Hello Daniel,
Saturday, December 01, 2001, 10:09:10 PM, you wrote:
Rac> Is this having 3 copies of the same chromosome? How does this happen?
Triploid (3N) means an organism has three sets of homologous chromosomes.
This contrasts with the majority of organisms that we are familiar
with (like humans) which are, throughout most of their lifecycle,
diploid (2N)(having two sets of homologous chromosomes). Remember that
egg and sperm are haploid (1N) (they have one set of homologous
chromosomes) and that fertilization restores the diploid condition.
(Don't confuse triploid with trisomy, which is having three copies of
a single chromosome, which can cause health and learning problems like
Patau Syndrome (trisomy of chromosome 13), Edward's Syndrome (trisomy
of chromosome 18), Down's Syndrome (trisomy of chromosome 21), etc...)
One reasonable theory that explains the origin of a triploid,
parthenogenetic lizard, like Cnemidophorus tesselatus(pattern class
C, Zweifel 1965), looks like this:
Cnemidophorus tesselatus Cnemidophorus sexlineatus
Parthengenetic Diploid Sexual Diploid
(pattern class D, Zweifel 1965) /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\/
Cnemidophorus tesselatus
Parthengenetic Triploid
(pattern class C, Zweifel 1965)
The diploid egg from a parthenogenetic C. tesselatus (pattern D) is
fertilized by the haploid sperm of a not-too-picky C. sexlineatus.
This results in the triploid, parthengenetic C. tesselatus (pattern
C). Does this make sense? Since C. tesselatus (D) is asexual, it's
eggs are also diploid (there is no 'meiosis' as you would normally
think of it).
Ota and Hikida (1989) point out that while this hybrid origin of
triploid lineages is a valid means of speciation in parthenogenetic
lizards, it can't explain the origin of H. stejnegeri (based on the
number and size of its chromosomes). They suggest that a possible
reorganization of the chromosomes within an existing triploid lineage
of the parthenogenetic H. garnoti-vietnamensis complex is more likely.
Ota et al. 1993. in Tropical Zoology 6 (1): 125-142 may have some more
info on this subject.
Chromosomal speciation is fairly common, particularly in plants and
amphibians. Consult a plant biology book or Duellman and Trueb's
excellent "Biology of Amphibians" for more info on cytogenetic
evolution.
I hope this helps.
--
Best Regards Reply To: [EMAIL PROTECTED]
Tony Gamble
University of Minnesota
Department of Fisheries, Wildlife, and Conservation Biology
###########################################################################
THE GLOBAL GECKO ASSOCIATION LISTSERV
WebSite: www.gekkota.com Archive: [EMAIL PROTECTED]/
The GGA takes no responsibility for the contents of these postings.
###########################################################################
