At least eight separate evolutionary events gave rise to jellyfish eyes !!!

During early evolution, as the animal-fungus ancestor diverged from the plant ancestor and then animals split off from fungi –– an obvious survival advantage would be to be able to “sense” one’s environment, i.e. any species able to see food, identify and avoid predators, and to recognize potential mates for reproduction would be better equipped to survive. Hence, the relevance of this topic to ‘gene-environment interactions.’ Lower organisms without eyes often can “sense” light vs dark (and thereby decide to move toward or away from this environmental signal).

Traits, such as eyes, represent a challenge to biologists who wish to understand the steps involved leading to evolution of such a complex organ as the eye, which has been established throughout evolution to have evolved at least 64 distinct times. Animal eyes are made of smaller building blocks –– including photoreceptor cells and pigment cells and sometimes having lenses or mirrors for improved spatial resolution. Components of eyes are seen across enormous evolutionary distances, leading to hypotheses that eye parts might have accumulated gradually, long before everything evolved into the complex eye. Eye evolution is primarily learned by studies of bilaterian animals [animals having a head (anterior) and a tail (posterior), as well as a back (dorsal) and a belly (ventral); therefore, they also have a left side and a right side; examples include arthropods, molluscs, and vertebrates]. Bilaterians invariably evolved with sophisticated neural machinery to process visual information. On the other hand, eyes are also seen in Cnidaria (animals having no anterior, posterior, dorsal or ventral sides’ e.g. jellyfishes, corals, and sea anemones), which have nervous systems of dispersed and condensed neurons for processing information locally; however, these animals have no typical bilaterian central nervous system.

How often eyes of varying complexity, including image-forming eyes, have evolved in animals with such simple neural circuitry remains open to speculation. Authors [see attached article] studied large-scale phylogenies of Cnidaria and their photosensitive proteins and coupled them with an extensive literature search on eyes and light-sensing behavior. Their results indicate that cnidarian eyes must have originated at least eight times –– with complex lensed-eyes having a history separate from other eye types. Compiled data show widespread light-sensing behavior in eyeless cnidarians. Comparative analyses support ancestors without eyes that already were able to “sense” light with dispersed photoreceptor cells.

The history of expression of photoreceptive opsin proteins supports the inference of distinct eye origins via separate assimilation of different non-visual opsin paralogs (two or more genes that were derived from the same ancestral gene and now reside at different locations within the same genome) into eyes. Overall, these data {see attached] show that eyes evolved repeatedly from ancestral photoreceptor cells in non-bilaterian animals that carried simple nervous systems, integrating existing precursors, similar to what occurred in Bilateria. This study thus underscores the potential for multiple, evolutionarily distinct visual systems –– even in animals having simple nervous systems.

Curr Biol 6 Aug 2o18; 28, 2413–2419
COMMENT: The ability to insert a vertebrate (e.g. mouse or human) transcriptional factor (TF) into the genome of an invertebrate (e.g. fly, worm or yeast) represents an amazing evolutionary discovery –– that dates back to (I’m guessing) the early 1980s. On PubMed I tried to find the original human TF shown to function in yeast, but there were more than 10,000 “hits” and I cannot find (or recall) the original TF or even what high-profile journal it was published in. DwN

COMMENT: Hi Dan, Yes, but developmental biologists were amazed, two decades ago, at the common genetic circuitry of Fly and Mammalian eyes, despite their very different structures, which argued for a common phylogenetic origin. Ectopic expression of mammalian Pax6 in flies is able to cause ectopic eye formation in the fly. And it’s a “fly-type of eye,” not a mammalian-type eye. Wow.

It would take a while to find the original report of eyeless/Pax6 (drosophila/mammal) dual-ability to induce ectopic eyers in Drosophila. Attached [at right] is one of the earliest papers [Development 1997; 124: 4819-4826] –– but not the most high-profile publication, as I recall.
COMMENT: Thank you, David. THOSE are the originals (by Walter Gehring, way back in Science, 1994-95).


From: David Nelson []
Sent: Wednesday, September 19, 2018 10:00 AM


1: Gehring WJ. The master control gene for morphogenesis and evolution of the eye. Genes Cells 1996 Jan; 1: 11-15. Review. PubMed PMID: 9078363.

2: Halder G, Callaerts P, Gehring WJ. Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. Science 1995 Mar 24; 267: 1788-1792. PubMed PMID: 7892602.

3: Quiring R, Walldorf U, Kloter U, Gehring WJ. Homology of the eyeless gene of Drosophila to the Small eye gene in mice and Aniridia in humans. Science. 1994 Aug 5; 265: 785-789. PubMed PMID: 7914031.

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