Chromosome Number Variation in Ocimum basilicum L. - J-Stage

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Summary Meiotic analysis documented variation in chromosome number in Ocimum basilicum L. (family: Lamiaceae) in the form of 2n 72 chromosomes in O.
© 2005 The Japan Mendel Society

Cytologia 70(4): 455–458, 2005

Chromosome Number Variation in Ocimum basilicum L. Moumita Mukherjee, Animesh K. Datta * and Gour Gopal Maiti Department of Botany, Cytogenetics and Plant Breeding Section, Kalyani University, Kalyani-741235, West Bengal, India Received October 11, 2005; accepted October 24, 2005

Summary Meiotic analysis documented variation in chromosome number in Ocimum basilicum L. (family: Lamiaceae) in the form of 2n72 chromosomes in O. basilicum var. citriodorum (lemon basil) and 2n52 chromosomes (a new number in Ocimum) in variety crispum. Persistent presence of secondary association of chromosomes in 87.88% and 69.39% metaphase I cells of var. citriodorum and var. crispum respectively suggested secondary polyploid nature of the species. Statistical analysis of cytological data revealed x12 as basic chromosome numbers for the varieties studied and polyploidy as well as aneuploidy might have been operative at different levels to give variable chromosome numbers within O. basilicum. Key words Ocimum basilicum var. citriodorum, O. basilicum var. crispum, Meiosis, Secondary association of chromosomes, Basic chromosome number, Chromosome number variations.

Ocimum basilicum L. (family : Lamiaceae) is an aromatic herb (native of Tropical Asia and naturalized throughout Tropical Africa, Asia, Europe and America) yielding essential oil of commerce and possessing antioxidant (Juliani and Simon 2002) and anticancerous (Aruna and Sivaramakrishnan 1996) properties. Karyomorphological studies performed in different varieties (O. basilicum var. minima, var. thyrsiflora, var. glabratum, var. purpurascence and var. difforme) and specimens collected from W. Africa and Tropical Asia revealed that the chromosome number in O. basilicum is 2 n48 (Vaarama 1947, Morton 1962, Pushpangadan et al. 1975, Pushpangadan and Sobti 1982). Mehra and Gill (1972) from meiotic chromosome analysis reported the same number (2 n48) for the species from Western India; however, chromosome count of 2 n16 has also been suggested (Sz.- Borsos 1970). Mukherjee and Datta (2005) documented 2 n72 chromosomes in O. basilicum from comprehensive meiotic chromosome analysis and the variety analyzed by the authors was O. basilicum vap. basilicum (sweet basil). The number 2 n72 had earlier been reported in Ocimum only for the species O. americanum L. (Pushpangadan et al. 1975). Critical analysis of taxonomic data of the genus Ocimum is provided that O. americanum is the synonym of O. basilicum L. (Morton 1962, Banerjee and Maiti 2000). Thus, there exist variations in chromosome number in the species. Present communication describes meiotic chromosome behaviour of O. basilicum var. citriodorum (lemon basil) and O. basilicum var. crispum as a part of research initiated in Ocimum species and varieties for their cytological characterization aiding to the identification of potent chemotypes. Materials and methods For meiosis , flower buds of suitable sizes from five randomly selected plants (raised in the Experimental plots of Kalyani University from seeds of var. citriodorum obtained from Medicinal Plant Garden, NIH, Govt. of India and those of var. crispum from National Bureau of Plant Genetic * Corresponding author, e-mail: [email protected]

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Cytologia 70(4)

Figs. 1–9. Figs. 1–3) Plant type of Ocimum basilicum var. citriodorum (1) showing 2 n72 chromosomes at diplotene (2–34 II4 I; 3–36 II). Figs. 4–9) Plant type of O. basilicum var. crispum (4) showing 2 n52 chromosomes at diplotene (5–24 II4 I), MI (6–chromosomes in 12 groups; 7–26 II) and AI (8–25 : 27; 9–20 : 32). Univalents were marked (→).

