Cytogenetic analysis of three catfish species of the family Pseudopimelodidae ( Teleostei , Siluriformes )

Cytogenetic analyses performed in Cephalosilurus apurensis, Microglanis aff. cottoides and Pseudopimelodus bufonius revealed that the three species have 2n = 54 chromosomes: C. apurensis presented six metacentric (M), 28 submetacentric (SM), 14 subtelolocentric (ST), and six acrocentric (A) chromosomes, while M. aff. cottoides showed 10M, 32SM, 10ST and 2A, and P. bufonius had 12M, 30SM and 12ST. The nucleolus organizer regions (NORs) were present on the short arm of a middle-sized ST pair, identified as pair 19, in C. apurensis NORs were found on the short arm of a middle-sized ST (pair 23) and on the long arm of a middle-sized ST (pair 22) in M. aff. cotttoides and on the short arm of three middle-sized ST pairs, identified as pairs 9, 10 and 11, in P. bufonius. C-banding revealed a very small amount of constitutive heterochromatin in the chromosomes of all species, including the NORs. The occurrence of 2n = 54 in all species of the family Pseudopimelodidae and its absence among species of the closely related Pimelodidae and Heptapteridae may be important in identifying Pseudopimelodidae species.

The order Siluriformes (catfishes) has 3,093 species, divided into 36 families and 478 genera, and distributed worldwide, except for the coldest areas in the Northern and Southern hemispheres (Ferraris, 2007).Recent phylogenetic studies showed that the old family Pimelodidae comprised three monophyletic units: Pimelodidae, Heptapteridae and Pseudopimelodidae (Lundberg et al., 1991;de Pinna, 1998).According to Ferraris (2007), Pseudopimelodidae is composed of the genera Batrochoglanis (five species), Cephalosilurus (four species), Lophiosilurus (one species), Microglanis (14 species) and Pseudopimelodus (five species).A new genus and species, Cruciglanis pacifisi, has been recently described by Ortega-Lara and Lehmann (2006).This family is widely distributed in South America and is considered the least known family among the naked Neotropical freshwater catfishes (Shibatta, 2003).Currently, the only species to have their karyotypes reported are Microglanis garavelloi (cited as M. cottoides -Vissotto et al., 1999a) and Pseudopimelodus mangurus (Martinez et al., 2004).The objective of the present study was to analyze the karyotypes of Cephalosilurus apurensis, Microglanis aff.cottoides and Pseudopimelodus bufonius.
The 2n = 54 present in Pseudopimelodidae contrasts with the modal 2n = 56 found in most catfish families (Oliveira and Gosztonyi, 2000) and specially with the diploid numbers found among representatives of Heptapteridae and Pimelodidae, which are closely related to Pseudopimelodidae (Sullivan et al., 2006) (Table 1).
Pseudopimelodid species have single or multiple Ag-NORs (Table 1).Cephalosilurus apurensis showed a single pair of Ag-NORs on the short arms of a middle-sized ST pair, identified as pair 19 (Figure 1a).The remaining pseudopimelodid species analyzed also had a single Ag-NOR: Lophiosilurus alexandrii showed Ag-NORs on the short arm of a SM (Marques, Garcia and Moreira Filho, personal communication); Microglanis garavelloi (Vissotto et al., 1999a) had Ag-NORs on the long arm of M; and Pseudopimelodus mangurus (Martinez et al., 2004) presented Ag-NORs on the short arm of SM/ST (Table 1).Single Ag-NORs were also identified in all species of Pimelodidae and all but one species of Heptapteridae (Table 1).This is also the most common condition in Siluriformes (Oliveira and Gosztonyi, 2000) and even in Teleostei (Klinkhardt, 1998).The Ag-NORs of M. aff.cotttoides were found on the short arm of a middle-sized ST pair, Martinez et al. 693   identified as pair 23, and on the long arm of another middle-sized ST pair, identified as pair 22 (Figure 2a).The Ag-NORs of P. bufonius occurred on the short arm of three middle-sized ST pairs, identified as pairs 9, 10 and 11 (Figure 3a).Multiple Ag-NORs were identified in one species of Heptapteridae (Vissotto et al., 1999a) and were not found among Pimelodidae (Table 1).The number and position of NORs are species-specific and do not seem to follow any pattern during karyotypic evolution.
C-banding showed the occurrence of a small amount of constitutive heterochromatin in the chromosomes of the three species (Figures 1b,2b,3b).In Cephalosilurus apurensis, positive C-banded segments were observed on the short arms of the largest ST pair (pair 19) and in the Ag-NORs.In Pseudopimelodus bufonius, C-banding evidenced segments on the short arms of the six larger SM pairs (pairs 9, 10, 11, 12, 13 and 14) and in Microglanis aff.cottoides, C-banding revealed positive segments on the short arms of one large ST pair (pair 23) and on the long arms of several ST pairs.The small amount of heterochromatic segments in the chromosomes of P. bufonius, M. aff.cottoides, and C. apurensis, as well as in other representatives of the family Pseudopimelodidae, P. mangurus (Martinez et al., 2004), and M. garavelloi (Vissotto et al., 1999a), suggests that this may be a characteristic of this catfish family.The occurrence of a very small amount of C-banded positive segments reported herein resembles the data reported for many teleost species, including siluriforms (Gold et al., 1990).
The presence of 2n = 54 chromosomes may be an important characteristic to differentiate Pseudopimelodidae species from species of Heptapteridae and Pimelodidae.Further analysis of additional Pseudopimelodidae species with different staining techniques will provide important information for a better understanding of the chromosome evolution in the group and will help to confirm the conservative nature of the diploid number in this fish family.

Figure 1 -
Figure 1 -Karyotype of Cephalosilurus apurensis (2n = 54) after: (a) conventional staining and (b) C-banding.In the inset, silver stained chromosomes showing the terminal Ag-NOR on the short arms of pair 19.

Figure 2 -
Figure 2 -Karyotype of Microglanis aff.cottoides (2n = 54) after: (a) conventional staining and (b) C-banding.In the inset, silver stained chromosomes showing the terminal Ag-NORs on the short arms of pair 23 and long arms of pair 22.