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

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.

Introduction

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 , has the greatest number of Hypostominae species (Reis et al. 2003).

Figure 1.

Phylogeny of the family Loricariidae proposed by Armbruster (2004).

The genus 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 (Zawadzki et al. 2010). Species of the genus display phenotypic plasticity that makes difficult to obtain diagnostic characters for the group (Armbruster 2004).

Recent studies suggested that the genus might be composed of some monophyletic groups (Muller and Weber 1992, Montoya-Burgos 2003, Armbruster 2004, Zawadzki et al. 2004, Alves et al. 2006). This suggestion is confirmed by extensive morphological variation in the genus combined with a largest variety of diploid numbers and karyotype formulae in Loricariidae (Artoni and Bertollo 1996, Alves et al. 2006), with diploid numbers ranging from 2n=52 in (Valenciennes, 1840) (Artoni 1996) to 2n=84 in sp. 2 (Cereali et al. 2008) (Table 1).

Table 1.

A summary of the cytogenetic data available for the genus . 2n = diploid number; M = metacentric; SM = submetacentric; ST = subtelocentric; A = acrocentric.

Species	Locality	2n	Karyotypic formulae	References
Hypostomus affinis (Steindachner, 1877)	Paraitinga River, São Paulo, Brazil	66	14M, 14SM, 12ST, 26A	Kavalco et al. (2004)
	Jacuí stream (SP)	66	14M, 14SM, 12ST, 26A	Fenerich et al. (2004)
Hypostomus albopunctalus (Regan, 1908)	Mogi-Guaçu River, São Paulo, Brazil	74	10M, 20SM, 44ST/A	Artoni and Bertollo (1996)
	Corumbataí River, São Paulo, Brazil	74	10M, 20M, 16ST, 28A	Present study
Hypostomus ancistroides (Ihering, 1911)	--	68	10M, 28SM, 30ST/A	Michelle et al. (1977)
	Araquá River, São Paulo, Brazil	68	18M, 10SM, 12ST, 28A	Alves et al. (2006)
	Corumbataí River, São Paulo, Brazil	68	16M, 4SM, 16ST, 32A	Present study
	Mogi-Guaçu River, São Paulo, Brazil	68	16M, 18SM, 34ST/A	Artoni and Bertollo (1996)
	Paranapanema River, São Paulo, Brazil	68	10M, 26SM, 32ST/A	Rubert et al. (2011)
Hypostomus prope auroguttatus (Kner, 1854)	Mogi-Guaçu River, São Paulo, Brazil	76	8M, 30SM, 38ST/A	Artoni and Bertollo (1996)
Hypostomus cochliodon (Kner, 1854)	Salobra river and Salobrinha stream (MS)	64	16M, 20SM, 28ST-A (male)/ 16M, 19SM, 27ST-A (female)	Cereali (2006)
Hypostomus emarginatus (Valenciennes, 1840)	Araguaia River, Mato Grosso, Brazil	52	16M, 30SM, 6ST	Artoni (1996)
Hypostomus goyazensis (Regan, 1908)	Vermelho River, Goiás, Brazil	72	10M, 16SM, 10ST, 36A	Alves et al. (2006)
Hypostomus prope iheringi (Regan, 1908)	Corumbataí River, São Paulo, Brazil	74	10M, 14M, 20ST, 30A	Present study
Hypostomus macrops (Eigenmann & Eigenmann, 1888)	--	68	10M, 14SM, 44ST/A	Michelle et al. (1977)
Hypostomus nigromaculatus (Schubart, 1964)	Tibagi River, Paraná, Brazil.	76	6M, 20SM, 50ST/A	Rubert et al. (2008)
	Mogi-Guaçu River, São Paulo, Brazil	76	8M, 20SM, 48ST/A	Rubert et al. (2008)
Hypostomus paulinus (Ihering, 1905)	--	74	10M, 20SM, 44ST/A	Michelle et al. (1977)
Hypostomus prope paulinus (Ihering, 1905)	Corumbataí River, São Paulo, Brazil	76	6M, 18M, 12ST, 40A	Present study
Hypostomus prope paulinus (Ihering, 1905)	Corumbataí River, São Paulo, Brazil	76	6M, 18M, 12ST, 40A	Present study
Hypostomus plecostomus (Linnaeus, 1758)	--	54	24M/SM, 12ST, 18A	Muramoto et al. (1968)
Hypostomus prope plecostomus (Linnaeus, 1758)	Orinoco River, Bolivar, Venezuela	68	12M, 16M, 12ST, 28A	Present study
Hypostomus regani (Ihering, 1905)	Mogi-Guaçu River, São Paulo, Brazil	72	10M, 20SM, 42ST/A	Artoni and Bertollo (1996)
	Paranapanema River, São Paulo, Brazil	72	10M, 18SM, 44ST/A	Rubert et al. 2011
	Araguá River, São Paulo, Brazil	72	12M, 18SM, 26ST, 16A	Alves et al. (2006)
Hypostomus strigaticeps (Regan, 1908)	Corumbataí River, São Paulo, Brazil	74	10M, 14M, 14ST, 36A	Present study
	Mogi-Guaçu River, São Paulo, Brazil	74	8M, 4SM, 62ST/A	Michelle et al. (1977)
	Paranapanema River, São Paulo, Brazil	72	10M, 16SM, 46ST/A	Rubert et al. (2011)
Hypostomus sp. 2	Salobrinha stream, Mato Grosso do Sul, Brazil	84	6M, 16SM, 62ST/A	Cereali et al. (2008)
Hypostomus sp. 3	Perdido River, Mato Grosso do Sul, Brazil	82–84	6M, 16SM, 64ST/A - 6M, 12SM, 66ST/A	Cereali et al. (2008)
Hypostomus sp. A	Rincão Stream, São Paulo, Brazil	70	18M, 14SM, 38ST/A	Artoni and Bertollo (1996)
Hypostomus sp. B	Mogi-Guaçu River, São Paulo, Brazil	72	12M, 18SM, 42ST/A	Artoni and Bertollo (1996)
Hypostomus sp. C	Mogi-Guaçu River, São Paulo, Brazil	68	10M, 18SM, 40ST/A	Artoni and Bertollo (1996)
Hypostomus sp. D1	Mogi-Guaçu River, São Paulo, Brazil	72	10M, 26SM, 36ST/A	Artoni and Bertollo (1996)
Hypostomus sp. D2	Mogi-Guaçu River, São Paulo, Brazil	72	14M, 20SM, 38ST/A	Artoni and Bertollo (1996)
Hypostomus sp. E	Mogi-Guaçu River, São Paulo, Brazil	80	8M, 16SM, 56ST/A	Artoni and Bertollo (1996)
Hypostomus sp. F	São Francisco River, Minas Gerais, Brazil	76	10M, 16SM, 50ST/A	Artoni (1996)
Hypostomus sp. G	Araguaia River, Mato Grosso, Brazil	64	14M, 24SM, 26ST/A	Artoni (1996)
Hypostomus sp. Xingu-1	Xingu River, Pará, Brazil	64	32M/SM, 32ST/A	Milhomem et al. (2010)
Hypostomus sp. Xingu-2	Xingu River, Pará, Brazil	66	32M/SM, 34ST/A	Milhomem et al. (2010)
Hypostomus sp. Xingu-3	Xingu River, Pará, Brazil	65	38M/SM, 26ST/A, 1b	Milhomem et al. (2010)

