CARYOLOGIA
Vol. 57, no. 2: 158-162, 2004
Chromosomal chimeras in the male track of Allium tuberosum Rottl. ex Spreng.
Geeta Sharma and R.N. Gohil*
Department of Botany, University of Jammu, Jammu − 180 006, India.
Abstract — During routine morphological and cytogenetic studies in the seed progeny of Allium tuberosum, an abnormal plant with high flower abortion in some inflorescences was noticed. Detailed somatic and meiotic studies revealed that although this plant possessed the normal chromosome complement (2n=4x=32), the PMCs in three inflorescences had varying number of chromosomes. While 67.77% PMCs scored had normal chromosome constitution, the number ranged from 8 - 33 in 32.22 % cells. Besides describing this anomaly in detail, the mechanisms likely
to be responsible for the same are also discussed.
Key words: Allium tuberosum, chimeras, male meiosis.
INTRODUCTION
A suspected autotetraploid with weak desynapsis
and apparently apomictic nature, A. tuberosum is cytologically a very interesting taxon. During the
course of our work on the species to fully understand
its nature and breeding behavior, chromosomal chimeras were observed in the anthers of some inflorescences.
The present report, based on the observations
made during two successive years in a population of
31 plants of the species raised from the seed, puts on
record the details of this anomalous behavior, hitherto unrecorded from this species.
MATERIALS AND METHODS
Karyotypic details and meiosis in male track of these
inflorescences were studied following the methods used
earlier (Sharma and Gohil 2003). Pollen stainability
was checked in 1% acetocarmine.
OBSERVATIONS
During the routine screening of seed progeny of
A. tuberosum, three inflorescences appeared abnormal. In all these inflorescences, compared to the rest,
* Corresponding author: R. N. Gohil;
e-mail: [email protected].
a great deal of flower abortion was noticed. As a result of this, while healthy growth leading to viable
seed formation was routine in other inflorescences,
these inflorescences did not produce any viable seed
at all. Interestingly, of the ten inflorescences arising
from the same rhizome, A. tuberosum being rhizomatous, only three showed this characteristic.
Studies on the somatic chromosomes from root tips
of this plant revealed that it is normal with 2n = 4x =
32. These chromosomes, on the basis of their gross
morphology, resemble those found in other plants of
this species.
Detail analysis on the chromosome behavior in
the male track of these inflorescences, however, revealed that their pollen mother cells had varied
number of chromosomes (Table 1). A perusal of Table 1 reveals that while 67.77% cells scored had normal (2n = 32) chromosomes, in 28.51% of the cells
scored the number ranged from 8 - 33. Since, only
very clear cells, with intact cell walls, where the deviation from the normal number could be detected
without any doubt alone were scored, there is every
possibility of the frequency of such cells being actually on the higher side. In fact, in ten more cells
(3.70%) also the number appeared at variance from
2n = 32. As the deviation was observed in PMCs at
all the stages of division, the frequency of cells with
various chromosome numbers has been calculated in
totality, irrespective of stage/stages. It is also quite
clear from Table 1 that in 85.71% of the deviant cells
the chromosome number is either equal to or more
than half the total chromosome complement of this
species viz. n = 16. It is also interesting to note that
chromosome number in most of the deviant cells is
less than the normal chromosomal complement (2n
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chromosomal chimeras in the male track of allium tuberosum rottl. ex spreng
Table 1 - Number and %age of pollen mother cells with variable number of chromosomes.
Number of chromosomes
Number of cells
% age of cells
8
10
12
15
16
19
20
22
24
25
26
28
29
30
31
33
Others with lesser no.
2
2
1
1
5
2
4
5
9
2
4
11
6
12
10
1
10
2.29
2.29
1.14
1.14
5.74
2.29
4.59
5.74
10.34
2.29
4.59
12.64
6.89
13.79
11.49
1.14
11.49
Fig. No.
1
2
3
4
Total number of PMCs = 270
PMCs with deviant no. of chromosomes = 87 (32.22%)
PMCs with normal chromosome number = 183 (67.77%)
= 32), except one where it is more. Moreover, most
of the deviant cells are with even number of chromosomes.
The phenomena of desynapsis and autobivalent
formation reported by earlier workers in A. tuberosum (Gohil and Kaul 1981), were observed in the
cells with deviant number of chromosomes too.
34.3% of the cells scanned at diplotene and metaphase I exhibit desynapsis. As the species is a tetraploid (auto- or segmental alloploid?), chromosomes
pair to form multivalent associations involving 3 - 8
chromosomes. Table 2 depicts the %age frequency
of chromosomes forming various associations both
in the PMCs of normal plants and those with chromosomal chimeras. In some of the chimeric cells observed at diplotene, metaphase I and anaphase I with
low number, chromosomes appeared morbid (Fig.
6). Such cells were also smaller in size. Because of
this reason, it becomes difficult to determine the exact stage of division in these cells.