Resources, Pusa, New Delhi, Accession No. IC212802; voucher specimens deposited in the Herbarium, Botany Department, Kalyani University) of O. basilicum var. citriodorum (lemon scented annual herb, leaves elliptic, broadened at middle, base cuneate to unequal–Fig. 1, flowers white; var. citriodorum is identical in phenotype to var. basilicum excepting that basilicum does not posses lemon scent and had light purple flowers) and O. basilicum var. crispum (plants diffusely branched, lateral branches more or less equal in length with the main shoot forming a corymbose appearance, leaves ovate, margin crisped-serrate, base equal, thick and somewhat fleshy–Fig. 4; flowers deep purple) were fixed (6 to 7 am) in Carnoy’s fluid and 3 changes were given in the fixative at an inter-

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Chromosome Number Variation in Ocimum basilicum L.

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val of 48 h. Anthers were squashed in 2% propionocarmine solution. Cytological data presented in the text has been pooled over the plants. Photomicrographs were taken from temporary squash preparations. Results and discussion Meiotic analysis O. basilicum var. citriodorum: The meiocytes had 2 n72 chromosomes (Figs. 2,3) always at MI (mean association: 35.76 II0.48 I per cell) mostly forming 36 II (86.36%, 132 cells scored). PMCs with 35 II2 I (7.58%), 34 II4 I (3.03%), 33 II6 I (1.52%) and 32 II8 I (1.52%) were also noted. Bivalent configurations assessed at diplotene (92 PMCs scored) showed prevalence of rod (range 25 to 35/cell, mean 30.10.21/cell) than ring (range 1 to 10/cell, mean 5.70.19/cell). Mean chiasma per bivalent was studied to be 1.150.01. The chromosomes at MI formed secondary groups (87.88%cells) of 3 (3.45%), 5 (3.45%), 6 (20.69%), 8 (13.79%), 9 (18.97%) and 12 (39.66%). About 77.78% AI cells (99cells scored) were cytologically balanced (36 : 36), while the rest had laggards (1 to 4 in 16.16% cells) and bridges (6.06%). O. basilicum var. crispum: PMC squashes revealed 2 n52 chromosomes (Figs. 5, 7, 8, 9) always at MI (mean association : 25.64 II0.73 I per cell) with predominance of 26 II (83.50%) formation (206 cells scored). Cells with 25 II2 I (8.74%), 24 II4 I (2.43%), 23 II6 I (1.46%), 22 II8 I (1.46%), 21 II10 I (1.94%) and 20 II12 I (0.49%) were also observed. Diplotene (148 cells assessed) showed more of rod bivalents (range 17 to 24/cell, mean 21.030.13/cell) than rings (range 2 to 9/cell, mean 4.770.13/cell) with an average of 1.180.01 chiasma per bivalent. About 79.61% MI cells demonstrated secondary chromosome associations and the chromosomes tended to form variable groups (Fig. 6) of 4 (4.88%), 5 (2.44%), 6 (23.17%), 8 (14.63%), 9 (7.32%), 10 (2.44%) and 12 (45.00%). AI cells were cytologically (26 : 26) balanced (96.03%, 126 cells examined) with occasional formation of laggards (1 to 2), unequal separation of chromosomes (25 : 27–Fig. 8 and 20 : 32–Fig. 9) and bridges in 1.59, 1.59 and 0.79 per cent cells respectively. Statistical analysis of cytological data relating to secondary groupings of chromosomes c 2 test revealed non-random assortment of the group classes (var. citriodorum: c 263.19 at 5 DF, p0.001; var. crispum: c 2166.18 at 6 DF, p0.001) with preference of 12 group class in citriodorum (observed: 12 group class 46, rest 70; expected: 12 group class 19.33, rest 96.67, total 116; c 244.16 at 1 DF, p0.001) and 12 and 6 in crispum (observed: 12 group class 74, rest 90; expected: 12 group class 23.43, rest 140.57, total 164; c 2127.34 at 1 DF, p