Hi Jamie,
I don't see why the original fusion that formed our chromosome 2
should have formed a reproductive barrier at all. Floating
polymorphism for Robertsonian fusions is quite common, and in many
cases apparently causes little or no reductiuon in fertility
(examples from rodents, spiders, grasshoppers, bovids,
cockroaches....). There is no clear pattern as to when fusions will
result in nondisjunction vs when they won't, but fusion trivalents
are more likely to be stable when the arms of the fusion are of
similar length (there is a difference in arm length on our chromosome
2, but it isn't extreme) and crossovers are relatively distal (don't
know if that is the case here) - the 14/21 fusion in humans is an
example of very unequal arm lengths and it often missegregates,
although people with this fusion still produce offspring and most are
normal (5-10% of offspring are tri 21, presumably many more lost
through spontaneous abortion, in addition to the other nondisjunction
products). So fixation of the chromosome 2 fusion is unlikely to have
required the very extreme circumstances you have outlined. If the
fusion floated in the population even for a relatively short time,
crossing over would have eroded any signal of the "founder event"
except for very close to the centromere. There must be enough data on
markers to establish if that is the case.
The fusion is a very obvious change in the karyotype, but I think it
is the inversions that are more likely to have been involved in
speciation events. Two of them specifically. As inversions preclude
recombination, we would expect to see less variation in the inverted
regions as the coalescence time is much more recent than for the rest
of the genome. Again gene mappers probably have those data already.
The interesting thing would be to compare the variation between the
inversions to get an idea of their order of occurrence - an older
inverted region would have had more time to accumulate variation. If
the inversions accumulate variation in a clock-like fashion, then
dating the inversions may also date the speciation events.
d
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Dave Rowell
Associate Professor in Evolutionary Genetics
School of Botany & Zoology
Australian National University
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