UNIVERSITY OF MICHIGAN

ANN ARBOR
DEPARTMENT OF ZOOLOGY

9 Janugry 1953

Dr. Joshua Lederberg
Department of Genetics
University of Wisconsin
Madison, Wisconsin

Dear Dr. Lederberg;

Thanks for the reprints. The recent Jeview
article was especially interesting and useful to me,
since I have been floundering around in the maze of
of facts in this field for some time, trying to make
some kind of sense out of them. I found much new and
valuable information in the review, but would like
to offer some criticisms of the section on insect
symbioses.

Many of the difficulties reviewers are having
in this field arise, in my opinion, from the uncritical
compllation by Steinhaus (Insect Microbiology). To
illustrate with an example dealt with by you, take the
important work of Wigglesworth and Brecher on Rhodnius.
From the account given in Steinhaus (pp. 234-235), the
reader would gather that there are intracellular
sy#bionts in this bug which Wigglesworth abehpest
thought to be transmitted via the egg, but later (1944)
turned out to be picked up from the environment by
the nymph. However, the 1944 paper of Brecher and
Wigglesworth definitely states that the intracellular
particles earlier thought to be the symbionts (figured
by Steinhaus on p. 234) were only "rows of cell gran-
ules", and that the actinomycete important in nutrition
is found in the gut lumen only. Thus, the example
of Rhodnius does not belong in the field of endo-
symbiosis.

It seems to me that the term endosymbiosis
is meaningless unless it is restricted to the intra-
cellular forms. Probably every animal has microbes
living in its digestive tract, and it is inevitable
that these provide comprnenets of the animal's nutrition,
But this has no bearing on the problem of cell function,
and one cannot keep the facts in order unless one

SOS 2g
UNIVERSITY OF MICHIGAN
ANN ARBOR
DEPARTMENT OF ZOOLOGY

considers the supposed intracellular microbes only on their
own merits, It is of course possible that there is a
broad transition between gut-dwelling and intracellular
"symbionts", but this has not been demonstrated to my
satisfaction. The root nodule bacteria provide very
tempting support for the view that bacterial intracel-
lular symbioses are common elsewhere, but it is pobbible
that these legume nodules represent some very special
case. The chlorellas also are quite persuasive. I got
curious about those in Paramecium, and found that they
have good, easily demonstrated nuclei, so they are the
real thing.

The example of Carter's pineapple mealybug as one
of cytoplasmic inheritance involving symbionts is more
persuasive in summary than it deserves. In the original
paper, it will be seen that he had to transfer very large
numbers of pineapple bugs to the grass in order to get
enough individuals back from the grass to work with; this
obviously implies very strong selection. Also, the
original pineapple bug colony was highly variable with
respect to the morphology of the symbionts, while the
recovered forms were not. Therefore, at least as far &s
the 1936 paper is concerned, the results could be inter-
preted as simple selection from an originally hetero-
zygous mealy bug population, and the "symbionts" as
cell particulates whose morphology is under control of
the mealy bug's chromosomal genotype.

My own present hypothesis on the subject of ondosym-
biosis (intracellular) in insects is that these large
"pacteroid” particles represent one or more kinds of mito-
chondria, that they are very widespread in insects, and that
they are involved in some as yet. unknown but very impor-
tant phase of insect physiology. I do not think that —
function involves the production of some kind of vitamin;
they are often massive in occurrence, and anyway, there
is no reason for thinking that the insects having them
do not get plenty of vitamins from their food and
bacteria-filled guts.

ti I hope that this criticism is constructive. Probably
these insect "bacteroids" are something outside the
important problems you are dealing with, and that Kappa
particles, rickettsias, bacteriophages, etc., are more

to the point.

Sincerely,yours,
“CUM.

aX anham

Assit. Prof. Zoology