FW: Clonal breakdown

John T Lonsdale john@johnlonsdale.net
Tue, 21 Nov 2006 14:29:09 PST
When an organism or cell is propagated asexually for any period of time it
inevitably picks up somatic mutations.  These are mutations in genes in
cells other than germline cells and they are not passed on by sexual
reproduction.  These mutations may be silent (not obvious) or cause a
change in the phenotype (the organism looks or behaves differently).
Somatic mutations may cause cancer or variegation, for example.  Many
cancers are the result of the acquisition of many somatic mutations,
each allowing the cell to escape from normal control mechanisms.  I can
only assume that John Bryan's 'clonal breakdown' is the actually
accumulation of multiple somatic mutations that cause the desirable
features of a plant to be lost, or undesirable ones gained.  'Clonal
breakdown' cannot be fixed.  In the case cited by JB he selected a
single individual from the population with desirable (or in reality
fewer undesirable) characteristics, put it into TC and propagated it
thereafter.  It will still have carried forward all the somatic
mutations it had previously acquired, and would continue to acquire new

Clonal drift is the same process as above.  It is a problem in many
areas.  For example human cells passaged in culture many times will
sooner rather than later start behaving differently to the cells that
were originally isolated.  This has a big impact if those cultured cells
are to be used in research because they no longer are representative of
the cells that would be found in the body.

Continued asexual reproduction, per se, does not cause a 'loss of
vigor', that is pure bunkum!

It is wrong to think that all viruses cause leaf streaking - infection
can have many different symptoms or none at all that are visible.
Humans carry many viruses that are silent for some, most or all of the

One of the aging processes in cells other than stem cells is (as Boyce
said) a gradual shortening of the telomeres (the caps at the end of
chromosomes).  Once they get past a certain point the cell detects this
and can become senescent or enters a pathway of programmed cell death
(apoptosis).  In the devastating disease Progeria, which causes
premature aging in humans, the telomeres become prematurely shortened.



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