Grace Hancock
Pine Crest School
Class of 2022

Sea slugs of the Elysia genus have exhibited the ability to photosynthesize, using the ability to survive months- long starvation periods. The sea slugs are able to sequester chloroplasts from algal prey, determined by electron micrograph imaging and sequencing of the rbcL gene.  The slugs retain only chloroplasts, separate from the nucleus or any algal cell components.  Despite lack of access to genes within the algal nucleus, required for the synthesis of photosynthetic proteins, the slugs are able to maintain the chloroplasts for months. This suggests the presence of algal nuclear genes somewhere within the slug.  Origins of the ability are disputed, with an initial hypothesis of horizontal gene transfer (HGT) between the slugs and the algae, in which photosynthetic genes would be introduced into the slug genome.  PCR of adult slug DNA and RNA confirmed the presence of algal nuclear genes.  However, if the presence of algal DNA was due to HGT, the sequences would also be located in sea slug egg DNA, before exposure to algal prey.  BLAST analysis of egg gDNA and algal cDNA identified no matches for shared photosynthetic genes, providing evidence against the HGT hypothesis. An  alternative theory has been proposed; photosynthetic genes may exist in extrachromosomal DNA carried within chloroplasts.  This extrachromosomal DNA would be introduced through consumption of algae, thus with sequences present in adult slugs but absent in egg genomes. Further research is required to confirm the presence of extrachromosomal DNA and investigate its function within the sea slug.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3708498/

https://pubmed.ncbi.nlm.nih.gov/22319135/

https://onlinelibrary.wiley.com/doi/full/10.1111/j.1744-7410.2006.00065.x