INTERACTIONS BETWEEN SOME RECESSIVE LETHAL AND VISIBLE
MUTATIONS OF THE FOURTH CHROMOSOME OF
DROSOPHILA M ELAN0GASTER
ELIZABETH AMBELLAN
AND
TIMOTHY PROUT
Columbia University, New York City and University of California, Riverside, Cdifornia
Received September 12, 1955
XPERIMENTAL populations of Drosophila melunoguster kept by DR. BRUCE
WALLACE
and his collaborators (1951) received chronic radiation of approximately 2,000r per generation. The effect on the fourth chromosome was analyzed
by these investigators by outcrossing males from the irradiated populations with
females carrying the dominant mutations, “cubitus interruptus” (ci”) and “eyeless”
(ey”) as markers. Male progeny were again backcrossed to ciD/eyD females. From
the progeny of this cross, males and females carrying the same dominant markers
and the fourth chromosome being tested, were then intercrossed. If these progeny
contained no wild type flies, the fourth chromosome in question must have been
lethal in double dose. By this method 13 different fourth chromosomes which were
lethal when homozygous were obtained from two experimental populations.
It was assumed because of the shortness of the fourth chromosome that it possesses
a relatively small number of loci capable of producing lethal or visible mutations.
Therefore possibly a large proportion if not all the lethal producing loci might be
included among these 13 lethals. Also if a single locus is capable of producing both
recessive lethal alleles and visible alleles, then again because of the shortness of the
chromosome, some of the known visible mutations of the fourth chromosome might
prove to be alleles of some of the 13 lethals.
In order to investigate these above possibilities the allelism within the lethals
was studied. Also studied were the interactions of these lethals with some of the
visible fourth chromosome mutations.
E
PROCEDURE
Eleven of the thirteen lethals were maintained as balanced lethal stocks with
ey”; two were balanced with ci”. I n order to study allelism all thirteen were crossed
in all possible paired combinations (78 such combinations in all). Two lethals were
considered allelic when no wild type progeny appeared from the cross between them.
In order to study lethal-visible interactions, first a number of fourth chromosome
visible mutants were investigated, and two stocks were chosen for study; they were
the two balanced lethal stocks, Cataract/cubitus interruptus (Cut/ci”), and bent/
Cubitus interruptus (btD/ciD).The expression of ci” differed in the two stocks. The
ci” of the btD/ciD stock showed missing segments on wing veins L4 and L5 distal
to the posterior crossvein and is hereafter referred to as ci.”’ The ciD of the Cut/ci”
stock with a piece missing only from L4 is referred to as ciD1.Each of these two stocks
was crossed to all thirteen lethals and the effects of lethal-visible interactions sought
among their progeny.
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ELIZABETH AMBELLAN AND TIMOTHY PROUT
TABLE 1
Allelism tests between the fourth chromosome recessive lethals and four “visible” mutants. Lethals with
the same letter behaved as alleles m’th each other. A minus sign indicates that the heterozygote between
the lethal and the “visible” dies; a plus sign indicates that such a heterozygote i s viable
Recessive lethals
Dominant “visibles”
Cat
bt
CiD2
b-7
b-9
d-12
a-l
a-2
0-3
a-4
b-6
_
_
_
_
+ + + + + + - +
-
_
-
8-8
6-10
6-11
e-13
_
ciD’
RESULTS
The results of the allelism crosses showed that there were only five different loci
represented among the thirteen lethals. One lethal locus, designated a, was represented five times; another, b, four times; another, c, was represented twice, and d
and e only once each.
The results of mating the two stocks, b t D / c i D * and Cat/ciD1, with the lethals are
summarized in table 1. The lethals are numbered 1-13 individually and are also
designated a, b, G , etc. showing their allelism relationships. In the body of the table,
a minus indicates a lethal interaction between the recessive lethal at the head of
the column with the visible mutant a t the left end of the row. A plus sign indicates no interaction.
The matings of b t D / c i D * with the lethals revealed the lethal interaction of btD with
all members of the b series. The expected class of b t D / b , appearing as either b t D or
wild type overlap, was missing from the offspring which were all c i D phenotypes.
This same mating also revealed a lethal interaction between ciD2 and all members
of the a series. The fact that in each case all members of the series interact with a
visible mutant in the same way confirms their allelism as determined by lethal
interactions with each other.
Matings of Cat/&’ with the recessive lethals showed a lethal interaction between
Cat and the a series. However, d also produced a lethal interaction with Cat.
The fact that Cat and ciD2 both interacted with the a series suggested that they
might be lethal with each other. Therefore, crosses Cat/ciD’ by b t D / c i D 2 were made,
and it was found that such an interaction took place; the Cat/ciD2 class was missing
from the progeny. c i D 1 / c i D 2 was also lethal.
ciD’ was found to be viable with all lethals, including the a series with which ciD2
produced a lethal interaction. This result adds to the morphological evidence differentiating c i D 1 from &*.
