysis on the algal genome (Caulerpa lentillifera) in which photosynthetic genes are clearly present. As a result, no algae-derived HGT was found in any of the analyses from P. ocellatus data. However, the same analysis found 73-93% of the photosynthesis-related genes from C. lentillifera data, suggesting enough sensitivity of our method. Our physiochemical analysis, moreover, showed that P. ocellatus type Black has a light-dependent ability to generate greater amounts of oxygen than their respiration, and that its fasting tolerance is enhanced in light environments. We, hence, concluded that P. ocellatus maintains photosynthetic activity for several months without algae-derived HGT. Comparative genomic analysis and cross-tissue RNA-Seq of P. ocellatus and E. marginata found multiple candidates for kleptoplasty-relating genes on the molluscan side.