chromosome number and sex
Schrum
divergence involve changes in chromosome number for animals that depend
on heterogametic sex for propagation? For example, let's take as a
"given" that all mammalian species originated from a common ancestor.
Since all mammals (and other animals) depend on sex for propagation, we
can assume that the evolution of heterogametic sexual reproduction
pre-dates the evolution of mammals. Now here is the leap: in order for
a male and a female to produce offspring, they must have the same
chromosome number, so that chromosomes can be properly aligned and
sorted during mitosis/embryogenesis. So how is it that most mammals
don't have the same chromosome number (i.e., humans have 23 pairs and
mice have 22 pairs, etc.)?
It seems to me that dependence on the union of gametes for reproduction
would "fix" chromosome number. Otherwise, we have to imagine that at
some point a male mammal had a mutation that altered it's chromosome
number, and this male mated with a female that had the same mutation.
Are there other explanations?
Christopher Mosley
Subject: Re: chromosome number and sex
Newsgroups: sci.bio.evolution
References: <7j6mtg$sf2$1...@darwin.ediacara.org>
Organization:
Yes, the "same number of chromosomes" is not absolutely necessary for
producing fertile offspring. A broken chromosome, creating two from one,
could still line up with the unbroken version. The mutation might then
be propagated. Or perhaps a union from 2 chromosomes accounting for
a decrease of one, but I don't really have the remotest idea what
can happen. Someone once posted something on a population of similar
animals (some kind of rodent, I think, with a fairly recent common ancestor).
This population was not at all uniform in chromosome number or form.
If I really did see an article like this here, such a "species" (with
an increased tendency to change chromosome number) might yield
some information on how these events actually occur and lead to speciation.
It suggests a more probable and gradual paridigm then the low probability,
instaneous speciation you are suggesting.
Clark Dorman
Schrum <sch...@mail.med.upenn.edu> writes:
> I'd like to post a question I have had for some time: how can species
> divergence involve changes in chromosome number for animals that depend
> on heterogametic sex for propagation? For example, let's take as a
> "given" that all mammalian species originated from a common ancestor.
> Since all mammals (and other animals) depend on sex for propagation, we
> can assume that the evolution of heterogametic sexual reproduction
> pre-dates the evolution of mammals.
Yes.
> Now here is the leap: in order for
> a male and a female to produce offspring, they must have the same
> chromosome number, so that chromosomes can be properly aligned and
> sorted during mitosis/embryogenesis.
No, this is the source of the confusion. Differing numbers of
chromosomes is not necessarily a barrier to reproduction. A good
example is that E. caballus (domestic horse), with 64 chromosomes
can interbreed with E. przewalski (Przewalski's horse) with 66
(see below).
A well documented example are the shrews, which are discussed at:
<http://meiosis.bionet.nsc.ru/Chrgeo.htm>. An interesting point
is that I got from this page was that shrew chromosomal
polymorphism was known in 1956. So, this is not exactly a new
discovery. Lemurs are also a common area of study:
<http://www.selu.com/~bio/cyto/diploid/Lemuridae2n.html>.
Finally, the mouse is known to have a diversity of chromosome
numbers; the following is an abstract for a paper on shrews in
Tennessee but mentions Mus musculus:
<http://www.karger.ch/journals/CCG/CCG763-4/CCG0153.htm> but also
see the reference to Mus domesticus below.
You can also use the change in chromosome numbers to get the
evolutionary history of related species. See the discussion of
dogs and their relatives at:
<http://www.kc.net/~wolf2dog/wayne2.htm>. And, of course, the
chromosomal relationships between humans and other great apes has
studied. See one of my favorite papers referenced below (Yunis
and Prakash).
Below, several posts discuss Robersian translocations. You can
also see a specific example at:
<http://www.pathology.washington.edu/Cytogallery/cytogallery.html>.
The Cytogallery has lots of neat stuff in general.
Of course, the above are all mammals (the example that you gave).
However, plants are well known for their wild and wooly
chromosome number changes that I won't even try to explain (since
I don't understand them) and examples are known for insects.
----------------------------------------------------------------------
From: Clark Dorman <cl...@s3i.com>
In-reply-to: ri...@southern.co.nz's message of Mon, 01 Jan 1996 02:30:05 GMT
Newsgroups: talk.origins
Subject: Re: Help answer a creationist's question about chromosome number changes
References: <4c579u$g...@orm.southern.co.nz>
Distribution:
--text follows this line--
In article <4c579u$g...@orm.southern.co.nz> ri...@southern.co.nz (Rick Harris)
writes:
> A creationist asked how it is that the number of chromosomes can
> change as new species evolves, because individuals with the new number
> would not be able to breed with any of the others and would run into
> problems due to inbreeding. I'm sure there's a simple explanation of
> this, but I'm damned if I can think of it right now.
