Department of Genetics, University of Wisconsin
Madison, Dece 26, 1951

Dear Cavalli:

I was very glad to hear from you, and to take this opportunity
to exchange best wishes for the New Year. I wish I could report that much
had Happened since the Cold Spring Harbor meetings, but I have been almost
immobilized with various distractions. I have the comfort of seeing progress
in, the laboratoryin the hands of my wife and my students. The genetic behavior

of lysogenic phage, on one hand, and of the entire heredity of Salmonella
on the other are very perplexing. You will doubtless have been reading
Evelyn Witkin's Microbial Genetics Bulletin, and will be au courant des
affaires. Mrs. Lederberg and I have completed one task that was much pore
interesting in comcept tham in application. The replica-plating technique
‘4s ‘mentioned in the last M.G.B., but in case you have not heargd or remem
bered about it, it is a method to make several copies on different media of
the growth on an initial plate. Velveteen fabric is used to make the pre-
gise trans er suameh asvtese hed tap inting process. Aside from the
‘great convenienés Or ropa plete 2ér finding mutants and testing recom-
_.binantse, the technaque makes it possible to isolate pre-adapted mutagts
without direct selection. A smoothly grown plate (inoculated with 10 ~10)
As veplicated to selective agar (eog. :ghage or streptomycin) plates. The
“Resistant mutants occur in super imposible positions, corresppnding to
the clones: ‘én the original plate ( an@. proving their occurrence). By taking
“inooutd from the indicated sites on the original plate, the resistant mutants
may ,he enrféhed by as much as 100-fold.,Such inocula are replated at lower
“dilutign aid the process reiterated tti1 the enrichment results in well-
*deolated epionies that give rise to fier cultures. The actual enrichment
ling ig neyér exposed to the selective’ ‘agent; in general the process is
“Like pedigtge selection of roosters br bulls for egg or milk prduction.
* complete “account: wE1T appear in the Jan. 1952 Jour. Bacteriology, but I
give this hasty summary in anticipa Eiyou may have an inmediate interest
in the ‘method. More. “recently, I haveetarted some work with actinomycetes.
Heterokaryosis prohably. occurs with moderate ‘pegularity in S. griseus; the
.. evidence for recombination ( stable progatrophs)is so far amhiguous, and I am
| just starting to look at ethe other "species".
I would’ welcoye an oppok $i ty to discuss your findings on the
possibility of heterothallipm in somgidetail.. I do not reach the same con-
clusions from the data you give although I. an also prejudiced by some of
our own findings. You will agree that any pair of the following can be crossed :
58-1618"; K-12; W-1177. In your matenfay, it ie the derived T+L+B,+ that is
unique, behaving differently from K~£2,; -in'eet-ce pas? Using this ae an indi-
.. gator, -you.then find that different t-1-B)- can be found (by recombination)
. Wien oan te crossed with the der .T 945) + I will ad@mit that you have

fs ential fertility, but I think bi y be premature to call it hetero-
$ i @aition that she accidentally picked up
7 2a Bell etrain which is now sterile wi ¥-10, W-677 etc. Like your stock, it
> can bo crotped™ with re-derived ne (obtained by segregation from diploids),
-S ~The. pieture: ig. ‘very similar: I think weycan infer that your TLB)+ strain suf
fered the same accident as our BeM- ¢ ime ateps from 58-161). If,ae I think
eS 4 Wain woud ‘be usefsl. te your further study of these relation-
‘¢'-ships,-ané you aPesplam@ine to bivide- this, ske will be pleased to send it
to you (if we can verify these old findings). Have you noticed that T-L-B,-
isolated by crosses on TLB, +m agar show the linkage peculiarities like
those of dégregdhts from persistent diploids.(p.13 and table 5b, my CSH ms.)
This was my conclusion in one experiment. From it, I have given up trjmg
to map with these cultures (one student is trying to straighten out some of
he anomalies), but we are slowly devloping new cultures, avoiding the use
of artificial mutagens. I have the same trouble with B- in 58-161: have
s you tried to recover B- recombinants on biotin agar, where sparing by
methionine would not interfere? Nothing new on cytology. Mies Lively
is spending most of her time on single cell isolations (to get the true
segregation ratios from diploide;cf. table 9 CSHms. I have heard a little
3 from Hayes: from this, I would judge (possibly prematurely) that he is
=o making unnecessarily complicated deductions.
“ We would like very much to visit Italy and yourself, even ata
a Congress. Ihe problem is purely financial. Let us know just what we should

send you, and we will try to do so by return mail. Wg dy
urs ssincergty,
_ Leger berg