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:
[email protected]
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.
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Received January 29, 2003; accepted June 3, 2003