The problem here is the statement: "individuals with the new
number would not be able to breed with any of the others". As it
turns out, this is not the case. During the reproduction, the
important point is not the actual number of chromosomes, but how
they line up and whether they can combine. I am not a biologist
and am particularly not a molecular biologist, but the answer
lies in a good molecular biology book. The one that I use is
"The Molecular Biology of the Cell" by Alberts et al. 1983.
There is now a 3rd edition.
In many normal humans (that can and do reproduce), there have
been an altered number of chromosomes because chromosomes have
their ends fused together. The chromosomes have an end cap
(can't find the official name right now) that does not contain
information, but can stick together. Since the chromosomes still
have all the information and can line up properly, everything is
ok. Usually, this is between the chromosomes with single-arms.
Also, normal humans can have whole segments switched between
between two chromosomes, as long as no material is lost (this is
called a chromosome translocation). See Alberts et al. for a
discussion.
The point though is that the situation is much more complicated
than simply that animals with different numbers of chromosomes
cannot reproduce. There is a species of deer that in particular
has a very wide number of chromosomes and they can continue to
reproduce.
A fascinating look at hominid chromosomes is in the paper:
Jorge J. Yunis and Om Prakash. "The Origin of Man: A
Chromosomal Pictorial Legacy", in Science, Vol. 215, 19
Mar 1982, p.1525-1530.
This paper answers the question: "How can man and chimpanzee be
related if they don't have the same number of chromosomes?" (23
pairs in man, 24 in great apes). No doubt, that is the _real_
question since your creationist is almost assuredly most
interested in how _he_ (or she) could have arisen from another
animal. What this paper has is a picture of all the chromosomes
of man, chimpanzee, gorilla, and orangutan lined up next to each
other and showing the 1000 band stage with the sections are all
labeled. Just by examining the picture for a couple of minutes
_clearly_ indicates that the chromosomes are remarkably similar.
The differences are equally interesting as the vast majority are
simple inversions of sections of chromosome. Chromosome 2 of
humans is shown next to two chimpanzee (and gorilla and
urangutan) chromosomes and the human one is twice as long as the
chimpanzee (and the other two as well) and all the bands match up
showing that the human chromosome is just a connection of the two
chimp chromosomes.
So thats your answer. Humans chromosome 2 is the combination of
2 chromosomes from the common ancestor with the chimpanzee and it
was able to reproduce fine, just as other animals are able to
reproduce with different numbers of chromosomes within their
species.
----------------------------------------------------------------------
From: howard hershey <hers...@indiana.edu>
Newsgroups: talk.origins
Subject: Re: Help answer a creationist's question about chromosome number changes Date:
2 Jan 1996 14:34:41 GMT
Organization: indiana university
Content-Type: text/plain; charset=us-ascii
X-URL: news:DORMAN.95D...@maverick.bu.edu
>In article <4c579u$g...@orm.southern.co.nz> ri...@southern.co.nz (Rick Harris)
>writes:
>> A creationist asked how it is that the number of chromosomes can
>> change as new species evolves, because individuals with the new number
>> would not be able to breed with any of the others and would run into
>> problems due to inbreeding. I'm sure there's a simple explanation of
>> this, but I'm damned if I can think of it right now.
Robertsian rearrangements are rather common in nature (found both
in differences between species and also *within* species as part
of the normal variation and chromosomal abnormalities. In fusion
(forming one chromosome from two), this involves two telocentric
(centromere at the end) chromosomes fusing to form one
metacentric chromosome with loss of the other centromere.
____. + ._____ --> ____._____
It can also involve fusion of two acrocentric (or acrocentric and
a telocentric chromosome). Sometimes the tiny chromosome formed
is lost; in other cases you have a metadicentric chromosome
formed. In the latter case one of the centromeres becomes
inactive for it to be stable.
a) ____._ + _.____ --> ____.____ + _._ (lost)
b) ____._ + _.____ --> ____._.____
All these processes are reversible, so that metacentric
chromosomes can become two telocentrics.
In addition, you can have simple chromosome trisomies (less
common in animals, more common in plants because plants tolerate
them better). In humans, the only aneuploidies that are viable
involve chromosome 21 (Down's) and the sex chromosomes.
