Genetic studies showed several years ago that the third chromosomes of wild strains of Drosophila pseudoobscura often carry suppressors of crossing-over. The salivary gland chromosome technique has made it possible to demonstrate not only that these are inverted sections, but also that there are many different inversions present in wild populations inhabiting different geographical regions. So far we have found at least fourteen different gene-sequences in wild stocks, and have found that in most geographical regions several sequences are present, though no single sequence appears to occur throughout the range of the species. There are a number of problems raised by these facts, most of which need further study; the present account is to be regarded only as a preliminary note

Dobzhansky, Th.

Sturtevant, A. H.

English

Caltech Authors - Main

History of the Species
Drosophila pseudoobscura, and Their Use in the Study of the 
Inversions in the Third Chromosome of Wild Races of
A. H. Sturtevant, and Th. Dobzhansky 
doi:10.1073/pnas.22.7.448 
 1936;22;448-450 PNAS
 This information is current as of December 2006.
 www.pnas.org#otherarticles
articles: 
This article has been cited by other
 E-mail Alerts
. click hereor
box at the top right corner of the article
articles cite this article - sign up in the 
Receive free email alerts when new
 Rights & Permissions
 www.pnas.org/misc/rightperm.shtml
tables) or in entirety, see: 
To reproduce this article in part (figures,
 Reprints
 www.pnas.org/misc/reprints.shtml
To order reprints, see: 
 Notes:
GENETICS: STURTEVANT AND DOBZHANSKY PROC. N. A. S.
INVERSIONS IN THE THIRD CHROMOSOME OF WILD RACES OF
DROSOPHILA PSEUDOOBSCURA, AND THEIR USE IN THE
STUDY OF THE HISTORY OF THE SPECIES
By A. H. STURTF,VANT AND TH. DOBZHANSKY
W. G. KERCKHOFF LABORATORIES OF THE BIOLOGICAL SCIENCES, CALIFORNIA INSTITUTE
OF TECHNOLOGY
Communicated June 8, 1936
Genetic studies showed several years ago that the third chromosomes
of wild strains of Drosophila pseudo6bscura often carry suppressors of
crossing-over. The salivary gland chromosome technique has made it pos-
sible to demonstrate not only that these are inverted sections, but also that
there are many different inversions present in wild populations inhabiting
different geographical regions. So far we have found at least fourteen
different gene-sequences in wild stocks, and have found that in most geo-
graphical regions several sequences are present, though no single sequence
appears to occur throughout the range of the species. There are a number
of problems raised by these facts, most of which need further study; the
present account is to be regarded only as a preliminary note.
One sequence has been arbitrarily designated as "standard," since it is
the one in which most mutant genes are known and is also the only sequence
that has been found in both of the cross-sterile races, A and B. This
sequence is commonest in race A on the Pacific coast, from British Colum-
bia to Lower California, but is found sporadically also in western Montana,
western Nebraska, central Texas, central New Mexico (Carizozo) and
southern Arizona (Santa Rita Mts.). In race B the standard sequence is
predominant in the Sierra Nevada and the Coast Range south of the San
Francisco Bay, California, but it has been found also in a single strain from
the Olympic peninsula, Washington. Several other sequences represent
single inversions with respect to the standard one; the "Arrowhead"
sequence, found throughout the range of race A in British Columbia and
the United States; the "Pikes Peak" sequence, found in race A from
western Nebraska south to central Texas and southeastern Arizona; the
"Klamath" sequence, found in race B from British Columbia south to
Mono county, California; and the "Sequoia" sequence, known only in race
B from Sequoia National Forest in California.
All the other known sequences differ from the standard by more than a
single inversion; one of them, "Cowichan" in race B from Vancouver
Island, differs from Klamath by a single inversion; "Chiricahua II," in
race A from southeastern Arizona, differs from Arrowhead by a single in-
version. All the others differ from all those so far named by more than a
single inversion.
448
VOL. 22, 1936 GENETICS: STURTEVANT AND DOBZHANSK Y
These facts give some information as to the historical relationships of the
sequences concerned. If the sequence of the loci in the standard be arbi-
trarily designated ABCDEFGH, we may represent Klamath as ADCB-
EFGH, Arrowhead as ABEDCFGH, and Pikes Peak as ABCGFEDH. It
is clear that the standard sequence could give rise to any of the three
others by a single inversion, or that any of the three could give rise to