DISCUSSION
The simplest assumption is that when two mutants show lethal interactions when
heterozygous, they are alleles. However a simple allelic relationship of “point”
mutations will not explain our data. One is forced therefore to more complex hypotheses. As will be seen below there are several such explanations, the differentiation
MUTATIONS I N DROSOPHILA CHROMOSOME
bP
“Visibles” ____
IC -----
I+- - - -
ci”‘
CiD2
4
+---------cat
I
________
225
____-____
$1
-f
Lethals
b
a
d
FIGURE1.-One hypothetical linear arrangement on the fourth chromosome of the lethals and
“visibles” studied, assuming deficiencies are involved.
of which requires further work. Unfortunately this necessary additional experimentation cannot be carried out by these authors on this material. Nevertheless,
it was considered worth while not only to present the foregoing data as far as they
go, but also to consider the several interpretations even though they may be indistinguishable.
First, a system of deficiencies could provide an adequate explanation. Figure 1
is a map showing the relationships of the mutants involved when deficiencies are
assumed. BRIDGESand BREHME(1950) state that the ci” mutation is “possibly a
deficiency”. Assuming then that our cP,
is a deficiency, then ci@ would be a deletion of greater length. This would explain the lethal interaction between c i D 1 and U ,
since U might be included in the area of extension by which c i D 2 exceeds ciD’.
If the ciD2 deficiency extended to include Cut then the lethal interaction of these
two would be explained, as well as the failure of any interaction between Cut and
CiD’.
The fact that Cat is inviable with d but that d is viable with c i D 2 and U with which
Cut interacts lethally, could be explained by assuming Cut is also a deficiency par-
tially including c i D 2 but extending to include d.
Assuming this system of deficiencies means that E t D could not be between c i D and
Cut since ciD2 deficiency includes both c i D and Cut, but not b t D . This arrangement
is a t variance with the conclusions of FUNG
and STERN(1952) who place btu between
c i D and Cut. These authors used three deficiencies of the right arm whose left breakage points were close together but essentially unknown, but whose right breakage
points could be arranged in a series each farther to the right than the preceding one.
As far as this series of right breakage points was concerned the first or shortest
deficiency included only ci, the second ci and bt and the third ci, bt, and Cut. (Actually spa was used, which is allelic to Cut.) Since, however, the left breakage points
were unknown there still remains the possibility of several other arrangements
including the one presented here.
However, FUNGand STERN’Sarrangement based on deficiencies is in agreement
with the sequence established by STURTEVANT
(1952) who used crossover criteria.
Furthermore, the system of deficiencies proposed above rests on no cytological
evidence. Even BRIDGES’original suggestion that c i D is a deficiency could not be
confirmed cytologically. I t seems likely therefore, that some factor other than deficiencies is involved.
An alternative explanation involves the possibility that the two balanced lethal
stocks, c i D 2 / b t D and ciD1/Cut have accumulated recessive lethals by mutation. Since
there is so little crossing over on the fourth chromosome the acquisition of nonallelic
recessive lethals on the two chromosomes of a balanced lethal stock is a definite
226
ELIZABETH AMBELLAN AND TIMOTHY
mom
possibility. If in the ci”*/btD stock the c i U 2 chromosome had acquired the U lethal
and the b t D the b lethal, and in the ciD‘/Cat stock the Cut chromosome had acquired
a and d lethals, then our findings would be explained. That these two stocks should
have acquired some of the same recessive lethals which were independently extracted
from DR. WALLACE’S
populations may seem unlikely, but this likelihood depends
on the total number of different loci capable of producing recessive lethals which
may be small.
A final explanation of our findings could involve a system of specific interlocus
interactions. To us this seems unlikely if only because in general such interactions
are the exception rather than the rule. A very special kind of interaction, however,
could be involved-namely, position effect. STERNand others (e.g. 1955) have studied,
in detail the position effects of the ci locus. If chromosomal rearrangements are
involved either in our lethal material or the ciD stocks, then our findings could have
still another and quite different interpretation.
I t should be made clear that these possible explanations (deficiencies, lethals, and
interaction) are not mutually exclusive, but a combination of them could be closer
to the truth.
Finally it should be pointed out that since there were only five different loci represented among the original 13 independently obtained lethals, there are probably a
relatively small number of loci on the fourth chromosome capable of producing
recessive lethals. Assuming 11’ loci all with the same mutation rate to lethality, it is
possible to write the probability of obtaining 5 different loci in a sample of size 13
as a function of N (LEWONTIN
and PROUT
1955). From consideration of this function
it is possible to state that the chances are very small that there are more than 20
loci on the fourth chromosome capable of producing recessive lethal alleles.
LITERATURE CITED
BRIDGES,C. B., and BREAME,K. S., 1950 Mutants of Drosophila melanogaster. Carnegie Inst.
Wash. Publ. 563.
FUNG,
S. T., and C. STERN,1951 Seriation of fourth Chromosome loci in Drosoplzila melanogaster.
Proc. Nat. Acad. Sci. U. S. 37: 403-404.
R. C., and T. PROUT,1956 Estimation of the different classes in a population. BioLEWONTIN,
metrics (in press).
STERN,C., and M. KODANI,1955 Studies on the position effect at the cubitus interruptus locus of
Drosophila naelanogaster. Genetics 40:343-373.
A. H., 1951 A map of the fourth chromosome of Drosopliila melanogaster, based on
STURTEVANT,
crossing over in triploid females. Proc. Nat. Acad. Sci. U. S. 37: 405-407.
WALLACE,
B., 1951 Genetic changes in populations under radiation. Am. Naturalist 85: 209-222.