LETTER TO THE EDITOR
Chromosomes of Drosophila recticilia
I n a recent issue of GENETICS (85: 85-93) a n article was published on
“Chromosomal polymorphisms of constitutive heterochromatin and inversions
in Drosophila” by VISUTBAIMAI.I would like to comment on the description of
the metaphase configuration of Drosophila recticiliu which was presented in
this paper. BAIMAIdescribed the karyotype of this species as consisting of two
pairs of large V-shaped chromosomes. two large J-shaped pairs, one pair of
small J-shaped chromosomes and a pair of dot chromosomes. This is in contrast
to m y previously published accounts in 1968 and 1971 in which the karyotype
was described as consisting of five pairs of rods and one pair of dots, with two
pairs of the rods appearing as double-length chromosomes. This earlier analysis
of the chromosomes of D. recticiliu utilized conventional aceto-orcein squash
preparations with no pretreatment. Figure 1 is a photograph of a n anaphase
configuration from one of these preparations (M71B2, Kaupo Gap, Maui).
BAIMAI’Stechnique for preparation of brain ganglia is a modification of the
Giemsa method which includes use of colcemid and hypotonic solution. It has
been known for a number of years that such pretreatment alters the usual metaphase chromosome configuration, resulting in X-shaped bodies for V-shaped
chromosomes and V-shaped structures for rod chromosomes. Staining procedures
designed to identify heterochromatic regions of the Drosophila mitotic chromo-
FIGUREI .-Aceto-orcein squash preparation of D. recticilia, Kaupo Gap, Maui. Collection
#M71B2. Original magnification 1215x.
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LETTER TO THE EDITOR
somes result in similar configurations. We have found that the large blocks of
heterochromatin in D.uirilis rods are compact and heavily stained; the sister
chromatids are closely paired in the heterochromatic regions, but separate in the
euchrolmatic regions.
It is my opinion that BAIMAIhas incorrectly interpreted the chromosome configurations for D.rectici2ia. On the basis of his diagrams and his discussion, it
appears that he considers that the centromeres of the major chromosomes are
loicated at the base of the euchromatic portions and that the blocks of heterochromatin form additional chromosome arms, resulting in V- or J-shaped chromosomes. However, on the basis of the previous aceto-orcein preparations and
the similarity in appearance of the D.recticilia chromosomes to rod-shaped
chromosomes of other Drosophila species, I believe that BAIMAIwas observing
rod-shaped chromosomes with large blocks of heterochromatin in the proximal
regions of the chromosomes.
In the analysis of Drosophila mitotic chromosomes, I am convinced that metaphase configurations should be based on observations from conventional acetoorcein smears as well as from specialized preparations, unless the technique
being utilized by the investigator is such that the location of the centromere is
readily observed.
FRANCES
E. CLAYTON
Department of Zmlogy
University of Arkansas
Fayetteville, Arkansas 72701