Add to that polyploidies, where whole chromosome sets are
duplicated. These are usually tolerated even better than
aneuploidies and, in fact, are a well-documented mechanism for
the generation of new species in a single generation
(hybridization + polyploidy in plants). Even animals have
polyploid species (a sister species of *Xenopus laevis*, the
clawed African toad, is a polyploid of *laevis*).
Plenty of ways to change chromosome numbers. There are even more
ways to change chromosome arrangement (translocations and
inversions and small duplications). And chromosomal abberations
are (by far) even more common than mutation at the DNA level.
----------------------------------------------------------------------
From: mtu...@snipthis.acpub.duke.edu (mel turner)
Newsgroups: talk.origins
Subject: Re: Changing Number of Chromosomes
Date: 1 Jun 1998 21:28:25 -0400
Organization: botany dept., duke univ.
Approved: rob...@ediacara.org
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In article <6kvhme$kl8$1...@nnrp1.dejanews.com>, rich...@asi-consulting.com
wrote..
>
>Hello! I've got a serious question about evolution. How is it possible for
>the number of chromosmes to evolve? Can a single species have an
>indeterminant number of chromosomes? If not, how can the number change? For
>example, if one mutant human had say 47 chromosomes, could he/she successfuly
>breed with a normal human? Has this question ever come up in this ng?
>Thanks!
Yes, the question has come up before. As it happens, there are
indeed many species known in which individuals vary in chromosome
numbers, [sometimes with a whole range of numbers in one
polymorphic species]. Chromosomal polymorphisms are actually
pretty common.
Sometimes there is a strongly interpopulational, geographic
component to the variation, suggesting speciation-in-progress.
Basically, chromosome number changes by a "fusion" or a
"splitting" of chromosomes doesn't necessarily prevent
interfertility of organisms with different numbers [or their
hybrids]; the homologous chromosome arms of the different sets
may still pair up at meiosis.
I posted a few refs on this issue a while back, here's the
headers [from dejanews (I searched for "robertsonian fusion",
which also found a few other relevant posts)]:
Subject: Re: Proposal...
From: mtu...@snipthis.acpub.duke.edu (mel turner)
Date: 1997/11/23
Message-ID: <658aoo$2ij$1...@news.duke.edu>
Newsgroups: talk.origins
A couple of excerpts from it [abstracts from Biological Abstracts]:
TI Mitochondrial DNA variation and the evolution of Robertsonian
chromosomal races of house mice, Mus domesticus.
AU Nachman-M-W; Boyer-S-N; Searle-J-B; Aquadro-C-F
SO Genetics 136(3): 1105-1120
PY 1994
AB The house mouse, Mus domesticus, includes many distinct
Robertsonian (Rb) chromosomal races with diploid numbers from 2n
= 22 to 2n = 38. Although these races are highly differentiated
karyotypically, they are otherwise indistinguishable from
standard karyotype (i.e., 2 n = 40) mice, [snip]
TI Cytogenetic analysis of autosomal polymorphism in Graomys
griseoflavus (Rodentia, Cricetidae).
AU Zambelli-A; Vidal-Rioja-L; Wainberg-R
SO Zeitschrift fuer Saeugetierkunde 59(1): 14-20
PY 1994
AB South American phyllotine Graomys griseoflavus specimens were
collected in eight localities of central Argentina and
cytogenetically analysed. These populations comprised the
following karyomorphs: 2n = 42, 41, 38, 37, 36, 35 and 34. These
chromosome polymorphisms resulted from Robertsonian fusions
(RFs). A pericentric inversion (PI) in two different autosomal
[snip]
----------------------------------------------------------------------
From: afor...@ouray.cudenver.edu (Queen of Potatoes)
Newsgroups: sci.bio.evolution
Subject: Re: Chimp/Human Compatibility
Date: 22 Jan 1996 23:52:49 GMT
Organization: University of Colorado at Denver
Approved: jo...@pogo.cqs.washington.edu
Originator: evolution@pogo
In article
<Pine.A32.3.91j.96012...@homer26.u.washington.edu>
Ronald Bates <ronb...@u.washington.edu> writes:
>The question was can they hybridize not can they produce fertile young.
>If I remember correctly there is an even greater difference between
>horses and donkeys and yet they don't need any "editing" to produce mules.
>
>On 18 Jan 1996, Onar Aam wrote:
>>
>> It's possible, but then some "editing" of the chimp DNA is needed. Part of
>> the problem is that chimps and humans don't have the same number of chromosomes.
