CARYOLOGIA Vol. 56, no. 4
Transcription
CARYOLOGIA Vol. 56, no. 4
Vol. 56, no. 4: 489-493, 2003 CARYOLOGIA Comparative cytogenetic analysis of the South American alligators Caiman latirostris and Caiman yacare (Reptilia, Alligatoridae) from Argentina PATRICIA AMAVET1, *, ROSA MARKARIANI1 and ALBERTO FENOCCHIO2 1 Universidad Nacional del Litoral,Facultad de Humanidades y Ciencias-Cátedra de Genética-Ciudad Universitaria, Paraje “El Pozo”, 3000 -Santa Fe, Argentina. 2 Universidad Nacional de Misiones- Facultad de Ciencias Exactas, Químicas y Naturales,Departamento de Genética-Félix de Azara 1552, 3300 - Posadas, Argentina. Abstract - One of the most important cytogenetic characteristics among the crocodilians is the diploid number, that ranges from 30 to 42 chromosomes and the fundamental number (FN) from 56 to 62, without sex chromosomes heteromorphisms. The purpose of this study was to characterise the chromosome complement of Caiman yacare and C. latirostris from Argentina through conventional Giemsa staining and simple banding techniques (CBC and NOR). The chromosome preparations were made from peripherical blood culture, and C- and NOR- banding were applied following routinary methods. The data obtained point out that both species share a similar chromosome number (2n = 42) and morphology with the complements composed by 12 pairs of telocentric chromosomes, 7 pairs of biarmed chromosomes and 2 pairs which could be considered microchromosomes. No conspicuous differences were revealed in the C-banding patterns and in both species the NOR’s was found on one of the small chromosome pairs. Key words: Cytogenetics; South American Alligators; Caiman latirostris; C. yacare. INTRODUCTION Living reptiles are amniotas non homeothermics, classifiable in 4 orders: Chelonia (turtles and tortoise), Squamata (snakes and lizards), Rhyncocephalia (New Zeland’s tuatara) and Crocodilia (crocodiles and alligators). Living crocodilians include three families: Crocodylia (crocodiles), Gavialidae (gharials) and Alligatoridae (caimans) (ZISWILER 1980). In Argentina, the only crocodilians cited belong to the Alligatoridae Family: Caiman latirostris (yellow- throated alligator) and C. yacare (jacare caiman). These species show some differences in their external morphology: C. yacare adults can reach * Corresponding author: fax: +54 0342 4575105; e-mail: pamavet@arnet.com.ar around 250 cm. long and are practically black, whereas C. latirostris as a length up to 300 cm and are lighter in colour. Skull morphology is different in both species, in Caiman yacare has a lengthened, triangular shape while C. latirostris shows a wide and dull snout (BRAZAITIS 1973). There are also differences in the geographic distribution of both species in Argentina, C. latirostris lives in aquatic environments in the Paraná River basin, it’s distribution is wider and reaches a higher latitude than C. yacare that is more related to the Paraguay River basin. However, the distribution patterns are partially overlapped among these species. C. latirostris lives preferably in shallow waters and vegetated marshes, whereas C. yacare preferes more open, deep and permanent waters (LARRIERA 1992). In Argentina the population of C. latirostris from Santa Fe province is listed in 490 the appendix II of the International Convention of Wild Species Traffic (CITES), as well as the entire population of C. yacare. The commercial use of both populations is allowed only under strict control, what turns them into species of zootechnical interest, representing a regionally important economic activity owing to the use of their leather. The Crocodilia Order has been widely studied in relation to morphophysiological and ecological aspects, with scarce cytogenetical antecedents. According LUI et al. (1994) crocodilians chromosomes were initially studied by Matthey, Matthey and Van Brinck, and Cohen and Clark. The available karyotypic data for this group were compared in a previous revision by COHEN and GANS (1970), that have reported diploid numbers ranged from 30 to 42 chromosomes with