Cavalies 12: 247-376

Rete

February 28, 1973

—

a
Dr. Arthur Koen)
Department o obiology

Indiana University
Bloomington, Indiana 47401

Dear Art,

This is just a couple of small comments on your "Atwood-Ryan-Novick"
paper. Properly speaking the model does not refer especially to pro-
karyote evolution but rather to unicellular organisms that tend to
proliferate by vegetative reproduction to the extent that linked complexes
are only rarely broken up. One would guess that the same principle would
apply pretty directly to paramecéum( (borrowing problems of macronuclear
conservation) but would have rather limited application to filamentous
bacteria owing to cooperation within a myceliun.

I would certainly not diminish the weight given to periodic selection
in the evolution of microorganisms. (Note, for example, the discussion in
the introduction to "Papers in Microbial Genetics", University of Wisconsin
Press, 1952 at page xv which also reminds me that Stocker also deserves
credit as a co-discoverer of periodic selection.) But in my old thinking
on this question (and I have not had a new thought on it in a long time)

I would classify periodic selection as a subset of the overall phenomenon

of periodic sampling -- namely the prevalent habit that one™® guesses should
be attributed to microbial ecology whereby large populations tend to be
generated cyclicly from very small inocula, for example the single spore

or single infectious organism that may initiate an epidemic. This kind of
sampling will have essentially the same forma? consequences for microbial
evolution whether it results from mutational advantage, whether frequency
dependent or not, or the exigenctes of inoculation.

i was interested in the assertion, page 9, that "the major selective
pressure is against the synthesis of unneeded protein". Is this a point
that you would like to discuss or document in anymore det&él? Do you give
no role to the synthesis of redundant DNA? There are mighty few examples of
auxotrophic mutants having a demonstrable selective advantage in complete
media. Levoff did describe one years ago. I understand, of course, how
regulation confounds the problem but it would be good to have some empirical
support even for an idea that is seemingly intuitively obvious. (I almost
forgot the example that is reported in my first paper with Francis Ryan,
PNAS 32:163, 1966, but perhaps for good reason since we were not able to
establish a growth advantage of the leucine~dependent genotype and the
actual mechanism of selection that operates there is still quite obscure.)

 
Dr. Arthur Koch ~2- 2/28/73

One issue that needs to be thought about is whether the advantageous
mutations involved in pericdic sééection have an absolute advantage or

one which is gene~frequency dependent. Werner Brewr had some evidence for
the latter in some of his Brucella population. Beery

Page 14. Your comment about the implied random character of Fox's
polypeptides: he is beginning to persuade me that the condensation is
far from random, a point that has an interesting bearing on the pre-
disposition of the cosmos to generate the kind of life we know.

Sincerely yours,

Joshua Lederberg
Professor of Genetics

JL/rr