In article <3od6df$cnp at iitmax.acc.iit.edu> Mark D. Garfinkel,
garfinkl at iitmax.acc.iit.edu writes:
>Unicellular organisms are not the only ones where the germline-soma
>division is absent, no? It wasn't all that long ago when a paper
>appeared in Nature describing rather vast forests as being composed
>of clonally related trees. Although I don't remember the paper in
>detail, the argument was that the trees were vegetative-
>propagation-derived. Cuttings, in other words, on an enormous
>scale, and not man-made.
Even in these clonal forests, there are some tissues which are
somatic, in the sense that their genetic information will never be
passed on to offspring. Leaves and bark, for example. I'm no
botanist, but I believe there are two or three plant tissue types
which do remain totipotent and thus capable of functioning as
'germline' cells, i.e., transmitting their genetic information to a
new organism.
My definition of germline-soma division is rather fuzzy, and the
Glossary of Genetics wasn't very helpful. So let me try to confuse
you, and maybe someone will enlighten me?
Ciliates have a clear case of germline-soma division. Each cell
contains two kinds of nuclei, micronuclei (germline) and macronuclei
(soma). The micronuclei are transcriptionally silent and their DNA
remains in a heterochromatic state. All the RNA is transcribed from
the macronucleus, so it is the information in the macronuclear DNA
that runs the cell. But during a sexual cycle, when two cells pair,
the macronucleus disintegrates. That information always dies during
a sexual cycle. The micronuclei undergo meiosis, meiotic products
are traded between the conjugating cells, and these fuse to produce
a new zygotic micronucleus in each cell. Mitosis produces a copy of
this micronucleus, which undergoes extensive modification and
amplification to become the new macronucleus.
So what? Notice that anything done to the macronucleus, either
during its construction or as a result of selection during asexual
proliferation (there may well be selectable variation between
clonally derived macronuclei) is not passed on to the next sexual
generation. The macronucleus can function as a scratchpad - the
scribblings may be useful now, but they are soon thrown away, and
the organism goes back to the original design. In a similar way
(analogy only! - and tenuous at that), the DNA in our immune system
cells is rearranged, and those changes are inherited by clones of
cells within our bodies. But the next generation of organisms
starts from the blueprints again.
Heterocysts of Anabaena(? the differentiated, nonreproductive
nitrogen-fixing cells on some filamentous algae) are effectively
somatic - they don't reproduce, they merely feed normal cells which
do reproduce. The DNA of B. subtilis spore mother cells becomes
somatic as soon as the spore nuclei have been produced, by this sort
of reasoning. Point is, it doesn't much matter what happens to that
DNA now, as long as it gets the spore nuclei properly packaged.
I'll stop with Salmonella IS200's: they are cleverly arranged in a
way which allows tidy duplication or deletion of the intervening
sequences by recombination between the ISs. Slightly damage an
important Salmonella gene, so that it only very inefficiently
performs its function, and soon Salmonella with repeated
duplications of that gene will show up. This not only allows the
Salmonella to thrive (using many copies of an inefficient enzyme),
but now the chance of reverting the damage (by mutation) is
multiplied by the number of copies. Once the damage has been
repaired, the ISs (plus selection) allow all the damaged copies to
be rapidly deleted, returning the Salmonella genome to its original
compact and efficient form (Ken Haack and John Roth).
In a way, the duplications that were eliminated were somatic - those
sequences only served to help the rest of the genome survive, they
did not get passed on themselves. Then again, one copy did survive,
and the process could be seen as an intragenomic method for
producing variation for selection to act upon . . . I think I'm
confused enough to stop now.
What is the essential characteristic of soma versus germ?
David Witherspoon, david at linkers.med.utah.edu
Oncological Sciences, Division of Molecular Biology and Genetics
5C334 SOM, University of Utah, Salt Lake City, Utah 84132
