Ask A Scientist Molecular Biology Archive

name         Graham H.
status       educator
age          40s
       
Question -   Yesterday I was asked by a student, aged 14, in essence 
the question was on the lines of: sex is determined by the presence of 
sex chromosomes, although the determining factor varies (i.e. X,Y system 
different in mammals compared with birds) there is a factor which 
nevertheless determines whether an individual is male or female.  So what 
happens:

(i)  in hermaphroditic animals;

(ii) and how can the temperature alter genetic instructions in certain 
reptiles during development in the egg;
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Graham,

You have asked very involved questions which are answerable, but maybe not
to a 14 year old ability to understand.
Basically (as basic as I can get to reach a 14 year understanding), I am
taking most of the below from a number of recent Scientific American
articles which may be available in your school's library.  There special
issue on Males and Females; and the article on the evolution of the Y
chromosome.  My discussion below would be heavily criticized for leaving
information out if put to task by experts.

1) In hermaphroditic animals?  Look up the earthworm exchange of sperm.  The
chromosomes related to sex are really no different than mammals.  These
animals connect to insure the proper exchange of sperm and they are stored
in specialize receptacles.  I not sure why you have included these organisms
in your question.  In simpler animals such as the Hydra sp. and sponges, it
is all a matter of timing.  Sperms are not released at the same time that
the same organisms eggs are released.  Maybe I can relate it to what you are
seeking below.  Simple animals have simple processes.  No advanced organism
uses hermaphroditic methods; except in genetic abnormalities or syndromes
which are mostly sterile.

2) Reptiles (and all classes other than Aves and Mammals) are heterothermic;
mammals are homothermic-> this is the key to the differences except with
birds.
a) Enzymes operate at specific temperatures including the ones that
determine sex.  This explains clearly why sex differences such as found with
alligators take place in these heterothermic animals.  Females have been
pretty much found to be the default (if you will) of most species.  Females
are far more important to the survival of a species than the males are.
Humans included!  Males develop due to a series of chemical changes that
once begun, result in the chemical sequences that result in the male
structures.  Once this series of reactions start, there is no going back.
In reptiles, the enzymes that will bring about the initial reactions to
begin the male development will only operate if at the right timing, the
temperature is conducive to allow the enzyme to operate that begins the male
development.  No correct temperature, the enzyme(s) responsible for male
determinants fail to start and you go to the default sex; a female.  Mammals
and birds are homothermic, so this will never happen for all the enzymes
have evolved to operate at a specific temperature; in humans about 37C.  Sex
has to come from specialized chromosomes instead.  Even so, we find that
there are human females with XY chromosomes (very rare) that the male
"startup" gene (called the sry gene) apparently did not work.  Default is
female.  This is a whole other area!

b) The problem with the 14 year old understanding is that the human XX, XY
process most discussed in middle schools makes it difficult to talk about
most other animals outside of mammals because many organisms work
differently such as discussed above, or at least appear to.  The
evolutionary process leading to the mammals required more detailed and
precise instructions because of the complexity of the group and, probably,
the reason that mammals carry the most amount of genetic code
introns(useless genes, 164 of these have been identified in human chromosme
20 alone) in their genome, development could get confused if a very ordered
arrangement of processes are not implemented.  
It is clear that reptiles actually developed the Y chromosome, and it is
clear that it arouse from the X chromosome due to a series of mutations.
However, many reptiles and birds have different sex determinants that
actually do the very same thing as we observe in mammals.  Regardless, of
the labels we give the chromosomes that determine sex in any animal, there
is one combination or "lack" of a combination that produces the chemistry
resulting in female reproductive structures and different chromosome
combination to allow for the chemistry resulting in a male.  Some reptiles
have evolved the temperature method of determining the sex and the sex
chromosomes are not as important; well for the sex determination anyway.
There are always exceptions.  Some reptiles are entirely female; no males
needed!  Plays badly with genetic variation, but works OK in some species.
Some fish change sex with age; being females takes more energy and works
best when you are young so when they get old, they become males.  Male
operation is energy cheaper and is more suitable for old age!  Some species
do not use sex unless there is a stress factor in their habitat; why change
your genetic instructions if the individual is well suited to its
environment.  I'm walking into ecology now and it is time to stop.

I apologize for I do not feel I have adequately answered your question.  I
have left out details that are important.  Entire courses in evolution
address your request and it is not easy to explain all the relationships
with out great details.

Steve Sample
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I did not have an answer to your question, so I asked a friend, Chris 
Borland, who did her graduate work on a hermaphrodite nematode (worm) 
called C. elegans.  Here i what she had to say:

"Well, in my favorite hermaphrodite species, C. elegans, there are 
actually two genders ­ hermaphrodites and males.  The hermaphrodites are 
“XX”, produce both eggs and sperm (each has an X), and self-fertilize 
internally (the eggs are already fertilized when they are laid).  The 
males have only one X chromosome ­ we call this “XO”, and are produced by 
nondisjunction.  Males arise at a pretty low rate normally (about 
1/1000).  They can mate with the hermaphrodites, and when this happens, 
the sperm from the male are used preferentially (more cross-progeny than 
self-progeny) and the next generation is 50% male (only half of the male 
sperm have an X)."

Chris Borland by way of Dr. Ticknor
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I see this as a question which needs answering on two fronts:  Biological
and philosophical.

Biologically,  humans all begin as females and early in gestation the y
chromasome gene called SRY is turned on which initiates a cascade of events
that leads to male phenotypic (outward) characteristics.  If this gene is
inactive for the period it needs to be (a matter of days) the fetus will
usually develop the outward signs of a female but the inner anatomy will
show that the gonads have no clear development to true female or male
forms...ovaries or testes.  The uterus is typically blind.  After birth,
many times, this is not detected and the child is raised as a female and
looks and acts as such.  When the time comes for puberty...no menstruation
occurs which usually will lead to a visit to the physician's office and
hopefully the proper guidance.  In many cases the girl develops into a woman
with even fewer pubertal problems (no skin blemishes etc.) and enters
womanhood with the only detriment being she will not be able to bear
children.

Philosophically it, in part, concerns what constitutes a person as an
individual and also what imparts their identity...If all the outward
appearances and behaviors of a person constituted those which were found in
females, does the presence of a "y" sex chromosome in someway overrule their
importance.  If the y chromosome is non-functional, what then?  There was an
interesting case of an Olympic female star who was stripped of her gold
medal(s?) after it was found she had a y chromosome...I believe in her case
she was "androgen insensitive" and the "sry" gene never made a difference.
Also she did not have the benefit of the male hormones that  might have
given her a competitive advantage.  Makes for a very interesting class
discussion.  When it comes down to it, I and most scientists I speak with on
this issue, have a difficult time coming to terms with what constitutes an
individual.  Heck I still can't even define what a living being is, and I
have been trying to find one for 40 years.  I suggest also that you get the
student to read some of the short stories of Isaac Asimov in the book "I
ROBOT".



Peter Faletra Ph.D.
Office of Science
Department of Energy
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