Meiotic abnormalities observed both at diplotene and metaphase I, also get reflected at anaphase
I, though in lesser frequencies. Table 3 depicts the
%age of cells with varying number of chromosomes
and their segregation patterns at anaphase I. Of 68
cells studied at anaphase I, 70.58% cells are with 32
chromosomes (16:16/17:15 segregations), 10.29%
cells are with 64 chromosomes (32:32 segregation)
due to autobivalent formation and 19.11% with
lesser number of chromosomes viz. 19, 20, 28, 30
and 31 with abnormal segregations (Table 3). As expected, micronuclei and/or chromatin bridges and
deformed nuclei were observed in nearly 50% cells
at telophase I. Pollen stainability in the flowers of
these inflorescences was as low as 0.07%. As mentioned earlier, flowers in these inflorescences either
did not set any seed or a few shrivelled seeds obtained failed to germinate.
DISCUSSION
Anomalous behavior of any kind in the most essential component of a cell, the chromosomes, is
Table 2 — Comparison of percentage of chromosomes involved in forming various associations in the PMCs of normal plants and
those with chromosomal chimeras at diplotene and metaphase I.
Associations
Normal plants
Plants with Chimeras
VIII
VI
V
IV
III
II
I
0.65
1.66
0.19
86.26
0.34
10.75
0.14
1.48
5.00
—
65.50
1.94
25.74
0.27
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sharma and gohil
LEGENDS
Pollen mother cells with different number of chromosomes in Allium tuberosum.
Fig. 1 — Pollen mother cell at metaphase I with 10 chromosomes (1 IV and 3 IIs). Figs. 2 - 5. Desynaptic PMCs with 15, 16, 28 and
32 chromosomes respectively. Fig. 6. PMC at diplotene with a few distorted chromosomes. Figs. 7 - 8. PMCs at anaphase I with 20
(11: 9 segregation) and 31 (17:14 segregation) chromosomes respectively (Scale = 10 µm).
both interesting and worth enquiry. The present report of chromosome chimeras in the male track of A.
tuberosum is an addition to its well documented cytological aberrations such as tetraploid nature (autoor segmental alloploid?), desynapsis, autobivalent
formation and suspected apomictic condition
(Mathur and Tandon 1965; Gohil and Koul 1973;
Gohil and Kaul 1981; Kojima et al. 1992). While
lot of information is available on all these aspects, the
present report, to the best of our knowledge, is the
first on the occurrence of chromosomal chimeras in
the species.
As has been brought out earlier, 32.22% of
PMCs in three inflorescences of the species had de-
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chromosomal chimeras in the male track of allium tuberosum rottl. ex spreng
Table 3 — Anaphase I segregation patterns in sixty-eight cells.
Chromosome number (2n)
Chromosome segregation
Number of cells
% age
19
20
28
30
10:9
11:9
15:13
15:15
16:14
17:13
17:14
18:13
16:16
17:15
32:32
2
1
2
2
1
1
2
2
45
3
7
2.94
1.47
2.94
2.94
1.47
1.47
2.94
2.94
66.17
4.41
10.29
31
32
64
viant number of chromosomes. In most of these
PMCs, the number observed was less than the somatic complement. Since, the phenomenon was observed during two successive years in the inflorescences of the same plant, it appears to have got established. The interesting part, however, is that only inflorescences arising from the same rhizome exhibit
the phenomenon.
The phenomenon of occurrence of chromosomal
chimeras is quite rare and has been reported only in a
few taxa. The only other such record in Allium is its
presence in the colchicine treated plants of Shallot
(Vodyanova 1980). Similar observations have also
been made in Saccharum hybrids (Nair 1972), Petunia axillaries (Padmaja 1988) and Capsicum annuum
(Harini et al. 1990 and Lakshmi et al. 1991). It is important to point out that in most of the earlier reported cases, plants with chromosomal chimeras
were the product of genetically unstable parents.
They were either induced hybrids such as Saccharum
(Nair 1972) and Petunia axillaries (Padmaja 1988)
or chemically treated one as reported in Capsicum
annuum (Harini et al. 1990). On the other hand,
tetraploidy in A. tuberosum is not an induced one. In
addition, in most of the earlier known cases, the
range of variation reported in the number of chromosomes in the PMCs was not to the extent as has
been observed in A. tuberosum. In the present case
chromosome number varies from 8 - 33. Another important point is that in A. tuberosum the aberration is
of spontaneous origin, that too confined to a few inflorescences. The very fact that chimeras were observed in two successive years and in the inflorescences arising from the same rhizome can be a
pointer towards some genic changes resulting in this
aberration. The present observations are somewhat
similar to the earlier report of such a phenomenon
observed by Sapre and Naik (1990) in Coix gigantea.
