Cytogenetics of species of the families Pimelodidae and Rhamdiidae ( Siluriformes )

Only 33 species among about 300 belonging to the families Pimelodidae and Rhamdiidae have been studied cytogenetically. The diploid number varies from 2n = 46 to 2n = 63 chromosomes, with the karyotypes often being of the meta/submetacentric type. As a result, there is generally a very elevated fundamental number. Departamento de Biologia Geral, CCB, Universidade Estadual de Londrina, Caixa Postal 6001, 86051-990 Londrina, PR, Brasil. Send correspondence to A.L.D.


INTRODUCTION
The family Pimelodidae is the most diversified of the neotropical Siluriformes, with about 300 species in 50-60 genera (Mees, 1974).Although several taxonomic revisions of this family have appeared, the systematics of these genera is still confused.
The subfamily Rhamdiinae was recently elevated to the category of family by Pinna (1993), based on a phylogenetic study of the order Siluriformes, which demonstrated the polyphyletic nature of the traditional family Pimelodidae.Some genera of the family Pimelodidae, including Pimelodella, Rhamdia, Imparfinis, Cetopsorhamdia and Rhamdella, were grouped into the new family Rhamdiidae based on morphological characters.Similarly, the subfamily Pseudopimelodinae is now considered as the family Pseudopimelodidae, and contains the genera Microglanis and Lophyosilurus (Pinna, 1993).
The current classification of the families Pimelodidae, Rhamdiidae and Pseudopimelodidae is therefore: Superorder: Ostariophysi Order: Siluriformes Sub-Order: Eusiluroidei Family: Pimelodidae Sub-Order: Rhamdioidei Family: Rhamdiidae Sub-Order: Loricarioidei Family: Pseudopimelodidae This classification was adopted in the present work, which summarizes cytogenetic data for species of the families Pimelodidae and Rhamdiidae mainly found in unpublished thesis and congress abstracts.The diploid number, haploid number, the presence of B chromosomes and other relevant data were examined (Tables I and II).The classification of the hydrographic basin was performed as described by Vari (1992).

DISCUSSION
The chromosomal number among the 33 species of the families Pimelodidae and Rhamdiidae studied cytogenetically ranged from 2n = 46 to 2n = 63 (Tables I and II).For a more conclusive analysis, the FN of some species of the two families was recalculated (in parentheses) and to others species the FN was calculated for the first time in the present study (asterisk), considering M and SM with 2 arms and ST-A with 1 arm.In some species it was not possible to describe the FN, and they were therefore not karyotyped.A relatively high fundamental number was observed in the two families (from 78 to 116) (Tables I and  II), with a frequently asymmetrical karyotype that included elements with one or two arms and a preponderance of metacentric chromosomes.

Cytogenetics of species of the families Pimelodidae and Rhamdiidae (Siluriformes)
Ana Cláudia Swarça, Lucia Giuliano Caetano and Ana Lúcia Dias

Abstract
Only 33 species among about 300 belonging to the families Pimelodidae and Rhamdiidae have been studied cytogenetically.The diploid number varies from 2n = 46 to 2n = 63 chromosomes, with the karyotypes often being of the meta/submetacentric type.As a result, there is generally a very elevated fundamental number.number also proved to be conservative with regard to the FN, as the majority of species have an FN = 96 (Table I).

Cytogenetics of species of the families Pimelodidae and Rhamdidae
The variability in chromosome number was greater among genera of the family Rhamdiidae.Thus, in Pimelodella 2n varied from 46 to 58 (Almeida- Toledo et al., 1992;Dias and Foresti, 1993;Swarça, 1998), in Imparfinis sp.there were 56-58 chromosomes (Fenocchio, 1993;Vicente et al., 1994;Vissotto et al., 1997) and in Rhamdia the number varied from 58 to 63 because of the presence of B chromosomes (Fenocchio and Bertollo, 1990;Fenocchio, 1993;Vissotto, 1995, Abucarma, 1998).However, 2n in this genus is most frequently 58 (Table II).The variation of the FN (from 78 to 116) was also greatest among member species of this family.Pimelodella sp., with 2n = 46, had an intraspecific variation of 78 to 92 in the FN (Toledo and Ferrari, 1976a;Braga, 1989, Vissotto, 1995;Silva et al., 1996;Swarça, 1998).Some of these variations could have been occurring due to different degrees of chromosome condensation leading to different chromosome classifications among authors.Others may be suffering rearrangements, as inversions, leading to a true polymorphism.
In the Pimelodidae family, B chromosomes have been identified in two genera: Bergiaria (Dias and Foresti, 1993) and Iheringichthys (Dias and Foresti, 1990;Vissotto, 1995;Silva et al., 1996).In the family Rhamdiidae, B chromosomes were observed in a species of Pimelodella (Almeida- Toledo et al., 1992) and in almost all of the species of the genus Rhamdia (Fenocchio, 1993;Vissotto, 1995;Maistro, 1996;Abucarma, 1998).This extra chromosome is highly conserved in the latter genus, and is therefore most likely an important characteristic in its karyotypic evolution.
Sexual chromosomes have been observed only in the Rhamdiidae family.In a species of Pimelodella the male individuals are characterized as heterogametic, possessing a sexual chromosomal system of the type XX/XY (Dias and Foresti, 1993).In Imparfinis mirini the females were heterogametic, thus displaying sexual chromosomic system ZZ/ZW (Vissotto et al., 1997).
Although the diploid number in pimelodids is relatively constant, the karyotypic evolution between these two families is more divergent than uniform.The Rhamdiidae family has a greater karyotypic variability than the Pimelodidae family, which corroborates the data of Pinna (1993), who grouped some genera of pimelodids into the new Rhamdiidae family.

Table I -
Cytogenetic data for the family Pimelodidae.

Table II -
Cytogenetic data for the family Rhamdiidae