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On-line version ISSN 1982-0224
Neotrop. ichthyol. vol.4 no.1 Porto Alegre Jan./Mar. 2006
Karyotype description of three species of Loricariidae (Siluriformes) and occurrence of the ZZ/ZW sexual system in Hemiancistrus spilomma Cardoso & Lucinda, 2003
Renildo Ribeiro de OliveiraI; Issakar Lima SouzaII; Paulo Cesar VenereII
IInstituto Nacional de Pesquisas da Amazônia INPA, Manaus, AM, Brasil
IIDepartamento de Ciências Biológicas e da Saúde, Instituto de Ciências e Letras do Médio Araguaia da Universidade Federal de Mato Grosso, 78698-000, Pontal do Araguaia, MT, Brazil. e-mail: firstname.lastname@example.org
The neotropical freshwater systems have a high number of catfish species (Siluriformes), and many of those are denominated "cascudos" in Brazil. Cytogenetic data about three "cascudos" species fished in the rio Araguaia are described in the present study. The Pterygoplichthys joselimaianus showed 2n=52, with 28 metacentrics (M) chromosomes, 16 submetacentrics (SM) and 8 subtelocentrics/acrocentrics (ST/A) in both sexes. Hemiancistrus spinosissimus showed 2n=52, with karyotype formulae 26M+22SM+4ST, in both sexes. Hemiancistrusspilomma also showed 2n=52, but in this species a ZZ/ZW sex chromosome system (25M+21SM+6ST in females and 24M+22SM+6ST in males) was observed. The cells from H. spinosissimus and P. joselimaianus showed one chromosome pair bearing Ag-NORs, while in the H. spilomma three chromosome pairs bearing Ag-NORs were detected. The data showed in this work reveal particular chromosomal characteristics, important for a good recognition of both Hemincistrus species, and also show the importance of the insertion of cytogenetic data on taxonomic phylogenetic studies.
Key words: Ancistrinae, cytogenetic, sexual chromosomes, Araguaia.
Os sistemas deágua doce neotropicais possuem um alto número de espécies de peixes Siluriformes, muitas das quais popularmente conhecidas como cascudos no Brasil. Dados citogenéticos sobre três espécies de cascudos, capturadas na bacia do rio Araguaia são descritos no presente trabalho. Pterygoplichthys joselimaianus possui 2n=52, com 28 cromossomos metacêntricos (M), 16 submetacêntricos (SM) e 8 subtelecêntricos/acrocêntricos (ST/A) em ambos os sexos. Hemiancistrus spinosissimus revelou 2n=52, com fórmula cariotípica 26M+22SM+4ST, nos dois sexos. H. spilomma também possui 2n=52, porém, nesta espécie foi observado um sistema de cromossomos sexuais do tipo ZZ/ZW (25M+21SM+6ST para fêmeas e 24M+22SM+6ST para machos). As células de H. spinosissimus e P. joselimaianus mostraram um par de cromossomos portadores de Ag-RONs. Em H. spilomma, três pares de cromossomos portadores de Ag-RONs foram observados. Os dados apresentados neste trabalho revelam características cromossômicas particulares, importantes para um bom reconhecimento de ambas as espécies de Hemiancistrus e mostram a importância da inserção dos dados citogenéticos nos estudos taxonômicos e filogenéticos.
The Siluriformes is the most diverse and widely distributed of ostariophysan groups, with approximately 30 families, 412 genera and over 2400 species (Nelson, 1994). Except for the families Ariidae and Plotosidae, is limited to the tropical freshwater systems (Burgess, 1989). The Loricariidae, regionally called "cascudos" or "acarís" is endemic to the Neotropical region and is the largest family of catfishes, with 646 species currently considered valid (Isbrucker, 1980; Armbruster, 2004). Nevertheless, the Loricariidae is not a very studied group. According to Oliveira and Gosztonyi (2000) only 56 loricariid species have been investigated at the cytogenetic level, and a range of diploid number among 2n=36 in Rineloricaria latirostris (Loricariinae) (Giuliano-Caetano, 1998) and 2n=80 in Hypostomus sp. (Hypostominae) has been reported (Artoni & Bertollo, 1996).
