ORIGINAL ARTICLE

Occurrence of multivalents and additional chromosomesin the pollen mother cells of Allium cepa L.

Geeta Sharma & Ravinder N. Gohil

Received: 10 October 2011 /Published online: 29 November 2011# Archana Sharma Foundation of Calcutta 2011

Abstract A group of six plants of Allium cepa L., a diploidspecies with 2n=16, growing in Kiharian village of Jammuprovince was studied for somatic and meiotic details. Whilethe somatic cells of these plants possessed normal chromo-some count, 14.75% of their pollen mother cells had 16plus 1–4 chromosomes. These plants were also peculiar inhaving 12.49 and 8.78% chromosomes associating asmultivalents (trivalents to hexavalents) at diakinesis and atmetaphase I respectively. Segregation of chromosomes wasirregular at anaphase I and pollen stainability very low.Besides, describing meiotic anomalies in detail, the mech-anisms likely to be responsible for the same are discussedin the present communication.

Keywords Allium cepa . Hyperploidy .Multivalentformation . Chiasma frequency

Introduction

Presence of complex chromosomal configurations in adiploid species, an indicator of translocation heterozygosity,is on record for a large number of plant taxa. Thoughmultivalents have also been observed in many diploidspecies of genus Allium [2, 5–9, 12, 14, 16], in fewnaturally occurring taxa, presence of meiocytes withvarying chromosome number is on the record. Alliumtuberosum (2n=4x=32) having pollen mother cells (PMCs)with 8–33 chromosomes and A. roylei (2n=2x=16) havingembryo sac mother cells with 16–19 chromosomes are thetwo exceptions [11, 13].

While documenting germplasm collections of species ofAllium from Indian gene centre, six plants of Allium cepa(2n=16) were collected from Kiharian village of JammuProvince (The Voucher specimen of the same has beenidentified and deposited in Jammu University Herbariumvide no. 13928). These plants were peculiar in having14.75% hyperploid PMCs (17–20 chromosomes) as well asin having multivalents. The present paper puts on record themeiotic details of these variants.

Material and methods

Karyotypic details and meiosis in male track of these plantswere studied according to the methods used earlier [12].Pollen stainability was checked by staining freshly dehiscedanthers in 1% acetocarmine.

G. Sharma (*)Department of Botany, University of Jammu,Jammu 180 006, Indiae-mail: geetaji@yahoo.com

R. N. GohilRavinder N Gohil, Centre for Biodiversity Studies, School ofBiosciences and Biotechnology, B.G.S.B. University,Rajouri, Jammu and Kashmir 185 131, India

Nucleus (December 2011) 54(3):137–140DOI 10.1007/s13237-011-0042-0

Observations and discussion

Studies on the somatic chromosomes from 4 to 5 root-tips of present plants revealed that these had 16chromosomes. Detail analysis of chromosome behaviorin the male track of these, however, indicated that while81.25% cells scored had the normal complement of 2n=16 chromosomes, 18.75% cells had 17–20 chromo-somes. Nearly half (47.5%) of the euploid cells andmajority of the aneuploid cells had multivalents inaddition to bivalents. Observations made regarding theoccurrence of various associations are presented in theTable 1, and Fig. 1.

Perusal of Table 1 revealed that 42.5% of the 40euploid cells scored at diakinesis had hexa-/quadrivalentsplus bivalents whereas remaining cells (57.5%) had eightbivalents only. Percentage of chromosomes involved inmulti- and bivalent formation were 12.49 and 87.5.Chiasmata were randomly distributed along multivalentsand bivalents, with average chiasma frequency per cellbeing 22.21. At metaphase I, 10 out of 27 (37.5%) cellsscored had tri-/quadrivalents, with percentage of chromo-somes involved in their formation being 8.78. Criticalanalysis of the multivalents at this stage for their shapesand the orientation revealed that except for a singlequadripartite structure that had chromosomes arranged inzig-zag configuration, remaining quadrivalents were ofopen ring type. Mean chiasmata frequency per cell wasless at this stage viz. 18.75.

Occurrence of 12.49% age chromosomes as hexa-/quadrivalents at diakinesis in the cells with normalcount indicate that a minimum of three non-homologouschromosomes had exchanged segments with each other.