a fundamental number varying from 56 to 62. These authors do not consider the occurrence of microchromosomes (m) in the Order, and no sex chromosome heteromorphism was found in 14 of the studied species. There were few data about chromosome banding of these species (LUI et al., 1994). In relation to the chromosome complements, COHEN and GANS (1970) and LUI et al. (1994) have described a karyotype for C. latirostris constituted of 12 telocentric pairs (T), AMAVET, MARKARIANI and FENOCCHIO 6 metacentric pairs (M) and 3 submetacentric pairs (SM). COHEN and GANS (1970) point out a complement which comprises 11 T pairs, 8 M pairs and 2 SM pairs for C. yacare. In this study were described, for the first time, the karyotypes of the two native alligator species from Argentina, C. yacare and C. latirostris. MATERIALS AND METHODS Six specimens of Caiman yacare (3 males and 3 females) and six specimens of C. latirostris (2 males and 4 females) were studied, these individuals were kept at the Experimental Zoological Station from Santa Fe, Argentina. Blood samples were obtained with heparinized siringe from the jugular intern vein at cervical vertebras level, following the method of TOURN et al. (1993). Cultures were developped in a culture medium (TC 199) containing phytohemaglutinin M (Gibco), bovine calf serum, and antibiotics and were incubated at 29ºC. Culture time was 72 hours for C. latirostris, and 96 hours for C. yacare samples. Colcemid, was added 1or 2 hour before the harvest. Afterwards, the material was treated for 45 minutes with hypotonic solution (0.075 M ClK), prefixed and then, fixed three or more times, using a 3: 1 methanol – acetic acid solution. Fig. 1 – Karyotypes of Caiman yacare (a) and Caiman latirostris (b). Giemsa staining. 491 CYTOGENETIC ANALYSIS OF THE SOUTH AMERICAN ALLIGATORS FROM ARGENTINA The chromosome preparations were made on slides previously demersed in methanol and then air dryied. The material were staining with 10% Giemsa for conventional analysis. The technique adapted for C- and NOR- bandings was based on SUMNER (1972) and HOWELL and BLACK (1980), respectively. Were analyzed in mean 23 metaphases per individual. The chromosomes were photomicrographied and classified following LEVAN et al.(1964), they were grouped in two principal categories, telocentrics (T) and metacentric- submetacentrics (M-SM) and a third including two small chromosomes (m), in decreasing order of size. RESULTS AND DISCUSSION Both species show a diploid chromosome number of 42 which can be classified as telocentric (T), metacentric (M) and submetacentric (SM) chromosomes. No differences were observed between males and females karyotypes, neither C. yacare and C. latirostris. Both species also have a similar cromosome morphology: 12 pairs of large and medium telocentric chromosomes (T), 7 pairs of medium sized metacentric- submetacentrics chromosomes (M Fig. 2 – Caiman yacare (a – b) and Caiman latirostris (c – d). NOR – partial metaphase plates – (a – c) and C-banding (b – d), respectively. 492 -SM) and 2 pairs of very small chromosomes (Fig. 1, a-b). As observed in other reptile species is very difficult to obtain chromosome bands, these two alligators also show a pale C-banding pattern (Fig. 2, b-d), and only one NOR bearing chromosome pair like was stated by LUI et al. (1994) (Fig. 2, a-c). The studied species share between them and with available data about the genus Caiman (COHEN and GANS 1970; LANCE 1994; LANG and ANDREWS 1994; LUI et al. 1994) cytogenetic characteristics, as chromosome number (2n=42), formulae and absence of distinguishable sexual chromosomes. In spite the opinion of COHEN and GANS (1970) it is possible to suggest the occurrence of microchromosomes in these two species. This suggestion is based on the analysis of the two smallest chromosomes of the complement, that are very short, with an almost indistinguisable shape. Thus, on the basis of the present data, the karyotypes of Caiman yacare and C. latirostris show a chromosome formulae composed by 12 T, 7 M-SM and 2m. The crocodilians have