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Chromosomes of a pestiferousland snail, Achatina(Lissachatina) fulica fulica(Bowdich) (Achatinidae:Pulmonata: Gastropoda)R.C. Choudhurya & Itu Mohapatraa

a Post-Graduate Department of Zoology, BerhampurUniversity, Berhampur-760 007, Orissa, IndiaPublished online: 31 Jan 2014.

To cite this article: R.C. Choudhury & Itu Mohapatra (1991) Chromosomesof a pestiferous land snail, Achatina (Lissachatina) fulica fulica (Bowdich)(Achatinidae: Pulmonata: Gastropoda), Caryologia: International Journalof Cytology, Cytosystematics and Cytogenetics, 44:2, 201-208, DOI:10.1080/00087114.1991.10797186

To link to this article: http://dx.doi.org/10.1080/00087114.1991.10797186

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CARYOLOGIA Vol. 44, n. 2: 201-208, 1991

Chromosomes of a pestiferous land snail, Achatina (Lissachatina) fulica fulica (Bowdich) (Achatinidae: Pulmonata: Gastropoda)

R.C. CHOUDHURY and ITU MOHAPATRA

Post-Graduate Department of Zoology, Berhampur University, Berhampur-760 007, Orissa, India

SUMMARY- Mitotic and meiotic chromosome preparations of Achatina (Lissacha­tina) fulica fulica were made by colchicine- citrate- flame drying- Giemsa technique. The diploid number 2n = 60 with the chromosome formula n = 28m + 2 sm and FN = 120 were obtained. On the basis of chromosome lengths the karyotype has been prepared and the morphometric analysis has been done. The haploid number n = 30 has been confirmed and the behaviour of germinal chromosomes have been studied. Conservativeness with regard to chromosomal change in Mollusca has been discussed.

INTRODUCTION

Molluscs, besides having an excellent fossil record, include 112,000 different species (JANUS 1965), but at the most, 0.5% of them are known karyologically. Except for a few countable number of recent studies, like that of NATARAJAN (1969), PATTERSON (1971), BABRAKZAI et a/. (1974, 1975), GoLDMAN et al. (1980), GoLDMAN and LovERDE (1983), BoTTKE (1982), VIT­TURI and CATALANO (1988) and PAGE (1988), most of the available reports are mainly intended to elucidate the chromosome numbers, often using meiotic preparations, being based on crude techniques. So, besides the detail study on the morphology of the mitotic and meiotic chromosomes of new species, the reinvestigation of old data is warranted which can encompass cytotaxonomy, cytogenetics, genetic toxicology and medical Zoology.

Out of the three subclasses of Gastropoda, Pulmonata includes more than 30000 species and is comparatively well represented cytologically. Unfortuna­tely, most of such studies lack details on the chromosomes and so, their origin and evolution are poorly understood. The giant African pulmonate land snail Achatina fulica fulica of the present study, though not an endemic species, tops the list of serious mollusc pests in most parts of India (RAuT and GHOSE 1984). In the present paper, we present our karyotype technique and the details of the mitotic chromosomes and behaviour of germinal chromosomes obtained from A. fulica fulica.

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202 CHOUDHURY and MOHAPATRA

MATERIAL AND METHODS

Specimens for the present study were collected from the horticultural gardens at Bhubaneswar, Orissa. They were identified by the Zoological Survey of India, Calcutta as Achatina (Lissachatina) fulica fulica (Bowdich).

Mitotic and meiotic chromosome preparations were made with a little modifica­tion to the cell suspension techniques of PRASAD and DAs (1978) and GoLDMAN et al. (1980) for gastropod chromosomes. Each living specimen was injected with 0.25 to 0.5 m1 of 0.025% colchicine solution directly through the apex. At least two hours after the injection, the specimens were sacrificed and the ovotestes masses were taken out and kept in distilled water at room temperature for 30 minutes and then in 0.8% sodium citrate solution for 15 minutes after proper mincing and flushing. The cell suspension was centrifuged at 2000 rpm for 5 minutes. To the centrifugant freshly prepared fixative (1:3 glacial acetic acid and methanol volume/volume) was added and recentrifuged to replace the fixative by a desired volume of fresh solution and the cells were resuspended. The fixed cell suspension was dropped on grease free, clean and prechilled slides in 50% alcohol and were immediately flame dried. The prepared slides were left overnight for complete air drying, then stained with 3% Giemsa (diluted in phosphate buffer, pH 6.8) for 30 minutes and washed throughly.