Andreata et al. (1992, 1993) described a XX/XY sexual chromosome determination in Pseudotocinclus tietensis and a ZZ/ZW in Microlepidogaster leucofrenatus, respectively. Structural chromosome polymorphisms in several other species of Hypoptopomatinae (Loricariidae) related to NORs position and C-banding pattern at both species and population levels was detected (Andreata et al., 1994). A ZZ/ZW system was also observed in Loricariichthyis platymetopon (Scavone & Júlio Jr. 1995), Hypostomus sp. from the Rio Araguaia (Artoni et al., 1998), and Ancistrus cf. dubius (Mariotto et al., 2004).
Ancistrinae has been recognized as a welldefined monophyletic group (Schaefer, 1987), but a recent study showed that the species of this group are more related to those of the tribe Pterygoplichthini and were grouped in the tribe Ancistrini (Armbruster, 2004). Both these tribes now belong to the subfamily Hypostominae (Armbruster, 2004). Chromosome data from Ancistrini are showed in Table 1. Except for Ancistrus n.sp.1 from the rio São Francisco (Acre, Brazil), with 2n=38 (Alves et al. , 2003), Ancistrus sp. from the rio Iguaçu (Paraná, Brazil) with 2n=48 (Lara, 1998), and Ancistrus cf. dubius from the rio Paraguai basin (MT), with 2n=44 (Mariotto et al., 2004) all other species have a diploid number of 2n=52. The only species of Pterygoplichthini karyotyped, Liposarcus anisitsi, also has 2n=52 chromosomes (Artoni & Bertollo, 1996).
In the present work, the karyotypes and active rDNA nucleolar location of the three species of Loricariidae are presented, as well as the description of ZZ/ZW sexual chromosome system in Hemiancistrus spilomma.
Material and methods
In the present study, the taxonomy proposed by Armbruster (2004) who considers the genus Glyptoperichthys as synonyms with Pterygoplichthys was applied. In this way, P. joselimaianus (Weber, 1991) (three females, four males and one unidentified sex) were sampled in the lago Quatro Bocas (15º23'20.1''S, 51º42'45.9''W), near the city of Araguaiana, Mato Grosso, Brazil. Voucher specimens were deposited in the fish collection of the Instituto de Ciências e Letras do Médio Araguaia, Universidade Federal de Mato Grosso, Brazil (ICLMA434). Hemiancistrus spinosissimus Cardoso & Lucinda, 2003 (two female and two males) and H. spilomma Cardoso & Lucinda, 2003 (eight females and two males) were catched in the rio Araguaia (15o50'15"S - 051o58'43"W), near the city of Pontal do Araguaia, Mato Grosso, Brazil. Voucher specimens were deposited in the Museu de Ciências e Tecnologia of Pontifícia Universidade Católica do Rio Grande do Sul (MCP36689, MCP36686, MCP36685).
Chromosomal preparations were obtained either from kidney cells after in vivo colchicine treatment (Bertollo et al., 1978), or from shortterm cell culture (Fenocchio et al., 1991). These were Giemsa stained for conventional analysis. The active rDNA nucleolar cistrons (Ag-NORs) were detected using AgNO3 staining (Howell and Black, 1980). The chromosomes were classified as metacentric (M), submetacentric (SM), subtelocentric (ST), and acrocentric (A), according to the arm ratio criteria adopted by Levan et al. (1964).
In both sexes of Pterygoplichthys joselimaianus, the diploid number was 2n=52 and the karyotypic formulae 28M+16SM+8ST/A (Figure 1). In both sexes of Hemiancistrus spinosissimus the diploid number was 2n=52 and the karyotypic formulae 26M+22SM+4ST (Figure 2a). H. spilomma also showed 2n=52 but, a ZZ/ZW sex chromosome mechanism was observed, with a karyotypic formulae of 25M+21SM+6ST in females and 24M+22SM+6ST in males (Figure 2b). The ZZ chromosomes are represented by the first submetacentric pair whereas the W chromosome is morphologically similar to the third metacentric pair.
