Re: [asa] chromasome fusion #2

Changes in chromosome number are not that unusual-rare enough to be a
useful genetic marker (e.g., Amphpeplinae are the only lymnaeids with
less than 18 chromosomes) but widespread. This can oocur via fusion
or division of chromosomes, or by extra/missing whole chromosomes
(including duplication of the entire set).

An example mentioned in one of my graduate classes was mice from
relatively isolated alpine valleys in Italy, where a number of
chromosome fusions have occurred.

There is no problem in making an embryo with one parent having fused
chromosomes and the other not. However, meiosis (making of gametes)
may be difficult in the heterozygous offspring, because in meiosis the
matching chromosomes have to line up. One chromosome lining up with
two is less reliable than the normal condition, so gamete production
could easily be lower. This also could produce meiotic drive, if the
one chromosome sorted properly more often than the two did.

In a small population, eventually you would have some individuals
homozygous for the new configuration, and it would generally be
advantageous for them to breed among themselves rather than with
unfused individuals.

Fusion of entire chromosomes end to end probably would not have much
effect on the activity of genes, but the new association between genes
newly on the same chromosome could have an effect.

Some genes can help sort out the presence of extra chromosomes. For
example, wheat is a hexaploid double hybrid. One gene enables it to
keep straight all the chromosomes and treat each pair from one
ancestral lineage as a distinct pair. Without it, it can't tell the
equivalent chromosomes form different ancestors apart and meiosis is a
lot messier. Similarly, seedless fruit at the grocery is often a
triploid mutant that can't grow seeds properly because of the problem
of sorting three chromosome sets. Corbicula (Asian clam), however,
can sort out three sets of chromosomes somehow.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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