Photomicrographs of well spread mitotic metaphases and different meiotic stages were taken. Camera lucida diagrams of 5 well spread mitotic metaphases were drawn to the closest accuracy and used for the morphometric analysis. Morphological nomencla­ture of the chromosomes was done by adopting the procedure of LEVAN et al. (1964). In the karyotype the chromosomes were arranged serially in order of their decreasing lenghts.

OBSERVATIONS

Mitotic chromosomes.

The diploid number 2n = 60 has been confirmed from 3 7 well spread mitotic metaphases (Fig. 1, A) in all the 9 specimens of the present study. The karyotype (Fig. 1, C) and the morphometric analysis (Table 1) indicate that all the chromosomes are biarmed and monocentrics with a median or submedian position of their centromeres. No clear cut size grouping is possible since the chromosomes show extreme gradual seriation. The absolute length of the chromosomes range between 6.42 ± 0.21 and 3.47 ± 0.03 micra. Their relative percentage lengths vary from 4. 79 ± 0.36 to 2.56 ± 0.07 (Table 1). The centromeric indices indicate that all are metacentrics except the 17th and 20th pair in the karyotype, which are submetacentrics. Moreover, the 20th pair is on the border line of submetacentric and metacentric. Thus, the chromosome formula is n = 28m + 2sm and the number of fundamental arms (FN) is 120. A few metaphases in polyploid condition, particularly tetraploids (Fig. 1, B), have been observed with much contracted chromosomes.

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CHROMOSOMES OF A PESTIFEROUS LAND SNAIL 203

Figs. lA-lC.- Mitotic chromosomes of Achatina fulica fulica. (A) Mitotic metaphase, (B) Polyploidy, (C) Karyotype.

Meiotic chromosomes.

The chromosomes of leptotene nucleus are diffused, slender thread like structures and bear densely stained granules, the chromomeres (Fig. 2, A). They are of unequal sizes and are irregularly spaced.

A polarized or the so called bouquet orientation of chromosomes (chromo­central type of orientation) has been observed (Fig. 2, B) which might be facilitating the synaptic process. During the bouquet orientation the chromo­somes are thickened and beaded.

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Figs. 2A-2H. - Meiotic chromosomes of Achatina fulica fulica. (A) Leptotene; (B) Bouquet orienta­tion of chromosomes; {C) Zygotene; (D) Pachytene; (E) and (F) Diplotene; (G) Diakinesis; (H) Metaphase I.

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CHROMOSOMES OF A PESTIFEROUS LAND SNAIL 205

In the zygotene stage (Fig. 2, C) the homologous chromosomes are paired forming bivalents with irregular hairy outlines and beaded appearances. The pairing is so tight and close that the individual chromosomes are not distingui­shable.

The pachytene stage shows clearly the close intertwining of paired homo­logues which are shortened and thickened (Fig. 2, D). Most of such elements are showing open ends indicating the beginning of separation of homologues which is to be followed immediately.

In the diplotene stage the bivalents are found further thickened and shortened (Fig. 2 E, F). The homologues are separated from each other except at a few specialized places of crossing over. The sites of such genetic exchange exhibit very characteristic configurations. Some of the elements are showing more than one chiasma.

From the diakinesis stages the haploid number of n = 30 has been con­firmed (Fig. 2, G). All the elements here are very much condensed and stained intensely with smooth outlines.

The metaphase-! elements (Fig. 2, H) are so much condensed that they appear as smooth round dots in the polar view and more or less dumb-bell shaped in the side view. Most of the bivalents show complete terminalization of chiasmata. The degree of terminalization is found greater between diakinesis and metaphase-! than in between diplotene and diakinesis.

DISCUSSION

The number of chromosomes (2n = 60 and n = 30) from mitotic and meiotic stages of A. fulica fulica of the present study confirms to that of the earlier report of NATARAJAN (1960) on the same species. However, except for the diploid number, there are considerable variation between the two groups of observations. The morphology of the chromosomes reported by NATARAJAN as 60 metacentrics hardly support his camera lucida diagram of 30 pairs of acrocentrics and rather purportedly support our findings. NATARAJAN has categorized the chromosomes as six larger pairs (4 pairs with median centro­mere) and the rest 24 pairs small with median centromere. But the chromo­somes, in our present observation, are extremely gradually seriated (Fig. 1, C and Table 1), so much so that their size categorization is hardly possible. Furthermore, NATARAJAN has reported the presence of two pairs of <<J» shaped chromosomes with unequal arms and sub-terminal centromeres among his larger category of chromosomes i.e., in the 5th and 6th position of his karyotype. Whereas, in our present finding, though there is the presence of two pairs of submetacentrics, they are on the 17th and 20th position in the karyotype (Fig. 2, C and Table 1) where the chromosomes have been arranged serially in order of their decreasing lengths.