In the present study, it was observed that P. joselimaianus and H. spinosissimus cells showed one Ag-NOR-bearing chromosome pair (inset of Figure 1 and 2a, respectively). The Ag-NOR signals were subterminally located on the short arm of pair 9 in P. joselimaianus (Figure 1, inset) and terminally on the long arm of the pair 17 in H. spinosissimus (Figure 2a, inset). Three Ag-NOR-bearing pairs were detected in H. spilomma, one located in the terminal regions of the long arm in pairs 2 and 18 and other at the terminal end of the short arm of pair 3 (Figure 2b, inset).
In Ancistrini, pericentric and paracentric inversions probably were the most important chromosome rearrangements that occurred in the karyotypic evolution of the group (Alves et al., 2003). The present data reinforce this hypothesis since all species analyzed also have 2n=52 chromosomes. These rearrangements keep the diploid number equal to 2n=52 chromosomes, metacentric and submetacentric, in most of the studied species (Table 1). Ancistrini has a lower diploid number when compared with Loricariinae (Alves et al., 2003), Hypostominae (Artoni & Bertollo, 1996), and Hypoptopomatinae (Andreata et al., 1994). However, these conclusions are preliminary because Ancistrini is a very specious subfamily and these data are only related to a few species from five genera. The species P. joselimaianus also has 2n=52, the same diploid number found in Liposarcus anisitsi (Artoni & Bertollo, 1996), other species of the tribe Pterygoplichthini. According to Armbruster (2004), the tribes Ancistrini and Pterygoplichthini are sister groups and compose a derived clade of the Hypostominae. The present cytogenetic data reinforce the putative relationship between these tribes suggesting that 2n=52 could be a primitive diploid number for them.
A few years ago the presence of a sexual differentiation at a chromosome level was considered an uncommon occurrence in fishes. Advances in the studies of Neotropical fishes, however, have shown a great number of new occurrences in different species and families. Among those species with differentiated sex chromosomes, 64% showed female heterogamety and 36% male heterogamety. Eighty percent of the sex chromosomes corresponds to simple systems (77% ZZ/ZW and 23% XX/XY) and twenty percent correspond to multiple sex chromosome mechanisms (Centofante et al., 2002). The presence of a ZZ/ZW sex chromosome system in H. spilomma (present study) and in Ancistrus cf. dubius (Mariotto et al., 2004) suggest that this system may has a wide distribution in Ancistrini.
Some fish groups have a predominance of one chromosome pair in their karyotype bearing just one active NOR, as some species of Pimelodidae (Dias & Foresti, 1993) and Loricariidae (Alves et al., 2003). On the other hand, occurrence of multiple NORs can be observed in some species of Hypostominae (Artoni & Bertollo, 1996). Among the species examined, Hemiancistrus spilomma is the only species thus far to show multiple NORs, as described in Table 1 and confirmed by the nucleolus analysis (Figure 2b).
It is early to draw conclusions on the significance of the ZZ/ZW occurrence associated with the multiplicity of Ag-NORs in Hemiancistrus spilomma, as well the maintenance of 2n=52 and single Ag-NOR in the majority of the Ancistrini species because this tribe is very specious (Fisch-Muller, 2003), and only a few species from five genus have been studied.
Nevertheless, our research contributes to new karyological information that can aid in understanding of the evolutionary history of this group of fish. Hemiancistrus spilomma and Hemiancistrus spinosissimus are morphologically very similar and can easily be confused. Since these species have particular chromosomal characteristics the present cytogenetic data may be used as markers for differentiating the two species in the middle rio Araguaia.
The authors are grateful to Andréia Benedita Poletto, Jason Leolino de Oliveira and Humberto Pereira Rego for their technical assistance. To the Dr. Francisco Langeani and Dr. Alexandre R. Cardoso for the identification of the specimens and Dr. Cláudio Oliveira for the valuable suggestions to improve the text. This research was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Proc. 552353/01-2) and Fundação de Amparoà Pesquisa do Estado de Mato Grosso (FAPEMAT, Proc. 2.1.01.00084/1998/07).
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Received August 2005
Accepted November 2005