Elysia chloractica – green sea slug

Today marks the birth Charles Darwin, whose theory of evolution by means of natural selection is one of the most elegant, powerful, and far-reaching scientific explanations ever produced. In honor of this auspicious occasion, it seems fitting to highlight some fascinating work conducted by researchers at the University of South Florida. A recent paper, whimsically titled “FISH Labeling Reveals a Horizontally Transferred Algal (Vaucheria litorea) Nuclear Gene on a Sea Slug (Elysia chlorotica) Chromosome”, Julie Schwartz and colleagues present direct evidence for horizontal gene transfer between the algae V. litorea and the green sea slug E. chloroctica.

For decades, researchers have been aware of the green sea slug’s tendency to hijack algae chloroplasts and incorporate them into their own cells (a phenomenon known as kleptoplasy). This alone is remarkable, simultaneously giving the slugs direct access to photosynthetic energy and shifting their color to a vibrant green, possibly allowing them to better camouflage themselves in their aquatic surroundings. The ability to commandeer chloroplasts from V. litorea, thereby attaining direct access to solar energy, is an absolutely extraordinary adaptation – the sort of thing that fills me with a delightfully invigorating sense of wonder at the diversity and complexity of life.

This, however, is not the end of the story for these neat little gastropods. Here, things get a little weird. Apparently chloroplasts aren’t the only thing green sea slugs steal from the algae they eat. It appears they also abscond with segments of V. litorea DNA. The relevant genes are then integrated into the slug’s chromosomes, likely as a mechanism to maintain the chloroplasts for months after they have been assimilated into the slug’s cellular structure. Researchers had suspected something like this might be the case for a while, but the FISH (fluorescence in situ hybridization) imaging technique deployed by the USF team allowed them to directly identify loci on a few slug chromosomes where V. litorea genes were present. Stranger still, the researchers were able to spot V. litorea genes on the chromosomes of a second generation of sea slugs that had experienced no direct contact with the algae in question. In other words, the adult slugs are able to pass genetic changes acquired during their life cycle on to a succeeding generation.

Fluorescence in situ hybridization – the green spots are the targeted genes

As is often the case with stimulating scientific research, this study raises at least as many questions as it answers. For instance, the mechanism by which the algae genes are incorporated into the slug chromosomes remains mysterious. It also seems to violate the longstanding, practically dogmatic prohibition against the inheritance of acquired characteristics. That fact alone gives me pause, and unraveling its consequences will likely take some doing. For now, let’s just take a moment to stand in awe at the breathtaking ingenuity of life and the remarkable process of evolution behind it.

Charles Darwin photographed by Herbert Rose Barraud in 1881