standard by a single inversion. But it is equally clear that Arrowhead
cannot have given rise directly (i.e., by a single inversion) either to Pikes
Peak or to Klamath. If Arrowhead was the original, ancestral sequence
then Pikes Peak and Klamath are related to it (and to each other) only
through the standard, unless one assumes the occurrence of a complex
............................. .... .............
Race A Rice B
PIKES PEAK SEQUOIA
STANDARD
ARROWHEAD KLAMATH
CHIRICAHUA E CWIcHAN
Race A Rxe B
............
.........:
FIGURE 1
series of coincidences (repeated breakages of the chromosome in exactly the
same places). This method enables us to construct a phylogeny of the
types concerned, though it does not give us a method of determining the
direction in which that phylogeny should be read. For the types named
the relations are shown in figure 1.
There is no direct method of determining which is the original sequence
here; but if any one be selected, then the whole course of the evolution is
thereby fixed.- However, it seems likely that the standard sequence is, in
fact, the ancestral one, for the following reasons among others: (1) it is
centrally located in the diagram, being the only type to which several
others are directly related; (2) it is the only sequence found in both races
(A and B); (3) it has a wide geographical range, being equalled in this
respect only by the Arrowhead sequence.
None of the other chromosomes of Drosophila pseudoobscura shows a
449
GENETICS: STURTEVANT AND DOBZHANSK Y PROC. N. A. S.
diversity of sequence at all comparable to that of the third chromosome.
Tan' has shown that races A and B differ by having three inversions with
respect to each other-one in each arm of the X-chromosome, and one in
the second chromosome. We have shown2 that the right limb of the X
often has an inversion associated with the "sex-ratio" gene, but the total
number of sequences known for this limb is three. In the course of the
present study three additional inversions have been found in wild strains:
one in the second chromosome of a few strains from western New Mexico,
one in the second chromosome of an A strain from Monterey county,
California, and one in a fourth chromosome from the Mexican plateau.
There are thus known from wild strains the following numbers of sequences:
the left limb of X, two; the right limb of X, three; the second chromo-
some, four; the third, fourteen; the fourth, two. The reason for these re-
lations remains unknown; it should, however, be pointed out that the
third chromosome which shows maximum variation within the species
pseudoobscura is also the chromosome which shows maximum differences
from the closely related species, Drosophila miranda.3
As stated, many localities yield several third chromosome sequences
from a single collection. This is perhaps related to the fact that from 15
to 20 per cent of the third chromosomes tested from wild stocks have been
found to carry lethal genes. This raises the question as to whether such
stocks are in a condition approaching that of balanced lethals. The prob-
lem is now being studied; but two facts make us hesitate to conclude that
diversity of sequence and frequency of lethals are causally related. (1) In
northern and western New Mexico and in the parts of Colorado, Utah and
Arizona near the point where these four states come together, extensive
tests have failed to show any sequence other than Arrowhead; yet lethals
were recovered here in a proportion not noticeably less than that found
elsewhere. The number of tests for lethals was, however, small. (2)
Preliminary tests indicate that the frequency of lethals is about the same
in the second chromosome as in the third, despite the much greater uni-
formity in sequence in the former (complete uniformity in race A stocks
from the regions studied).
' Tan, C. C., "Salivary Gland Chromosomes in the Two Races of Drosophila pseudoob-
scura," Genetics, 20 (1935).
2 Sturtevant, A. H., and Dobzhansky, Th., "Geographical Distribution and Cytology
of 'Sex-Ratio' in Drosophila pseudoobscura and Related Species," Genetics, 21 (1936).
3 Dobzhansky, Th., and Tan, C. C., "Studies on Hybrid Sterility. III. A Com-
parison of the Gene Arrangement in Two Species, Drosophila pseudo6bscura and
Drosophila miranda." In press.
450


Inversions in the third chromosome of wild races of Drosophila pseudoobscura, and their use in the study of the history of the species

https://authors.library.caltech.edu/9528/1/STUpnas36.pdf

Inversions in the third chromosome of wild races of Drosophila pseudoobscura, and their use in the study of the history of the species

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main