Cytogenetic studies in 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 were karyotyped and the results employed to discuss the karyotype evolution of the genus.PageBreakPageBreak

Material and methods

Cytogenetic analyses were performed on chromosomal preparations obtained from six species. Five species were collected in the Corumbataí River, São Paulo, Brazil: (Ihering, 1911) (6 males and 4 females) (LBP 2544), (Regan, 1908) (5 males and 6 females) (LBP 2547), (Regan, 1908) (6 males and 7 females) (LBP 2545), prope iheringi (Regan, 1908) (5 males and 4 females) (LBP 1674), and prope paulinus (Ihering, 1905) (5 males and 6 females) (LBP 2548). One species of prope plecostomus (Linnaeus, 1758) (3 males and 2 females) (LBP 2198) was collected in the Orinoco River, Bolivar, Venezuela. Vouchers were deposited in the fish collection of Laboratório de Biologia e Genética de Peixes (LBP), UNESP, Botucatu, São Paulo, Brazil.

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).

Results and discussion

has karyotype with 2n=68 (16M, 4SM, 16ST, 32A) and terminal Ag-NORs on the short arm of the chromosome pair 1 (M) and pair 10 (SM) (Fig. 2a).

Figure 2.

Giemsa stained karyotypes of a , 2n=68 b , 2n=74. Ag-NOR-bearing chromosome pairs in the insets. Bar = 10µm.

has 2n=74 (10M, 20SM, 16ST, 28A) and terminal Ag-NORs on the short arm of the chromosome pair 15 (SM) (Fig. 2b).

prope iheringi has 2n=74 (10M, 14SM, 20ST, 30A) and terminal Ag-NORs on the long arms of the chromosome pairs 23, 24, 25, 30 (A) (Fig. 3a).

Figure 3.

Giemsa stained karyotypes of a prope iheringi, 2n=74 b , 2n=74. Ag-NOR-bearing chromosome pairs in the insets. Bar = 10µm.

prope paulinus has 2n=76 (6M, 18SM, 12ST, 40A) and terminal Ag-NORs on the long arm of the chromosome pair 20 (A) (Fig. 4b).

Figure 4.

Giemsa stained karyotypes of a prope plecostomus, 2n=68 b prope paulinus, 2n=76. Ag-NOR-bearing chromosome pairs in the insets. Bar = 10µm.

prope plecostomus has 2n=68 (12M, 16SM, 12ST, 28A) and terminal Ag-NORs on the short arm of the chromosome pair 16 (ST) (Fig. 4a).PageBreak

has 2n=74 (10M, 14SM, 14ST, 36A) and terminal Ag-NORs on the short arm of the chromosome pair 14 (ST) and on the long arm of the chromosome pairs 21, 22 e 24 (A) (Fig. 3b).

The genus 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 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 (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 (, prope plecostomus, and prope paulinus); and the three others had multiple Ag-NORs (, prope iheringi, and ). 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 PageBreakconsidered 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 (Jenyns, 1842) (Moreira-Filho and Bertollo 1991, Maistro et al. 1998, Alves and Martins-Santos 2002). On the other hand one of the most important problems associated with the study of the genus 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 sp., which reflects our poor taxonomic knowledge of the group. Among the species, the high diploid number is coincident with a high the number of uniarmed chromosomes (Table 1), suggesting thePageBreak 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 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 .