eklhad@ihnet.UUCP (K. A. Dahlke) (09/13/85)
< The line eater > What is the earliest animal (on the evolutionary scale) that exhibits morphological asymmetry against a backdrop of bilateral symmetry? A friend nominated a particular crab possessing one large claw, for fighting. Certainly all mammals, birds, etc have the beating heart on one side, and the liver on the other. These popular asymmetries are consistent (i.e. the liver belongs on the right). There is a preferred (correct) orientation. Are simple asymmetries, such as the crab's claw above, consistent? What about that funny fish with his eyes on one side? Is it always the same side? Why did these asymmetries evolve? Couldn't mammals have a beating heart behind the sternum, and a small liver on each side? Couldn't the crab have two fighting claws? What biological mechanism begins the asymmetry in the developing embryo? Is the mechanism different for consistent/inconsistent asymmetries? What goes wrong in the few humans who are reversed (i.e. liver on the left, etc)? How common is this? Has it been documented in other animals? I don't see much traffic on this newsgroup, which is unfortunate. There are so many interesting things to talk about. -- This .signature file intentionally left blank. Karl Dahlke ihnp4!ihnet!eklhad
todd@scirtp.UUCP (Todd Jones) (09/17/85)
> > What is the earliest animal (on the evolutionary scale) > that exhibits morphological asymmetry against a backdrop of > bilateral symmetry? > A friend nominated a particular crab > possessing one large claw, for fighting. I may be totally off (if so, flame), but I recall that Paramecia are basically asymmetrical except for their gullet. Surely a creature such as this existed, in some form, before the crustacean mentioned. -todd jones
mrh@cybvax0.UUCP (Mike Huybensz) (09/17/85)
In article <295@ihnet.UUCP> eklhad@ihnet.UUCP (K. A. Dahlke) writes: > What is the earliest animal (on the evolutionary scale) > that exhibits morphological asymmetry against a backdrop of > bilateral symmetry? Bilateral symmetry and subsequent asymmetry have probably evolved several times independently. There a quite a number of unicellular organisms which are essentially bilaterally symmetric but exhibit morphological asymmetry. Diatoms of many types have paired valves (shell-like exoskeletons of silica) one of which fits inside the other, like a pillbox. (Though of course they aren't animals.)_ Various protozoans have left and right handed individuals, which usually breed true. > A friend nominated a particular crab > possessing one large claw, for fighting. > Certainly all mammals, birds, etc have the beating heart > on one side, and the liver on the other. There are huge numbers of other asymmetries. One of my favories is a mite (mites are a specialty of mine) whose males are quite literally hill-side circlers (mythical animals with legs longer on one side, who live on hill sides because they can only stand upright there.) These mites live between barbs of feathers (which are asymmetric spaces.) > These popular asymmetries are consistent (i.e. the liver > belongs on the right). > There is a preferred (correct) orientation. > Are simple asymmetries, such as the crab's claw above, consistent? > What about that funny fish with his eyes on one side? > Is it always the same side? The asymmetries are usually fairly consistent within a species. Some figures I've seen are around 1 in 10,000 being reversed. In a few cases (such as the mites) the handedness seems to be random. (The fish you are thinking of are flounders.) Snails also tend to be consistent, though there are occaisional species which have a reversed spiral, and occaisional individuals. > Why did these asymmetries evolve? Got me. > Couldn't mammals have a beating heart behind the sternum, > and a small liver on each side? > Couldn't the crab have two fighting claws? The other claw is the feeding claw, usually. > What biological mechanism begins the asymmetry in the developing embryo? > Is the mechanism different for consistent/inconsistent asymmetries? > What goes wrong in the few humans who are reversed > (i.e. liver on the left, etc)? How common is this? > Has it been documented in other animals? I think human reversals are about one in a thousand. Don't know the rest. > I don't see much traffic on this newsgroup, which is unfortunate. > There are so many interesting things to talk about. Quite so. -- Mike Huybensz ...decvax!genrad!mit-eddie!cybvax0!mrh
shipman@nmtvax.UUCP (09/20/85)
> From: eklhad@ihnet.UUCP (K. A. Dahlke) > What about that funny fish with his eyes on one side? > Is it always the same side? I remember hearing that there are two taxa of flounders, left- eyed and right-eyed. > What biological mechanism begins the asymmetry in the > developing embryo? A more general question: how do cells in the embryo know "where they are" on the body plan? How does a cell know it's going to be part of a big toe and not an eyelid? I've been trying to find someone who will venture an answer to this one for years. -- John Shipman/Zoological Data Processing/ucbvax!unmvax!nmtvax!shipman
thau@h-sc1.UUCP (robert thau) (09/22/85)
> I remember hearing that there are two taxa of flounders, left- > eyed and right-eyed. There are both left-eyed and right-eyed flounder, but single species can (and do) have left-eyed and right-eyed individuals; they are not two taxa. (Oddly enough, in at least one species extending across the Pacific, the percentage of left-eyed flounder is about 50% near Japan and only about 10% near California). > A more general question: how do cells in the embryo know "where > they are" on the body plan? How does a cell know it's going to > be part of a big toe and not an eyelid? I've been trying to > find someone who will venture an answer to this one for years. > -- > John Shipman/Zoological Data Processing/ucbvax!unmvax!nmtvax!shipman I've read some recent embryology; *nobody* knows yet.
tmb@talcott.UUCP (Thomas M. Breuel) (09/22/85)
In article <772@nmtvax.UUCP>, shipman@nmtvax.UUCP writes: > A more general question: how do cells in the embryo know "where > they are" on the body plan? How does a cell know it's going to > be part of a big toe and not an eyelid? I've been trying to > find someone who will venture an answer to this one for years. Yes. That is what developmental biology is all about. Developmental biologists have been trying to *find* an answer to this one for more than a century. Seriously, though: there is probably no magic going on. Positional information may given by chemical gradients, by electric fields and by cell surface interactions. But these are only the physical means by which a mechanism of pattern formation and morphogenesis can be implemented. There have also been many suggestions as to the nature of the mechanism itself. They all await experimental verification. Thomas.
werner@aecom.UUCP (Craig Werner) (09/28/85)
> > A more general question: how do cells in the embryo know "where > > they are" on the body plan? How does a cell know it's going to > > be part of a big toe and not an eyelid? > > -- > > John Shipman/Zoological Data Processing/ucbvax!unmvax!nmtvax!shipman > I've read some recent embryology; *nobody* knows yet. > -Robert Thau There are two so-called theories of morphogenesis. One is called the 'European Model' -- your ancestors decide your fate, the other is the 'American Model' -- your neighbors determine what you become. In fact, in one organism, the nematode C. elegans, the complete lineage of every cell has been determined, and it appears to follow no discernable rhyme or reason at the cellular level. Maybe, at the molecular level ..... -- Craig Werner !philabs!aecom!werner "The proper delivery of medical care is to do as much Nothing as possible"