Department of Genetics, Universi $y of Wisconsin
ue Madison 6, Wisconsin

' June 27;> 1951

Dr. D. Lewis,
John Innes Horticultural Institution,
Bayfordbury, Hertford, Herts.,

Engja nd.

Dear Dr. Lewis:

The 4lst annual reppeb of your institution was just
received here, and I was especially interested to read
the tantalizingly condensed summary of your experiences
with mtations at the S locus. An expjanation came to
my mind for the “temporary"omitations which I hoped you
might comment upon.

My Botany is nono too sure, but if I am not too far off,
the mature pollen grain in Oe. organensis contains two
nuclei- one the tube nucleus, the other a gensrative nucleus
which divides in the pollen tube or earlier to produce the
two sperms. Is it not likely that the phenotype of the
male gametophyte is controlled by the tube nucleus rather
than the generative? If so, a mutation occurring subsequent
to meiosis might produce a change enabling " the pollen grain
carrying the mtated allele to pass the incompatibility sieve",
although there would be no alteration detectable in the next
generation. This would not account for the remarkable family
referred to pn p. 16, linesé&10.

This notion would, however, also account simply for

the surplus of clones of single mutants referred to in your
paper on spontaneousf{ mutation rates, since such clones would
result either from mutations at the last meiotic/ or the first
postmeiotic mitosis. Half these clones, however, would have

to carry "temporary" mutations, whereas such mitations should
not be characteristic af larger clusters presumably resulting
from premeiotic mtations.

Yours aineerely >
Joshua Lederberg,

Associate Professor of
Genetica