Q: "You said ferrets have 40 chromosomes and European polecats have 40,
but steppe polecats have 38. You mentioned some scientists think they
may be the same species even though they have different number of
chromosomes. Do any other species have different numbers of
chromosomes?"
A: Only when they dig deep into their jeans. Just Levi it at that, ok?
It is more common than you might think. The black rat, rattus rattus,
shows geographical variation due to a Robertsonian rearrangement causing
progressive fusion of chromosomes (2n = 38, 2n = 40, and 2n = 42). It
is considered to be a single species. Wild tufted deer (Elaphodus
cephalopus) have a karyotype between 2n =46, 47, and 48. The karyotype
of Microtus longicaudus (longtail vole) ranges between 2n = 56 and 2n =
70; a condition also seen in Microtus mandarinus. Most Burchell's zebras
have 44 chromosomes, but some have 45. Sika deer, Cervus nippon, have
been reported to have a karyotype chromosome count between 2n = 64 and
68. North American mice of the genus Peromyscus tend to have different
karyotype counts, with some species, such as Peromyscus boylii, the
karyotype is reported as "the fundamental number being 54-56." European
wild boar have been reported to have chromosome numbers of 2n = 36, 37,
and 38. Ctenomys minutus, a South American rodent, has karyotypes of
2n = 42, 45, 46a, 46b, 47, 48, 49, 50; it has eight different chromosome
counts! Different chromosome counts between members of a single species
is far more common than you might think. When there are different
karyotype counts in a single species, it is termed a "polymorphic
karyotype" or "chromosome number polymorphism." In a very general
sense, it is most noted in species having a wide geographic range, with
different chromosome counts at different ends of the distribution. You
know, like with the eastern and western varieties of the polecat.
However, sometimes the different counts are relatively isolated.
On the other hand, Hsu and Benirschke report that there is no discernable
difference between the karyotypes of the lynx, bobcat, and domestic cat.
They all have the same number of chromosomes, and the morphology of one
is pretty much the same as the others. There are numerous examples of
this type of similar karyotypefar more than listed above that show that
polymorphic karyotypes are more common than generally recognized. The
identification of the ferret's progenitor was basedfor the most parton
karyotype morphology; because both the ferret and the European polecat
have 40 chromosomes that look, for all intents and purposes, similar.
Thus, based mostly on karyotype, the ferret was made a subspecies of
the European polecat and renamed Mustela putorius furo. Using identical
standards of investigation, the lynx, bobcat, and housecat would be
subspecies of each other. As you can see, much of the distinction
between karyotype groups is subjective.
In fairness, Gregor Mendel may have first described genetics nearly a
century and a half ago, but Watson and Crick's description of DNA was
nearly a century later. The computers and other technology necessary
to really look deep into the genome are of relatively recent invention.
The lessons we learn in the next couple of dozen years will rewrite much
of what we know about genetics today. Species theory still lags behind
somewhat, and I am sure that as evolutionary genetics is incorporated
into the concept, it will change, at least subtly, in many ways to
reflect theoretical advances. Until then, the use of genetics has not
been able to discern from which polecat the ferret was domesticated.
Bob C [log in to unmask]
PISTOL: Master Fer! I'll fer him, and firk him, and ferret him. Discuss
the same in French unto him.
BOY: I do not know the French for fer, and ferret, and firk.
William Shakespeare. 1600 Henry V, Act IV, Scene IV, Line 26 to 28.
[Posted in FML issue 4781]
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