UNIVERSITY OF CAMBRIDGE

PROFESSOR R. A. FISHER, Sc.D., F.R.S. DEPARTMENT OF GENETICS WHITTINGEHAME LODGE
MISS M.F.1. SPEYER, B.Sc. 44 STOREY'S WAY
Research Assistant and Secretary CAMBRIDGE
Tel, 55822
23/4/49

Dear Dr. Lederberg,
Thank you very mueh for your letter and your reprints. Unfor-

tunately ,your cultures have not come : from the description of the content on

the envelope containing the reprints, I take it that the cultures were also there.

But there were two large holes in the envelope, and the cultures must have eloped

through these during the travel. Woulé it be a great trouble to you to send them
again ? All of the strains you had sent me were useful to me.

Your sending of BMB powder was providential, because it con-
vinced me excellently of how good a medium it is and how useful it may be for my
purposes. All successive trials,however, to reproduce this medium with my ingre-
dients failed, I believe, because/methylene blue does not work. I an now waiting

have ordered because they
for the Difco products »Which /should be more reliable,being made on purpose for

EMB agar.

My work since I last wrote you was »volarised by an interesting

. Making the c esses
finding, of a mutant strain giving © very high frequency of recombination.

4
your usual way », it gives mupxts a frequency of recombination higher than 10 ..
In special conditions I could obtain up to 60 % of recombinants,in a cross compa~
rable to that of 58-161 x W 583 in the presence of aneurin.

The analysis of crossing-over predusts frequencies ,however,
‘ wad of
Shows significant discrepancies 6 those obtained with the usual strains, though

I hope to be able to explain these discrepancies 2s due to the formation of mixed

prototroph coloniesyas a consequence of multiple matings within each colony.
Using sparingly the EMB powder you sent me, I could test a

200 protortrpphs obtained in the cross between your strains W 583 and 58-161, for

Lactose fermentation, virus resistance , b, independence ,and I got results en-

tirely comparable to those you give ,for a similar cross,in your 1947 paper on

Genetics. This surprised me a little,after all, because you wrote me that in

the W 583 strain the Gal ~ mutation is epistatic to Lact, so I should have ob-

tained less Lac + than sEpected | But perhaps the linkage between Gal and Lac is
very close. Unfortunately for all work on fermentation I must wait until I can
reproduce satisfactorily the EMB agar.

I have hopes that the new strain,of “hich I told you above,may help
to settle many questions ,like that of postzygotic reduction etc.,i8, as I am
trying to do, I shall be able to follow the mating at the microscope. I canht
already obtain recombination using one marker only on each side,instead of
the usual two. I am also trying,in view of this, to select with phage.

Thank you véry much for all your suggestions, which spafe me a lot of

work. I shall let you know about fubther developments, 25 soon as there will be

any.

Yours very sincerely bunt: 6 uf