Post by Richard Fitzpatrick
Deborah Stevenson wrote...
Post by Deborah Stevenson Post by Richard Fitzpatrick
David Winsemius wrote...
Post by David Winsemius Post by Helge Moulding
I'm not convinced that the number of chromosomes is a reliable
indicator of genetic or phylogenic distance.
You're not? Really? Think about it on the level of high school
biology at least. We are not talking about distance, rather
breeding feasibility. A difference in chromosomal number is
certainly a barrier to effective development in an ovum fertilized
by a sperm from a species with a different chromosomal number.
Cells that cannot figure out how to pair up their chromosomes are
going to have some very strange looking mitotic figures. A very
effective killfile mechanism.
Um, David? The very article you quoted earlier notes that while
most cats have 38 chromosomes and ocelots (and Geoffroy's?) have 36,
there is at least one report of a ocelot(36) x puma(38) mating
That's not that hard. It's making *fertile* offspring that's more of
A horse has 32 pair of chromosomes, a donkey 31, and they interbreed
all the time. Contrary to popular belief, mules aren't invariably
sterile, but reproduction is pretty rare. As a process for
perpetuating the gene pool, it's not a great idea.
No argument there. But David W didn't say that. He said the
different number of chromosomes is a barrier to effective ovum
development. That would prevent any off-spring, not just make the
Perhaps a difference in chromosomal number is not a totally effective
interspecies barrier, and I erred in attributing the barrier
to mitotic considerations when it is more from meiotic mechanisms, but
I remain convinced that it is a cross-species barrier. Even when the
same species of egg and ovum get together, there is significant wastage.
There are barriers besides chromosomal number as well:
If you want to get a bit weirded out, do a search on "interspecies
fertilization" on PubMed. Human spermatozoa are tested by running them
into hamster ova.
In duck interspecies matings, the efficiency of the fertilization is
an economic concern:
Also a concern in yak-cattle crosses (a new personal best in wierd
New Zealand insects are recognized as different species on the basis of
The researchers used hybridization of two chromosome races to develop
evidence regarding the mechanism of insect speciation.
Post by Richard Fitzpatrick
Really, I'm not trying to irritate David (hi, Dave! *waves*) or argue
whether hybrids will breed or not. It's just that in this thread some
AFU long-termers whom I respect have been displaying unusual (for
them) lack of clarity in their thinking and/or a hitherto-absent
tendency to read what they want into the various cites and articles.
So sue me. Is anyone suggesting that horse-donkey hybridization means they
are the same species? Or that their almost universal sterility doesn't
imply a significant biological distance? (I have read one web page that
claimed that the Goeffrey cat X domestic cat hybrid was fertile. I also
found a MadSci answer that says there is a horse hybrid that is fertile
despite a difference in chromosome number:
I pointed out the disparate number of chromosomes in Felidae and canines as
data that I thought would inform the original poster's question
regarding "potential for breeding" as a measure of "similarity". Moulding
then challenged the idea that chromosomal number was a good measure of genetic
distance. It is not a complete measure, since there are only some many
integers below the largest number counted in a normal animal cell. If
two species have different chromosomal number, they are "really" different
species. I have yet to see a cited example of an animal species that has a
variable number of chromosomes. In humans, somatic mutations do create cell
colonies whose cells have variation in chromosomal number. They are called
"cancer". Search on aneuploidy + neoplasm.
Perhaps I should have specified fertile offspring and the capacity for "breeding
true" as a feature of the barrier, but we were talking about speciation and
genetic distance. Such a framework implies that any process need be heritable.
Hybrids are just at an intermediate genetic distance. I stand by my assertion that
if two animals have different chromosomal numbers, that they are almost
always going to have trouble propagating their joint features. The exceptions
noted (human Turner's (XO), human Klinefelter's (XXY), XXXY, autosomal trisomy,
monosomy, tetrasomy) all have greatly reduced chances of reproductive success,
as do most (two possible exceptions found so far) of the hybrids across species
with different chromosomal number. That seems to me to be fairly well
characterized as a biologic barrier. The vast majority of non-dipoid chromosomal
reassortments in animals end up biologically barred from reproduction.
David "but plants are really different" Winsemius
If the statistics are boring, then you've got the wrong numbers.