Karyotypic diversity and evolutionary trends in the Neotropical catfish genus Hypostomus Lacépède, 1803 (Teleostei, Siluriformes, Loricariidae)

Abstract The family Loricariidae with 813 nominal species is one of the largest fish families of the world. Hypostominae, its more complex subfamily, was recently divided into five tribes. The tribe Hypostomini is composed of a single genus, Hypostomus Lacépède, 1803, which exhibits the largest karyotypic diversity in the family Loricariidae. With the main objective of contributing to a better understanding of the relationship and the patterns of evolution among the karyotypes of Hypostomus species, cytogenetic studies were conducted in six species of the genus from Brazil and Venezuela. The results show a great chromosome variety with diploid numbers ranging from 2n=68 to 2n=76, with a clear predominance of acrocentric chromosomes. The Ag-NORs are located in terminal position in all species analyzed. Three species have single Ag-NORs (Hypostomus albopunctatus (Regan, 1908), Hypostomus prope plecostomus (Linnaeus, 1758), and Hypostomus prope paulinus (Ihering, 1905)) and three have multiple Ag-NORs (Hypostomus ancistroides (Ihering, 1911), Hypostomus prope iheringi (Regan, 1908), and Hypostomus strigaticeps (Regan, 1908)). In the process of karyotype evolution of the group, the main type of chromosome rearrangements was possibly centric fissions, which may have been facilitated by the putative tetraploid origin of Hypostomus species. The relationship between the karyotype changes and the evolution in the genus is discussed.


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The subfamily Hypostominae with about 386 species (Reis et al. 2006) is the largest one in the family Loricariidae. The subfamily Hypostominae can only be recognized as monophyletic with the inclusion of the old subfamily Ancistrinae and the exclusion of some genera more related to the subfamily Neoplecostominae (Armbruster 2004). This subfamily is divided into five tribes: Corymbophanini, Rhinelepini, Hypostomini, Ancistrini, and Pterygoplichithini (Armbruster 2004) (Fig. 1). The tribe Hypostomini, with the only genus Hypostomus, has the greatest number of Hypostominae species (Reis et al. 2003).
The genus Hypostomus is the most representative in the family (Weber 2003, Hollanda Carvalho et al. 2010 with 126 species distributed from Central America to southern South America . Species of the genus display phenotypic plasticity that makes difficult to obtain diagnostic characters for the group (Armbruster 2004).
Cytogenetic studies in Hypostomus are relatively well documented (Table 1). In a review of genus cytogenetic data by Bueno et al. (2011) the relations between diploid number and karyotypic formulae of genus were established. However, several problems were not yet solved, including the pattern of karyotype evolution in Hypostomini. In the present study, six species of Hypostomus were karyotyped and the results employed to discuss the karyotype evolution of the genus.  Chromosome preparations were obtained from kidney tissues using the technique described by Foresti et al. (1993). Silver staining of the nucleolus organizer regions (Ag-NORs) was performed according to the technique proposed by Howell and Black (1980). Chromosome morphology was determined on the basis of arm ratio, as proposed by Levan et al. (1964) and the chromosomes were classified as metacentrics (M), submetacentrics (SM), subtelocentrics (ST) and acrocentrics (A).
The genus Hypostomus seems to be the karyotypically most derived genus in Loricariidae (Rubert et al. 2011), the variation of diploid number observed in the six species of Hypostomus analyzed (2n=68 to 2n=76) confirms this hypothesis. All species analyzed exhibited a large number of acrocentric chromosomes, reinforcing the hypothesis that higher diploid numbers are positively related to higher number of acrocentric chromosomes in Hypostomus (Artoni and Bertollo 2001). According to Oliveira and Gosztonyi (2000), high diploid numbers may represent a derived characteristic in siluriforms.
Three species had single Ag-NORs (H. albopunctatus, H. prope plecostomus, and H. prope paulinus); and the three others had multiple Ag-NORs (H. ancistroides, H. prope iheringi, and H. strigaticeps). All species presented terminal Ag-NORs, a marked characteristic of the species of this genus. The occurrence of multiple Ag-NORs is the most common characteristic among the Hypostomini, however, this phenotype is considered a derived characteristic among siluriforms (Oliveira and Gosztonyi 2000), which usually predominate single Ag-NORs.
Differences in the karyotype formulae or in the number and position of Ag-NORs are common in species that do not present extensive migration behaviour, since isolated populations are more commonly involved in inbreeding processes, which makes the fixation of chromosome rearrangements easier (Almeida-Toledo et al. 2000). This kind of phenomenon has been extensively documented in fishes as in Astyanax scabripinnis (Jenyns, 1842) (Moreira-Filho and Bertollo 1991, Maistro et al. 1998, Alves andMartins-Santos 2002). On the other hand one of the most important problems associated with the study of the genus Hypostomus is the correct species identification due to the large number of species as well as the close morphological similarity among species (Armbruster 2004). Thus, Table 1 shows many samples identified as Hypostomus sp., which reflects our poor taxonomic knowledge of the group. Among the Hypostomus species, the high diploid number is coincident with a high the number of uniarmed chromosomes (Table 1), suggesting the occurrence of a large number of centric fissions in the karyotypic evolution of the group (Artoni and Bertollo 1996). This hypothesis is reinforced considering that the species of Rhinelepini, the sister group of Hypostomini, has 2n=54 chromosomes (Alves et al. 2003, Alves et al. 2005, Alves et al. 2006. The occurrence of a polyploidy event in the origin of the tribe Hypostomini may explain the existence of duplicated centromeres and telomeres that could have been activated in the centric fissions rearrangements. Thus, in the ancestor of Hypostomini an extensive process of chromosome fusions should have occurred changing a putative original karyotype with 2n=108 chromosomes into a karyotype with 2n=54 chromosomes. The alternative hypothesis that species of Hypostomus with high diploid numbers are the most primitive, suggesting that new chromosome fusions are reducing the diploid numbers in the genus, is not corroborated by the phylogenies available for the genus (Montoya-Burgos 2003, Armbruster 2004). Considering that the available phylogenies for the genus Hypostomus are very limited regarding the number of species and precise fish identification, further phylogenetic studies including karyotyped fishes are fundamental for a better understanding of the chromosome evolution in Hypostomus.