Friday, May 24, 2019

Pogonophora "eye"?

For many years I carried 3 x 5 cards in my shirt pocket so I could jot down things I wanted to remember or explore.  I just came across the following one recently; it was tucked in with some other paper's.  Read as follows.

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1/16/2003  Jan 16, 2003  JGE idea

1   Pogonophora "eye" as possible intermediate in having retina reverse molluscan condition

2   Pogonophora (nearly) "straight line" in evolutionary tree with Pre-Cambrian divergences of other groups at various Post-extinction events

3   TATA box distribution

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On the back of the card I had only a black spot and an x spaced about two inches apart.  the spot and the x are a simple device to demonstrate our blind spot where the optic nerve goes through the retina.  By closing one eye and looking at one mark about six to ten inches in front of your open eye and maneuvering the card until the other mark disappears in the blind spot when the brain fills in the void,

If the x in on the right and I look at it with my left eye, the dot disappears about six inches from my eye when the marks are almost horizontal.

The squid eye does not have a blind spot because the nerve fibers from the retina lie behind the light sensitive retinal cells.  All vertebrate eyes presumably have a blind spot because the nerve fibers from the retina run over the retina until they form the beginning of the optic nerve as they pass through the retina.  There, the blind spot is not apparent to us because the other eye fills out the image in the brain.

The Pogonophora must have retained enough of the eye genes to provide a base for rebuilding the eye.  But the inversion as compared to the annelid, mollusk, arthropod line made the light sensitive retinal cells and the nerve fibers reversed in position.

TATA box things would be worth examining if you are a molecular biologist looking for answers to some related evolutionary steps.

Pineda et al., 2000, Proc. Natl. Acad. Sci. USA, 97:4525-4529, say "previously demonstrated expression of Pax-6 in planarian eyes, suggest that the same basic gene regulatory circuit required for eye development in Drosophila and mouse is used in the prototypic eye spots of Platyhelminthes and, therefore, is truly conserved during evolution."

Joseph Engemann   Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan    May 24, 2019

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