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TABLE 1

Absolute length Mean relative Chromosome of the chromosome percentage Centromeric Chromosome

number in 11m with length with index morphology standard error standard deviation

1 6.42 :1:: 0.21 4.79 :1:: 0.36 43.4 m 2 5.85 :1:: 0.30 4.41 :1:: 0.48 43.0 m 3 5.74 :1:: 0.27 4.38 :1:: 0.17 42.8 m 4 5.10 :1:: 0.23 3.85 :1:: 0.23 41.2 m 5 5.05 :1:: 0.13 3.82 :1:: 0.19 43.7 m 6 5.01 :1:: 0.28 3.80 :1:: 0.22 40.9 m 7 4.96 :1:: 0.17 3.71 :1:: 0.27 41.1 m 8 4.92 :1:: 0.25 3.67 :1:: 0.33 45.7 m 9 4.73 :1:: 0.11 3.48 :1:: 0.21 44.3 m

10 4.69 :1:: 0.14 3.44 :1:: 0.18 42.5 m 11 4.65 :1:: 0.22 3.41 :1:: 0.24 42.8 m 12 4.57 :1:: 0.19 3.33 :1:: 0.11 47.6 m 13 4.52 :1:: 0.12 3.30 :1:: 0.16 43.2 m 14 4.51 :1:: 0.09 3.29 :1:: 0.14 42.3 m 15 4.47 :1:: 0.13 3.25 :1:: 0.32 48.1 m 16 4.42 :1:: 0.11 3.18 :1:: 0.21 47.9 m 17 4.40 :1:: 0.18 3.18 :1:: 0.31 26.3 sm 18 4.31 :1:: 0.26 3.07 :1:: 0.16 43.1 m 19 4.22 :1:: 0.12 3.05 :1:: 0.15 39.7 m 20 4.11 :1:: 0.16 3.04 :1:: 0.34 3(.3 sm 21 4.06 :1:: 0.15 3.02 :1:: 0.09 38.2 m 22 4.06 :1:: 0.17 3.01 :1:: 0.17 44.3 m 23 4.02 :1:: 0.10 2.99 :1:: 0.22 49.2 m 24 4.01 :1:: 0.07 2.97 :1:: 0.26 45.5 m 25 3.96 :1:: 0.09 2.93 :1:: 0.14 41.7 m 26 3.92 :1:: 0.06 2.91 :1:: 0.18 43.6 m 27 3.91 :1:: 0.12 2.91 :1:: 0.23 39.4 m 28 3.72 :1:: 0.19 2.78 :1:: 0.29 42.1 m 29 3.56 :1:: 0.23 2.63 :1:: 0.12 44.7 m 30 3.47 :1:: 0.03 2.56 :1:: 0.07 45.8 m

The differences in the karyotype of the species in the two sets of observations might be due to the erratic observations by NATARAJAN because of the crude technique of sectioning for chromosome studies, or might be due to geographical variations, as the two sites of specimen collection Annamalainagar for the study of NATARAJAN, Bhubaneswar for the present study, are far away from each other (around 2000 kms.).

Basing on the organization of kidney and ureter, the order Stylommato­phora of Pulmonata has been divided into four suborders as Orthurethra, Mesurethra, Heterurethra and the most specialized Sigmurethra (Pn.sBRY 1900). The suborder Sigmurethra is divided into three infraorders: Holopo-

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CHROMOSOMES OF A PESTIFEROUS LAND SNAIL 207

dopes, Aulacopoda and Holopoda; of which, the Holopodopes includes several primitive «Achatinidae like» genera and families (BAKER 1962). The chromo­some numbers of Holopodopes, so far investigated, are n = 25, 29, 30, 31 (PATTERSON 1969). However, cytological information for this group is too fragmentary to draw any conclusion concerning their cytotaxonomic relation­ships.

In general, conservativeness with regard to chromosomal change is evi­dent in most mollusc groups at different taxon levels. Many higher taxa have characteristic chromosome numbers with only a few species having wide deviations from the basic haploid set. And most such variations within a taxon are seldom greater than ± 1 or 2 bivalents (PATTERSON 1969). So it appears that the mechanisms for addition or deletion of chromosomes in them operate at low or inefficient level. However, the karyotypic evolution in them cannot be detected only from the chromosome numbers. A massive data on the detail morphology of mitotic and meiotic chromosomes and their banding pattern studies are essential for such study.

Acknowledgements.- We are grateful to Prof. C.C. DAs, HEAD, P.G. Department of Zoology, Berhampur University for providing laboratory facilities and to Dr. N.V. SUBBA RAo, Zoological Survey of India, Calcutta for the identification of the specimen.

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