Although, as expected, the percentage of chromosomesforming multivalents was less at metaphase I than atdiakinesis, 8.78% multivalents still retained as tri-/quadrivalents. This is possible if the chiasmata onceformed in the interchanged regions remain unresolved atmetaphase I. Though multivalent formation has earlierbeen reported in A. cepa by Koul [7] in a clone fromAgra, latter collection differed from the present in lackinghexavalents and in having quadrivalents of open- and zig-zag configurations in equal proportion. Hyperploids cellswere also missing in the latter clone.

Aneuploid cells with 17 chromosomes were differentfrom the other hyperploid cells in having a very smallsized additional chromosome. This chromosome wasfound lying singly at metaphase I and not in associationwith the normal chromosomes. In having smaller size,its presence in a few cells and non-pairing with thenormal chromosomes of the complement are some ofthe features that this chromosome shares with the Bs[4]. Amongst Alliums, chromosomes resembling Bs areearlier on record for A. ampeloprasum, A. paniculatumand A. schoenoprasum [3, 9, 14].

Two-four additional chromosomes in the remaininghyperploids cells were nearly of the same size as thenormal chromosomes of the complement. Like the euploidcells, later cells also had multivalents plus bivalents(Table 1), though the number of chromosomes involved inmultivalent formation varied in the cells with differentchromosome counts. While PMCs with 18 chromosomeshad a single quadrivalent, in the cells with 20 chromo-somes, three quadrivalents were present. Hyperploid meio-cytes with few extra chromosomes can originate as a resultof non-disjunction of chromosomes during the premeioticmitosis, probably as a result of spindle related abnormal-ities. Sapre and Naik [10] held same view for explaining theorigin of deviant type PMCs with ±1–4 chromosomes inCoix gigantea.

At anaphase I, 15 euploid cells were available. Ofthese, 10 (62%) showed regular segregations whereasremaining exhibited unequal disjunctions such as 7:9 or6:10. According to Swanson [15], there are three possiblearrangements viz. one alternate and two adjacent, wherebyfour chromosomes constituting a quadrivalent can segre-gate. Burnham [1] however opined that for each adjacenttype arrangement, there is an equal opportunity of analternate type orientation. While the alternate arrangementproduces viable gametes, adjacent arrangements result inproducts deficient for some and duplicate for othersegments and hence, inviable gametes. In the presentcytotypes, very low pollen stainability probably is owingto predominance of open ring quadrivalents whichundergo adjacent segregations.

Table 1 Chromosome associations observed in 80 PMCs of A. cepa

Chr. Number Meiotic Stage(No. of PMCs)

Associations(No. of PMCs)

Figs.

16 Diakinesis (40) 1VI+1IV+3II (2)

1IV+6II (15)

8II (23)

Metaphase I (27) 1IV+6II (8) 1a, b

1III+6II+1I (2)

8II (17)

16+1small chrs. Metaphase I (3) 1IV+6II+1I (2) 1c, d

8II+1I (1)

18 Diakinesis (7) 1IV+7II (4)

9II (3)

20 Diakinesis (3) 3IV+4II (3) 1e, f

138 Nucleus (December 2011) 54(3):137–140

Presence of hyperploid meiocytes and multivalentspredominately of alternate type seem to have adverselyaffected the reproductive capacity of the present cytotype,

most probably by formation and fusion of geneticallyimbalanced gametes. Very low pollen stainability (4.5%)and sterility manifest these observations.

Fig. 1 Pollen mother cells ofAllium cepa L.: a and cPollen mother cells at metaphaseI with16 (1IV+6II) and 17chromosomes (1IV+6II+1 smallchromosome); b and d Explan-atory diagrams for figures a andc; e A Pollen mother cell atdiakinesis with 20 chromosomes(3IV+4II); f Explanatory dia-gram of figure e . (quadrivalentsare highlighted using biggerarrows and small chromosomein figure c is highlighted usingsmall arrow). Bar=10 μ

Nucleus (December 2011) 54(3):137–140 139

Acknowledgements The authors are grateful to the Head,Department of Botany, University of Jammu, Jammu for providing thenecessary facilities and Council of Scientific and Industrial Research,Govt. of India, New Delhi for the award of post-doctoral fellowshipto GS.

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