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Chromosome Identification in Papaya (carica Papaya)

M.A. Hague
Department of Genetics and Plant Breeding, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

Abstract/ Executive Summary:

An experiment was conducted to investigate the general cytological condition of papaya. Seed germinated healthy roots were collected from the three different strains of papaya cultivars (P-1, P-2 & P-3). Slides were prepared following 2.5 hours of pretreatment by a saturated aqueous solution of monobromonaphthalene (MBN), hydrolysis in 10% HC1 for 10 minutes at 60°C, Mordanting in 2% iron-alum for 4 minutes and staining in 0.5% haematoxyline for 4 minutes-schedule. Chromosome preparations were done so as to obtain them in well-spread condition and in one focal plane of the microscope objective lens. Chromosome measurements were done from the prints of the photomicrographs of the chromosome plates. In all the three papaya strains 2n = 18 chromosomes were found. The diploid complement contained two pairs of satellited chromosomes. The range of the total length of the individual chromosome was 9.3 11 to16.0 pt in P-1, 7.21...t to 16.0 v. in P-2 and 9.211 to 18.0 pi, in P-3.

Though the general composition of the haploid karyotype in the three papaya strains studied was the same, yet some variation in respect of length and arm ratio of the individual chromosome exists. Thus, it is apparent that some degree of karyotypic variation is occurring among the strain's studied. It is also assumed that karyotypic differences occurring in these- papaya strains could be due to structural changes of some chromosomes.

Keywords: Individual chromosome, Karyotype, Papaya

Location: Department of Genetics and Plant Breeding, Bangladesh Agricultural University, Mymensingh, Bangladesh

Start Date: 00-04-2001

End Date: 00-12-2002

Program Area: Knowledge Management

Commodity: Papaya, Chromosome

Objectives:

Considering the aspects mentioned above the present research programme has been undertaken with the following objectives. 1. To study the general cytological conditions in some indigenous papaya collections. 2. To identify individual, chromosome through karyotype analyses in root tip complements of some indigenous papaya collections.

Methodology:

Root tip collection and fixation Viable seeds of all three accessions were germinated in the laboratory of the Department of Genetics and Plant Breeding, Bangladesh Agricultural University, Mymensingh, Bangladesh from April 2001 to December 2002. Healthy and vigorously growing roots of 1.0-1.5 cm long were collected. It required about 8 to 10 days to obtain roots of such length: Roots thus, collected were thoroughly washed. The roots were then pretreated in a saturated aqueous solution Of monobromonaphthalene (MBN) for 2.5 hours with occasional stirring. Such MBN pre-treatment was done to get an increased number of prometaphase plates. The duration of MBN pre-treatment was ascertained from preliminary trials. MBN pre-treated roots were then thoroughly washed and fixed in freshly prepared acetic alcohol (1:3). The fixed roots were left in the fixative until these were used for slide preparation. Slides were prepared within about two weeks of fixation of the roots. The fixation of roots and slide peroration was repeated as and when needed.

Slide preparation Fixed roots were thoroughly washed and these were hydrolyzed in small vials with 10% HC1 at 60°C in an oven for 10-12 minutes. After hydrolysis, the roots were allowed to cool down and then they were washed thoroughly. The roots were then treated with a 2% aqueous solution of iron alum (Ferric Ammonium sulphate) for about 4 minutes for mordanting. Iron alum-treated roots were then washed thoroughly. The roots were then stained in 0.5% haematoxyline solution for about 4 minutes and washed several times until the coloring of the water stopped.

For slide preparation, the stained root was kept on a clean slide. The meristematic tip portion of about 1.0-1.5 mm was excised and the remaining back portion of the root was thrown away. The excised tip portion was squashed in 0.25-0.5% aceto-carmine solution after putting the cover glass. The cover glass was previously coated with a thin layer of egg albumin, which was dried by flickering once or twice over a spirit lamp. The slide was then dried over a flame, followed by a repeated heat-cool-press process. The process was continued until the chromosomes were satisfactorily spread and were in the same focal plane of the microscope objective lens. Sufficient flattening was carried out by gentle pressing on the smear using the back portion of the needle for clean observation Of all the chromosomes in the complement. Satisfactory slides were placed upside down on a watch glass submerged in 45% acetic acid in a large size petridish. A varied range. of time would require getting the coverslip detached itself from the slide. Immediately after the separation of the cover glass containing the cells on its adhesive coated surface was passed through 50%, 70% and 100% alcohol, one minute in each step respectively. The cover glass was again passed through absolute alcohol for one minute. The coverslip was mounted in a small amount of Euparal on a clean slide. The slide was then left flat for next few days, during which time the mounting would get stift enough, making the slide a permanent one.

Photomicrography and measurement of chromosomes. PhotomIcrography of the chromosome plates was done from permanent and temporary slides. Photomicrographs of the selected chromosome plates were taken with the aid of Olympus research microscope model BX40 using plan 100X objective. Fujicolour 100 ASA DX Film was used. Films processing and printing were made from commercial centres. Chromosome measurements were made form prints and chromosome length was converted to micron (µ) based on calculated print magnification.

Source of Information: J. Bangladesh Agril. Univ. 2(1): 25-30,2004 ISSN 1810-3030

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

All papaya strains studied contain 2n =18 chromosomes. The chromosome is considerably longer, 8µ-181.1. During slide preparation bringing the chromosomes in one focal plane of the objective lens in the well-spread condition of the chromosomes posed a major problem. To attain such a condition .one or more chromosomes would tend to break apart in the primary and secondary constriction region. This would lead to erroneous observations. Therefore, utmost care had to be exercised during slide preparation and selection of chromosome plates in undistorted conditions.

The cytological investigation of the three papaya strains (P-1, P-2, and P-3 revealed the presence of 2n = 18 chromosomes in the root tip cells. Previous reports in this respect on papaya strains in different countries are in agreement with the present findings (Sharma et al. 1988. Vidal et al. 1993). In any instances of either strain-no deviation in the 2n number was encountered. However, cell preparations for such observations must be satisfactory enough so as to avoid any misconclusion.

Knowledge Management: Journal