KARYOTYPE OF A SEI WHALE
TOSHIO KASUYA
The chromosomes of the Mammalia have been studied for a long period, and many
species of mammals except those belonging to the Lemures, Proboscidea, Hyracoidea and Sirenia were analyzed their karyotypes (Matthey, 1949).
In the order Cetacea, Makino (1948) first reported a study on the chromosomes of Dall's porpoise, Phocoenoides dalli (True). He concluded that the diploid
number of the chromosome is 44 and it has sex chromosomes of XY type. And
also he pointed out the resemblance between the karyotype of the Dall's porpoise
and that of the Ungulata rather than that of the Carnivola.
Nowosielski-Slepowron and Peacock (1955) reported a study on the chromosomes of the blue, fin and sperm whales. In this report chromosome numbers
were not decided and were only suggested the approximate diploid number 48 in
the above three species.
I report here a study on the chromosomes of the sei whale based on a testes
sample obtained from the North Pacific.
I am much indebted to Dr. M. Sasaki of the Hokkaido University, who gave
me valuable suggestions on the cytological technique. Greater thanks are due to
Prof. T. Hibiya of the University of Tokyo who kindly discussed the draft.
MATERIAL AND METHOD
The material used in this report was collected from a sei whale captured in
the northern part of the North Pacific in the summer season of 1966 by a Japanese
whaling factory ship. In the external characters and internal organs of this whale,
no abnormality was found.
The animal had been dead about 4 and a half hour when the testicular material
was collected. After removal from the animal, the piece of testicular tissue was
minced with sharp scissors on a small petri dish without adding any saline. 2 ml of
0.6% hypotonic sodium citrate solution was added to about 0.5 ml of the tissue shred
and mixed. Then it was allowed to stand at the room temperature for about 40
minutes. Then, the cells suspending in the hypotonic solution were fixed by adding
6 ml of acetic alcohol (acetic acid 1 : methanol 3) for about 30 minutes. The suspension was kept undisturbed until relatively larger tissue shreds had settled down
to the bottom of the tube. Then the upper layer of dispersed cell suspension was
separated.
After centrifugation at 3,000 r.p.m. for about 3 minutes, the supernatant was
decanted, and 6 ml of fresh fixative was added. The fixative was changed three
times at the interval of about 20 minute or more. The cell pellet of the final centri83
84
T. KASUYA
fugation was resuspended in 2 ml fresh :fixative.
Some preparations for microscope were made on the factory ship after
the method of Sasaki (1964). But most of the preparations were made by the same
method at The Whales Research Institute, after the cell suspension was stored in a
freezer for about 4 months.
For staining the slides, diluted Giemsa solution ( 1 : 20) was applied for 10 to
20 minutes.
NUMBER OF CHROMOSOMES
Chromosomes in spermatogonia
The material provided fairly numerous spermatogonial nuclei undergoing
division. And many of these were relatively free from overlapping of chromosomes,
which allowed the counting of number. Most of these nuclei contained one or
two of the nucleoli stained pale, which suggested that these cells are not in the
stage of metaphase but probably in the late prophase.
Spermatogonial nuclei with diploid chromosomes were easily recognized from
those of the spermatocyte by their large chromosome number and the simple shape
of each chromosomes.
TABLE I.
Number of chromosomes
Number of occurrence
Percentage of occurrence
NUMBER OF CHROMOSOMES OBSERVED IN THE
SPERMATOGONIAL NUCLEI
40
41
42
43
44
45
46
47
48
49
Total
2
2
38
3
4
1
54
3.7 3.7 1.9 1.9 70.3 5.4 7 .4 1.9 1.9 1.9 100.0
By the treatment of the hypotonic sodium citrate solution, the chromosomes
in some spermatoganial nuclei revealed the chromatids and the position of the attachment of fibre. They were used for the morphological investigation mentioned
later.
The number of diploid chromosomes was counted after their relative arrangement in a nucleus was sketched. In some nuclei which seemed difficult to count
the chromosomes, above procedure was repeated independently two or three times.
And the coincided number was taken.
The chromosome number was counted on 54 spermatogonial nuclei by this
method. Its results is shown in Table 1. It shows that about 70 percent of the
nuclei examined gave counts of 44, though the counts falls relatively wide range,
41 to 49 in diploid number. And it can be concluded that 44 is the diploid number
of the chromosomes of the sei whale.
Chromosomes in spermatocytes
The spermatocyte nuclei undergoing division were not common comparing
with those of spermatogonia. Four primary spermatocytes were observed with
spreaded chromosomes. And only one secondary spermatocyte nucleus was observed
with the chromosomes spreading fairly well.
85
KARYOTYPE OF A SEI WHALE
Though, only from these spermotocyte nuclei the final conclusion on the number
of chromosomes of the sei whale could not obtained, all of them suggested the diploid
number of chromosomes to be 44.
