Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site cybvax0.UUCP
Path: utzoo!linus!philabs!cmcl2!harvard!think!mit-eddie!cybvax0!mrh
From: mrh@cybvax0.UUCP (Mike Huybensz)
Newsgroups: net.bio
Subject: Re: Morphological Asymmetry
Message-ID: <757@cybvax0.UUCP>
Date: Mon, 16-Sep-85 21:41:12 EDT
Article-I.D.: cybvax0.757
Posted: Mon Sep 16 21:41:12 1985
Date-Received: Fri, 20-Sep-85 01:07:23 EDT
References: <295@ihnet.UUCP>
Reply-To: mrh@cybvax0.UUCP (Mike Huybensz)
Distribution: net
Organization: Cybermation, Inc., Cambridge, MA
Lines: 69
Summary: 

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


Brought to you by Super Global Mega Corp .com