[Molecular-evolution] Human Chromosome Two: Evidence of First-degree Consanguity in Human Evolution

[Dave Rowell]

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

-- 
--------------------------------------------------------------
Dave Rowell
Associate Professor in Evolutionary Genetics
School of Botany & Zoology
Australian National University
___________________________________