MORPHOLOGY OF THE CHROMOSOMES
Morphological measurements were made on the enlarged photograph of 10 spermatogonial nuclei which showed fine structure of the chromosomes, in parallel
with microscopical observation.
After the total length and the length of the shorter arm of each chromosomes
were measured, the total length was standardized by the following formula.
R.L.
TABLE 2.
= Total length of a chromosome
X l,OOO
Length of (21A's+X)
R.L. : Relative length of a chromosome
A: Autosome
X : X chromosome
RELATIVE LENGTH, PERCENTAGE OF SHORTER ARM AND
RATIO OF ARMS1l
Relative length
Serial No.
1
2
3
4
5
6
7
8
9
10
11
12
x
13
14
15
16
17
18
19
20
21
y
1)
2)
range
mean
76-102
67-83
62-78
58-65
53-58
49-53
47-49
44-48
43-47
42-46
41-46
39-44
39-45
37-42
34-42
34-41
29-39
29-38
27-35
25-34
24-27
17-22
7-12
86
73
67
60
55
51
49
46
46
44
43
42
42
40
39
38
35
35
32
30
27
20
9
Percentage of
shorter arm
Ratio of
arms•l
18
30
23
26
26
23
32
32
44
34
37
46
43
35
32
40
40
32
33
41
44
29
4.5
2.3
3.3
2.8
2.8
3.4
2.1
2.1
1.3
1.8
1.7
1.2
1.4
1.9
2.1
1.5
1.5
2.1
2.0
1.4
1.3
2.4
Mean value obtained from ten spermatogonial nuclei.
Calculated from the former column.
86
T.KASUYA
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KARYOTYPEOFA SEI WHALE
87
t
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ea
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ve.
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nt
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eka
r
y
o
t
y
peoft
hes
e
iwhal
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sc
ompar
e
d wi
t
ht
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t of t
h
e Da
ll
'
s
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r
p
o
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e(Makino, 1
9
4
8
)s
omer
e
s
embl
ane
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ef
ound,namel
yt
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o
t
a
lnumbe
r
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h
r
omos
omewitht
h
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e
n
g
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r
e
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p
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n
d
i
n
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th o
r1
1
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nt
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ec
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s
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e
i
wha
l
e1
2
th)
,Y c
hromosomeswi
t
hs
hor
t
e
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e
ngt
hand autosome
sc
omp
o
s
e
dofme
t
a
c
e
n
t
r
i
ca
nda
c
r
o
c
en
t
r
i
cc
le
me
n
ts
. Butwhe
ncompar
e
dpr
e
c
i
c
el
y
,s
omed
if
r
er
e
nc
e
s
a
r
ef
ou
n
d
. I
nt
h
ec
a
s
eoft
heDal’
sporpoi
s
e(
Mak
i
no,1
9
48)t
h
e5t
h and 6t
h
e
leme
n
t
sar
em
e
t
a
c
e
n
t
r
i
cc
hromos
ome
s and o
t
h
e
rr
el
a
ti
v
e
l
yl
a
r
ge
rc
hromosomes
a
r
et
h
eac
r
o
c
e
n
t
r
i
c,a
ndhes
ug
g
e
s
tt
heme
mbe
r
soft
he9
th t
ot
h
e2l
s
tp
a
i
r
st
o be
a
c
r
o
c
e
nt
r
i
c
.
I'
88
T. KASUYA
Among relatively larger autosomes of the sei whale, the 9th and 12th are
metacentric or V shaped chromosomes. And I think that these differences ofkaryotypes between the two species are not unreasonable, when it is considered that they
belong to the different suborder or Odontoceti and Mysticeti.
REFERENCES
LEwrs, K. R. & JoHN, B. (1963). Chromosome marker. 489 p. J. & A. Churchill, London.
MAKINO, S. (1948). The chromosomes of Dall's porpoise, Phocoenoides dallii (TRUE), with remarks on the
phylogenetic relation of the cetacea. Chromosoma, 3, 220-31.
- - (1963). The chromosomes ef man. 199 p. Kinokuniya, Tokyo. (In Japanese).
MATTHEY, R. (1949). Monographies scientifiques, Les chromosomes des vertebres. 356p. E. Rouge, Lausanne.
- - (1951). The chromosomes of the Vertebrates. Advances in Genetics, 4: 159-80.
NowosrnLsKr-SLEPOWRON, B.J. A. & PEACOCK, A. D. (1955). Chromosome number in the blue, fin and
sperm whales. Proc. Zool. Soc. Edimb. B. 65: 358-68.
SASAKI, M. (1964). Notes on polyploidy in human male germ cells. Proc. Japan Academy, 40 (7): 553-57.
EXPLANATION OF PLATES
Fig. 1-3.
Fig. 1.
Fig. 2.
PLATE I
Spermatogonial nuclei of the sei whale at the late prophase of the division.
PLATE II
Spermatogonial nucleus of the sei whale at the late prophase of the division.
Spermatocyte nucleus at the second division.
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