a considerable variation of chromosome numbers but an almost stable FN, ranged from 56 to 62. One group constituted by the genera Paleosuchus, Melanosuchus and Caiman has a common diploid number of 42 chromosomes with large T but without large M. This situation could be considered “primitive” or “ancestral” and COHEN and GANS (1970) have advanced the hypothesis that the karyotype of all the crocodilians are derived from Paleosuchus by means of some pericentric inversions and Robertsonian translocations –fusions-. Especifically in relation with the genus Caiman, these authors suggested that C. latirostris could have given rise to C. crocodylus (= C. yacare) by two inversions. The present data does not support this hypothesis, because the karyotypes of these species are very similar and principally due to the fact that the group of telocentrics, biarmed and the small (m) chromosomes are constituted by the same number of elements, 12, 7 and 2 pairs, respectively. On the other hand, due to the small size and differences in chromosome condensation of the biarmed elements it is very difficult a clear distinction between M and SM chromosomes and consequently evolutive inferences only on the basis of these primary data seems to be very AMAVET, MARKARIANI and FENOCCHIO specullative. Therefore, the application of additional techniques as fluorescence, restriction enzime banding as well as FISH could help in a better karyotypic characterization, improving cytotaxonomic comparisons. REFERENCES BRAZAITIS P., 1973 – The identification of Living Crocodilians. Zoologica, 58: 59-101. COHEN M.M. and GANS C., 1970 – The chromosomes of the order Crocodilia. Cytogenetics, 9: 81-105. GORMAN G.C., 1973 – The chromosomes of the reptilia, a Cytotaxonomic interpretation. In A.B. Chiarelli and E. Capanna (Eds.) “Cytotaxonomy and Vertebrate Evolution”, pp. 349-424. Academic Press, London. HOWELL W.M. and BLACK D.A., 1980 – Controlled silver staining of nucleolus organizer regions whit protective colloidal developer: a 1- step method. Experientia, 36:1014-1015. LANCE V.A., 1994 – Environmental sex determination in reptiles: Patterns and processes. Journal of Experimental Zoology, 270: 1-2. LANG J.W. and ANDREWS H.V., 1994 – TemperatureDependent sex determination in crocodilians. Journal of Experimental Zoology, 270: 28-44. LARRIERA A., 1992 – La conservación y el manejo del jacare de papo amarelo en la Argentina. In: L. Verdade and A. Lavorenti (Eds), “Anais do II Workshop sobre Conservaçao e Manejo do jacare-do-papo-amarelo (Caiman latirostris)”, p. 8-17, CIZBAS / ESALQ/ USP, Piracicaba, Sao Paulo, Brasil. LEVAN A., FREDGA K. and SANDBERG A.A., 1964 – Nomenclature for centromeric position on chromosomes. Hereditas, 52: 201-220. LUI J.F., TAPIA VALENCIA E.F. and BOER J.A., 1994 – Karyotypic analysis and chromosome biometry of cell cultures of the yellow throated Alligator (Caiman latirostris DAUDIN). Revista Brasileira de Genetica, 17: 165-169. SUMNER A.T., 1972 – A simple technique for demonstrating centromeric heterochromatin (Preliminary notes). Expl. Cell Res., 75: 304-306. TOURN S., IMHOF A., COSTA A.L., von FINCK M.C. and LARRIERA A., 1993 – Colecta de sangre y procesamiento de muestras en Caiman latirostris (Informe de avance). In: A. Larriera, A. Imhof, M.C von Finck, A.L. Costa and S.C. Tourn (Eds), “Memorias del IV Workshop sobre Conservación y Manejo del yacaré overo (Caiman latirostris)”, p: 25-30. Fundación Banco Bica, Santo Tomé, Santa Fe, Argentina. CYTOGENETIC ANALYSIS OF THE SOUTH AMERICAN ALLIGATORS FROM ARGENTINA TROIANO J.C. and ALTHAUS R., 1993 – Hallazgos hematológicos en Caiman latirostris (Crocodylia, Alligatoridae) en condiciones de cautiverio. In: A. Larriera, A. Imhof, M.C von Finck, A.L. Costa and S.C. Tourn (Eds), “Memorias del IV Workshop sobre Conservación y Manejo del yacaré overo (Caiman latirostris)”, pp. 12-14, 493 Fundación Banco Bica, Santo Tomé, Santa Fe, Argentina. ZISWILER V., 1980 – Vertebrados. Zoología especial. Tomo II. Ed. Omega, Barcelona, España. Received January 29, 2003; accepted June 3, 2003