Nature Highlights featuring jellyfish eye evolution
"Jellyfish and their kin have no brains and make do with rudimentary nervous systems. But an analysis now shows that these simple sea creatures evolved eyes multiple times, transforming basic precursor cells into a wide range of useful visual systems.
Using DNA sequences, Natasha Picciani and Todd Oakley at the University of California, Santa Barbara, created an evolutionary tree of Cnidaria, the large grouping — or phylum — that includes jellyfish, sea anemones and corals. They then incorporated information about the species’ light-sensing abilities. The team found that the common ancestor of today’s cnidarians could probably detect light and dark, but lacked specialized eyes.
Among the descendants of that eyeless ancestor, at least eight separate evolutionary events gave rise to eyes, including some with lenses and others with simple structures called eye cups. At least two major types of cnidarian eye use different molecular systems for light detection, suggesting that different lineages co-opted ancestral genes independently to enable vision."
"The eyes have it" by Kelle Freel over at Molecular Ecology
Eyes are pretty darn complicated, which makes them for studying complex trait evolution. Maybe the first time I realized how interesting eyes are when I saw this by the oatmeal about the amazing-ness of the mantis shrimp (are they your new favorite too?), or when I first listened to Colors (or the update) by Radiolab (which also mentions the majestic and clearly magical mantis shrimp).
Eyes exist at different levels of complexity, at their most basic they might have some photoreceptors, pigments, or maybe even lenses or mirrors. As Picciani et al., (2018)from the Oakley lab , point out, many researchers focus (no pun intended) on the evolution of eyes in bilaterian animals, essentially the animals that have a right and left side (like us). As you might imagine, these visual systems are incredibly intricate, and unraveling their evolution is quite the challenge.
The phylum Cnidaria includes plenty of marine creatures you might be familiar with, such as corals, anemones, box jellyfish, siphonophores, and true jellyfish. It might be surprising to realize that even these critters can have simple eyes. The article by Picciani and colleagues reports that eyes actually evolved multiple times among the Cnidaria. Amazingly even in organisms with verysimple nervous systems, light sensing and basic visual systems have evolved more than once.
The authors constructed a massive phylogeny spanning 1,102 species of Cnidaria using 5 genes (6,629 nucleotides), and overlaid morphological data regarding the presence (or absence) of eyes collected from previously published literature. They found that eyes evolved between 8 (on the conservative side) and 13 times in Cnidaria! They outlined evidence supporting multiple origins for eyes, and that sensitivity to light was broadly present across the Cnidaria.
The authors then delved into the various structures and levels of complexity that each eye evolution event resulted in, as their initial model of ancestral state evolution considered only if the eye was there (present) or not (absent). Using detailed descriptions of structure and eye development they found that “morphological details are often different among eyes (they) infer to be of separate origin, as expected in the absence of strong convergent evolution.”
Picciani et al., also concluded that sensitivity to light developed before eyes (makes sense) and finally they analyzed genes (called opsins) involved in the development of vision. From an extensive phylogeny of opsins the authors determined that the results supported their conclusion that separate eye evolutionary events occurred across the Cnidaria.
This nifty paper answered many questions while raising a bunch more, setting the stage for interesting follow up studies. Next time you see a jellyfish trying to evade the “Pokémon” robot arm, maybe you’ll wonder why that critter didn’t see it coming.
"UCSB researchers find that eyes originated in jellyfish at least eight separate times, helping to illuminate how evolution produces complex visual organs"
By Julie Cohen
Eyes detect light and convert it into electrochemical impulses in neurons. The process is the precursor to vision in complex beings. Yet some simple organisms, such as jellyfish (cnidarians), also have many types of eyes that use the same photoreceptive proteins, called opsins, as animals like flies and humans.
Seeking to understand how many times jellyfish eyes originated during their evolution, UC Santa Barbara evolutionary biologist and a team of his students compiled a large genetic data set from which they inferred relationships among cnidarian species. By creating a tree that delineated those relationships, the researchers were able to determine that eyes originated at least eight separate times within this group. The team’s findings appear in the journal Current Biology.
“Eyes are one of the most complicated organs that scientists study,” explained co-author Oakley, a professor in UCSB’s Department of Ecology, Evolution, and Marine Biology. “Other animals with complicated nervous systems evolved eyes more than once. Cnidarians have much less complicated nervous systems, so it was surprising to find that eyes have evolved so many times within this phylum.”
The investigators coupled their genetic data with an extensive literature search to tease out the historical expression of photoreceptive opsin proteins in cnidarians. In previous research, Oakley’s lab found that opsins are used outside of eyes in cnidarians to trigger the firing of stinging cells called cnidocytes. The new study showed that not only was there widespread light-sensing behavior in eyeless jellyfish, but that complex, lensed eyes in this group had a separate history from other eye types.
“With the number of species we included in our cnidarian tree of life, it is now possible to see that eyes originated many times,” said lead author Natasha Picciani, a graduate student in the Oakley Evolution Lab. “The morphological details and opsin proteins are different, which supports the idea that those eyes evolved independently. Our study demonstrates that simple building blocks can be used to build complex things in different ways.”
These findings demonstrate the concept of convergent evolution — the independent origin of similar features in species of different lineages. Convergent evolution — which also occurs in plants — creates analogous structures with similar form or function that were not present in the last common ancestor of the group.
“Now that we know where these eye origins occur in the cnidarian family tree, which lineages possess eyes and whether those eyes belong to the same or different origins, we know where to expect to find more differences in future research,” Picciani noted. “When we conduct more detailed morphological studies, we’ll know which species to target to make appropriate comparisons.”
UCSB co-authors are Jamie Kerlin, Noemie Sierra, Andrew Swafford, Desmond Ramirez, Nickellaus Roberts and Johanna Cannon. Marymegan Daly from The Ohio State University is also a co-author.
This work was supported by a grant from the National Science Foundation to the Oakley Evolution Lab and through funding from a doctoral scholarship to Picciani from Scientists Without Borders.