For general information purposes, two species do not need to have
the same number of chomosomes to produce an offspring (sometimes
even a fertile offspring). Since equines have been used to
illustrate hybridity in this thread, I offer the following
information:
E. caballus (domestic horse) has a diploid chromosome count of 64.
E. asinus (common donkey) has a diploid count of 62.
E. hartemannae (Hartman's Zebra) has a diploid count of 32
E. przewalski (Przewalski's horse) has a diploid count of 66
E. hemionus (Onager or Asiatic Ass) has a diploid count of 56
E. grevyi (Grevy's Zebra) has a diploid count of 46
E. burchelli (Grant's zebra) has a diploid count of 44
To my knowledge the only crosses that do not result in living offspring are:
Przewalki x Grevy's zebra
Przewalski x Hartman's zebra
Grevy's zebra x onager
Grevy's zebra x Grant's zebra
The number of diploid number of chromosomes a hybrid has will be
half the diploid count of each parent added together. Hence,
Przewalski x Equus will have 65 while a donkey x a horse will
have 63.
----------------------------------------------------------------------
David L. Rosen
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Chromosome number is not a hard hybridization barrier. Attached references
are studies of modern populations of animals and plants with varying chromosome
number, well as some saltations that could interbreed. My favorite are the New
Mexican gophers.
BTW: I never found a follow up on those mules that foaled. Did any of
the foals later foal?
Also, can anyone explain how those New Mexican gophers split into more
than two polykaryotic populations? Sounds a little like punctuated evolution
to me. However, that is a description and not a real mechanism.
Please mail directly to me.
d-j-...@erols.com
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Here are some studies of populations of similar organsisms
with a varying chromosome number, supposedly generated by
a recent mutation. Listing order is for author, title of
article,volume, pages, year. These articles show that
changes in chromosome number aren't necessarily fatal,
and that resulting descendants can sometimes even be
viable. Small differences in chromosome number aren't
even absolute hybridization barriers, although they can
make hybridization more difficult.
Rodents (natural populations):
Charles S. Thaeler, Jr., "Four Contacts Between Ranges
of Different Chromosome Forms of the Thomomys Talpoides
Complex," Syst. Zool. V. 23, pp. 343-354 (1974).
David J. Schmidly and Gilbert L. Schroeter,
"Karyotypic Variation in Peromyscus Boylii (Rodentia:
Cricetidae) from Mexico and Corresponding Taxonomic
Implications," Syst. Zool. V. 23, pp. 333-342 (1974).
Plants
(Trogopogon, a natural population):
Marion Ownbey, "Natural Hybridization and Amphiploidy
in the Genus Tragopogon," American Journal of Botany
V. 37, pp. 487-498 (1950).
Douglas E. Soultis and Pamela S. Soltis, "Alloploid
Speciation in Tragopogon: Insights from Chloroplast
DNA," Amer. J. Botany. 76, 1119-1124 (1989).
(A book on artificially induced mutations like triticalee)
Ed. YP. S. Bajaj, Biotechnology in Agriculture and Forestry 2,
Springer-Verlag, Heidelberg 1986).
Note: The drug colchicine can induce chromosome pair doubling.
It is mostly used in plants. Triticale was created using
cochicine to permit hybridization between wheat and rye.
Fertile Mules
Franklyn Eldridge and Yoshiko Suzuki, "A mare mule -
dam or foster mother," Journal of Heredity V. 67, 353-360
(1976).
Franklyn Eldrich and William F. Blazak, "Horse, Ass, and
Mule Chromosomes," Journal of Heredity V. 67, pp. 361-367
(1976).
For a more
Chimpanzees have 24 pairs of chromosomes, humans have only
46. The following article shows a junction on the human
chromosome where two chimpanzee like chromosomes seems to
have been joined. Other junctions with rearrangements are
also photographed.
Jorge J. Yuni and Om Prakash, "The Origin of Man: A
Chromosomal Pictorial Legacy," Science V. 215, 1525-1540
(3/19/82).
Alot of hybridization barriers seem to follow Haldane's Rule,
which states that if only one gender of a hybrid is fertile it
is only the homozygous gender. In us, the homozygous gender is
female. This seems to apply to the few fertile mules shown
above. A description of Haldane's Law is in:
Michael Turelli, "The Causes of Haldanes Rule," Science 282, 889-891
(10/30/98).
Note that the Science V. 215 article and the second Journal of
Heredity V. 67 articles have photographs of chromosomes.
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