Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!decwrl!pa.dec.com!hollie.rdg.dec.com!psw.enet.dec.com!winalski
From: winalski@psw.enet.dec.com (Paul S. Winalski)
Newsgroups: sci.bio
Subject: Re: Reconstructing cells from DNA
Message-ID: <1991Apr15.185049.3959@hollie.rdg.dec.com>
Date: 15 Apr 91 18:50:49 GMT
References: <18637@csli.Stanford.EDU> <1991Apr13.203239.22379@hollie.rdg.dec.com> <1991Apr14.211248.2072@alchemy.chem.utoronto.ca>
Sender: news@hollie.rdg.dec.com (Mr News)
Reply-To: winalski@psw.enet.dec.com (Paul S. Winalski)
Organization: Digital Equipment Corporation
Lines: 52

In article <1991Apr14.211248.2072@alchemy.chem.utoronto.ca>,
mroussel@alchemy.chem.utoronto.ca (Marc Roussel) writes:
>
|>In article <1991Apr13.203239.22379@hollie.rdg.dec.com>
|>winalski@psw.enet.dec.com (Paul S. Winalski) writes:
|>>I don't think that there are any cases of non-nucleic-acid-based hereditary
|>>mechanisms in any known organisms.  Such mechanisms might be possible, but
|>>living organisms don't make use of them.
|>
|>     I don't think that right, unless you want to extend the meaning of
|>"hereditary mechanisms" to the point of meaninglessness.  Most
|>biological morphogenesis in multi-celled organisms is now known to be
|>the result of complex spatio-temporal interactions (chemical waves and the like)
|>between embryo gene products and the chemical environment provided by
|>the egg and/or uterus.  The number of digits that a person (or other 
|>mammal) develops is now known to be determined by chemical waves at
|>certain stages of development rather than by direct coding.

Granted.  However, the effects of these morphogenetic mechanisms are not
inheritable.  To use your example, suppose that we fiddle with the uterine
chemistry in such a way as to alter the number of digits on the developing
embryo's hands.  The baby will duly be born with excess or missing digits,
but that morphological characteristic will not be inherited by any of the
child's offspring.  See the history of thalidomide for proof that this is
the case.  Now, if we instead affect the spatio-temporal-chemical interactions
involving the development of the digits by introducing a point mutuation in
the DNA encoding a certain protein critical to the process, we can get a change
in the number of digits that *is* inherited by future generations (e.g.,
polydactyly in domestic cats).

By "hereditary mechanisms" I meant means of transmitting persistent (i.e.,
inheritable by future generations) changes to morphogenesis.  I know of no
instances of such mechanisms that are not nucleic-acid-based.  This includes
some examples where the mechanism of the developmental characteristic involves
the chemical environment of the egg.  Consider the left-hand-spiral mutuation in
the pear whelk (genus Busycon).  The shell of this gastropod normally has a
clockwise spiral when viewed from the top.  There is a mutation that results
in a counterclockwise spiral.  It behaves as a simple Mendelian recessive
gene, but with one difference.  The factor that determines the direction of
the shell spiral comes from the egg cytoplasm and therefore is dictated by the
genotype of the female parent, not by that of the zygote.  Thus, if you mate
two snails heterozygous for the shell spiral gene (Ss x Ss) to produce a female
snail homozygous for the recessive left-hand spiral (ss), that snail's shell
will have a right-hand spiral, as determined by the mother's genotype.
However, all offspring of this snail will have counterclockwise-spiralling
cells, regardless of their genotype (Ss or ss).  Here we have a case where
cytoplasmic factors result in a phenotype that seemingly contradicts the
genotype, but it is only the DNA-encoded information about this characteristic
that is passed on to other generations.  I am not aware of any exceptions
to this principle.

--PSW