A Genome of Photosynthetic Animals Decoded

Photosynthetic Sea Slug, Plakobranchus ocellatus Type Black

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The photosynthetic sea slug, Plakobranchus ocellatus type black, (left) and an electron micrograph showing sequestered algal chloroplasts within a sea slug cell (right). Credit: Taro Maeda

Genome analysis exposes chloroplast acquisition without gene transfer in photosynthetic sea slugs.

Plants, algae, and some germs have the ability to carry out photosynthesis, which is the procedure of changing sunshine energy into sugar. Animals are usually not able to utilize this procedure to get energy, however there are a couple of recognized exceptions to this. Some sea slugs use up chloroplasts from the algae that they take in into their cells. These chloroplasts maintain their capability to carry out photosynthetic activity within the animal cells for numerous months, and hence supply them with photosynthesis-derived nutrition. This procedure is called “kleptoplasty,” and it has actually brought in much attention due to its incredible individuality in making animals photosynthetic for over 50 years.

A pushing concern is how these sequestered chloroplasts maintain their photosynthetic ability without algal nuclei. Since the genome of the algal nucleus encodes the majority of the proteins needed for photosynthesis, chloroplasts separated from algal cells immediately lose their photosynthetic ability. Nevertheless, algae-eating sea slugs maintain this photosynthetic ability for months. There have actually been many disputes about the systems underlying the phenomenon of sequestered chloroplasts keeping photosynthetic abilities over the long term. A commonly accepted hypothesis accounting for kleptoplasty is the horizontal gene transfer of the photosynthesis genes from algae to sea slug.

A group of scientists at the National Institute for Basic Biology (NIBB), in addition to partners from 7 other Japanese organizations, have actually released the genome of the sea slug, Plakobranchus ocellatus type black, in eLife. “Since the sea slug is a non-model organism, its genome analysis was very tough in comparison to model organisms such as the mouse and the fruitfly. Furthermore, there was no high-quality genetic information for them. This situation consequently hindered the verification of the hypothesis of algae-derived horizontal gene transfer,” stated Shuji Shigenobu, a genome researcher and teacher at NIBB who is the matching author of the paper, “But we succeeded in accurately revealing the genome information of the sea slug”. Scientists are prepared to settle the arguments worrying the horizontal transfer of algal genes to the animal nucleus based upon freshly revealed genome information. “We looked at the genome very carefully, but we found no evidence of photosynthetic genes encoded on the sea slug genome, ” he stated.

“We are embarking upon a new challenge to answer the question: how does the sea slug retains this function without horizontal gene transfer?” stated Taro Maeda, the very first and co-corresponding author of the paper. “Our genome data also provides clues to this. We have found several candidate genes related to the long-term maintenance of photosynthetic activity. These genes related to protein metabolism, oxidative stress tolerance, and innate immunity should be subsequently highlighted in future studies.”

The systems underlying kleptoplasty are still evasive. Further understanding of this phenomenon might lead us to ingenious biotechnologies, which, for instance, might bestow photosynthetic capabilities to other numerous animal cells in the future.

Reference: “Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus” by Taro Maeda, Shunichi Takahashi, Takao Yoshida, Shigeru Shimamura, Yoshihiro Takaki, Yukiko Nagai, Atsushi Toyoda, Yutaka Suzuki, Asuka Arimoto, Hisaki Ishii, Nori Satoh, Tomoaki Nishiyama, Mitsuyasu Hasebe, Tadashi Maruyama, Jun Minagawa, Junichi Obokata and Shuji Shigenobu, 27 April 2021, eLife.
DOI: 10.7554/eLife.60176

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