Re: What animals or species have 4 chromosomes?

Hi Jim,

  There are several ways to interpret your question, so I'm going to
briefly digress and discuss genome structure.  At the most basic level, the
genome of an organism is made up of DNA.  Eukaryotic genomes are assembled
into one or more chromosomes, or contiguous pieces of DNA.  In humans there
are 24 chromosomes: 22 autosomes and two sex chromosomes (X and Y). 
However, most of the cells in a human body actually carry two copies of the
genome (2n = 46), and therefore two copies of each autosome and either two
X's (46 XX) or an X and a Y (46XY).  The term for organisms like mammals
and birds that spend most of their life cycle with two copies of the genome
per individual is diploid.  Organisms which spend most of the life cycle
with one copy are known as haplois, and organisms which have four copies of
the genome per cell are called tetraploid.  

  So, there are three possible ways to interpret your question.  First,
which organisms are tetraploid, carrying four copies of each chromosome? 
Birds and mammals are always diploid, which might be a side effect of using
sex chromosomes to determine sex.  However, tetraploidy is not unusual in
fish, reptiles and amphibians.  For example, some salmon are tetraploid, as
are Xenopus, the African frogs used in many classical developmental biology
experiments.  Ploidy is quite flexible in plants, where tetraploid lineages
can be produced experimentally from diploid.  Examples of tetraploid plants
include alfalfa, coffee, peanuts and McIntosh apples.  Perhaps the most
interesting application for tetraploid plants is crossing tetraploid and
diploid lines to produce triploid hybrids, which are conveniently seedless
(like watermelons http://cuke.hort.
ncsu.edu/cucurbit/wmelon/seedless.html).

  Second, we can interpret your question as which organisms have four
chromosomes in each cell?  This could be a haploid organism with one copy
of each of four distinct chromosomes (n=4), a diploid organism with two
copies of two (2n = 4), or a tetraploid organism with four copies of just
one (4n = 1).  Unfortunately I can't find any examples for this answer.

  Finally, we can interpret your question as which organism's genomes have
a haploid chromosome count of n = 4?  A favorite model organism in the
genetics community is the fruit fly, which falls in this category.  I'm
sorry I can't seem to find any other examples offhand.

  Regards,
  Chris

  Useful web sites

Further discussion of ploidy: h
ttp://core.ecu.edu/biol/summersk/summerwebpage/biol2300/GenChromos.htm

Lists of chromosome counts for a variety of species: 
http://www.seps.org/oracle/oracle.archive/Life_Science.Gen
etics/1999.08/000934474627.6939.html

Discussion of ploidy in plants: http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/chromnumber/number6.htm