In both these cases (Coix gigantea and A. tuberosum), chimeras have been observed in some inflorescences of the same plant only. These, however, differ
Fig. No.
7
8
in range of variations. While in Coix gigantea the deviant PMCs had either one / two chromosomes in
addition or missing, in the present case no PMCs, except one, are with number more than 2n. While describing this anomaly in Coix gigantea, Sapre and
Naik (1990) have tried to explain the origin of such
PMCs as a result of nondisjunction of chromosomes
during premeiotic mitosis, either in the normal
course of division or after somatic pairing. Although
the explanation seems quite feasible, it will not be inappropriate to record that it lacks any proof. Moreover, Chennaveeraiah and Wagley (1985) held endoreduplication responsible for mosaicism. This
phenomenon, though quite frequent in both pollen
mother cells and megaspore mother cells of A. tuberosum (Gohil and Kaul 1981; Kojima et al. 1992)
does not seem to be responsible for the anomaly in
present case, as it is a normal feature noticed regularly in this species. As such, if endoreduplication
leads to mosaicism then all the plants of A. tuberosum should express this aberration.
While documenting and analysing the variations
observed in the number of chromosomes in PMCs, it
is important to point out that cytomixis is the most
frequent cause attributed for this aberration. Cytomixis has been reported to result in PMCs with a
much wider range in chromosome numbers. In Astragalus subuliformis (2n = 16), PMCs were found to
have 4 - 32 chromosomes (Ashraf and Gohil 1994).
In view of this, one cannot ignore the possibility of
cytomixis as one of the reasons behind the variation
observed in the number of chromosomes in the
PMCs. Since cytomixis is always inferred from the
presence of cytoplasmic connections between the
PMCs, the possibility of these connections having
formed at earlier stages of meiosis and lost subsequently cannot be ruled out. Chromosomal chimeras, whether formed as a result of disturbances during the premeiotic mitosis or early or later stages of
meiosis, are quite interesting and warrant further in
depth investigations.
162
sharma and gohil
Acknowledgements — The authors are grateful to
the Department of Botany, University of Jammu,
Jammu for providing the necessary facilities and DST,
Govt. of India, New Delhi for financing the project.
REFERENCES
Ashraf M. and Gohil R.N., 1994 — Cytology of legumes of Kashmir Himalaya. V. Cytomixis and chromosome migration in Astragalus subuliformis DC.
The Nucleus, 37: 119-122.
Chennaveeraiah M.S. and Wagley S.K., 1985 —
Chromosome mosaicism in cultivars of garden crotons (Codianeum variegatum Blume). The Nucleus,
28: 8-13.
Gohil R.N. and Kaul R., 1981 — Studies on male and
female meiosis in Indian Allium. II. Autotetraploid
Allium tuberosum. Chromosoma, 82: 735-739.
Gohil R.N. and Koul A.K., 1973 — Bearing of meiotic
irregularities on karyotype alteration in some polyploid taxa of genus Allium. Advancing Frontiers in
Cytogenetics, Hindustan Publ. Corp., 136-142.
Harini I., Lakshmi N. and Prakash N.S., 1990 — A
new report of chromosomal chimaera in Capsicum
annuum L. Cytologia, 55: 649-654.
Kojima A., Nagato Y. and Hinata K., 1992 —
Apomixis in Allium tuberosum and Allium ramosum. In: The genus Allium- taxonomic problems
and genetic resources. (eds, P. Hanelt, K. Hammer
and H. Knupffer) Institut fur Pflanzengenetic
und Kulturpflanzenforschung, Gateresleben, Germany.161-165.
Lakshmi N., Prakash N.S., Harini I. and Srivalli T.,
1991. - Meiotic studies in some mutants of red pepper
(Capsicum annuum L.). J. Indian Bot. Soc., 70: 347350.
Mathur M. and Tandon S.L., 1965 — Cytology of an
autotetraploid Allium tuberosum. Ind. J. Hort., 22:
382−384.
Nair M.K., 1972. - Cytogenetics of Saccharum officinarum L., Saccharum spontaneum L. and S. officinarum X S. spontaneum hybrids I. Chromosome mosaics. Cytologia, 37: 565- 573.
Padmaja V., 1988 — Studies on manifold abnormalities
at meiosis in Petunia (2n=14). Cytologia, 53: 199204.
Sapre A.B. and Naik A.S., 1990 — Formation of aneuploid gametes through nondisjunction during premeiotic mitoses in Coix gigantea. Cytologia, 55: 71-77.
Sharma G. and Gohil R.N., 2003 — Cytology of Allium roylei Stearn. I. Meiosis in a population with
complex interchanges. Cytologia, 68: 115 − 119.
Vodyanova D.S., 1980 — Production of high yielding
shallot forms by means of colchicine treatment and
their use in breeding. Selskokhozyaistvennaya. Biologiya, 15: 145-146.
Received 1.7.2003; accepted 21.9.2003