Hypostomus boulengeri (Eigenmann & Kennedy, 1903)

Paiva, Suzana de, Porto, Fernanda Errero, Codognotto, Flávio José, Fernandes, Carlos Alexandre, Rossi, Margarida Maria Vieira, Borin-Carvalho, Luciana Andreia, Portela-Castro, Ana Luiza de Brito, Zawadzki, Claudio Henrique, Renesto, Erasmo & Martins-Santos, Isabel Cristina, 2024, Allozyme and cytogenetic analysis in two species of Hypostomus (Siluriformes: Loricariidae) from the Paraguai River basin, Brazil: occurrence of B microchromosome and intrapopulation heterochromatic polymorphism in H. boulengeri, Neotropical Ichthyology 22 (2), pp. e 230117-e 230117 : e230117-230117

Cytogenetic data. Individuals of

Hypostomus boulengeri

presented a diploid number of 2n = 68 distributed in 14m +22sm+10st+22a and a fundamental number (FN) equal to 114 ( Fig. 2A ). In addition to the basic karyotype, all male and female individuals presented a variation from zero to one B microchromosomes in the somatic cells without homology with the other chromosomes ( Fig. 2A , in the box). These elements are smaller than any chromosome of the standard A complement and presented 6% of frequency in the cells analyzed ( Tab. 2 ). Hypostomus cochliodon showed 2n = 64 with the karyotypic formula of 16m +22sm+18st+8a and FN = 120 ( Fig. 3A ). There were no karyotypic differences between males and females in both species.

Analysis of the nucleolus organizer region performed with Ag-NOR and 18S rDNA FISH techniques in H. boulengeri showed a multiple NOR located on the short arm of three pairs of submetacentric chromosomes (pairs: 9, 10, and 14; Fig. 2A , in the box) and in the telomere region on the long arms of pair 24 ( Fig. 2A , in the box). In H. cochliodon, the single NOR was detected on the long arm of the first pair of acrocentric chromosomes at the telomeric position (pair 29; Figs. 3A, C , in the boxes). However, in some individuals of this species, additional staining was detected in the short arm in one of the homologs of the NOR organizing pair (NORs in both telomeres, Figs. 3A, C, in the boxes).

In H. boulengeri , the C-banding revealed pericentromeric heterochromatin blocks in most metacentric, submetacentric, and subtelocentric chromosomes and the short arm of pairs 9, 10, and 14 coinciding with the NOR regions ( Fig. 2B ). Furthermore, a conspicuous heterochromatic segment can also be observed in the long arm of some acrocentric chromosomes in both sexes (pairs: 24, 25, and 26), and the number of chromosomes containing this type of heterochromatin varied among individuals in the population (two to six chromosomes; Figs. 2C–F ), characterizing a numerical polymorphism. Additionally, some individuals have heterochromatic B microchromosome ( Fig. 2B , in the box). Heterochromatin in H. cochliodon was evidenced mainly in the pericentromeric regions of metacentric and submetacentric chromosomes and blocks on the short arm of the submetacentric and subtelocentric chromosomes ( Fig. 3B ). Further, the NOR region was C-banding positive ( Fig. 3B , in the box).

Allozyme data. Nine enzymatic systems allowed the analysis of 15 loci of the species H. boulengeri and H. cochliodon , presenting 30 alleles; among these were diagnoses (Idh-A and Gcdh-A; Tab. 3 ). In H. boulengeri , several exclusive alleles were detected with variable frequencies at the loci: Aat-A- a and c, Adh-A- b and c, Gpi-B- a and d, G3pdh-A- b and c, G3pdh-B- b and Idh-B- a. Regarding the genetic variability of the two populations, values of 0.2461 and 0.0309 were found for the average expected heterozygosity (H e) ( Tab. 3 ) for H. boulengeri and H. cochliodon , respectively.

Figures

FIGURE 2 | Karyotypes of Hypostomus boulengeri subjected to A. Giemsa, B. C-banding and G. FISH with 18S rDNA probe (yellow). Polymorphism of the heterochromatin in pairs 24, 25, and 26 are shown in C, D, E, and F. The Ag-NOR- bearing chromosomes are boxed beside the karyotype stained with Giemsa. The B microchromosomes are boxed beside the karyotypes stained with Giemsa and C-banding. Scale bar = 10 µm.

FIGURE 3 | Karyotypes of Hypostomus cochliodon subjected to A. Giemsa, B. C-banding, and C. FISH with 18S rDNA probe (yellow). The Ag-NOR-bearing chromosomes are boxed beside the karyotype stained with Giemsa. Note one of the homologs of pair 29 with marking in both telomeres after stained Ag-NOR (Box in A), C-banding (Box in B), and FISH with 18S rDNA probe (Box in C). Scale bar = 10 µm.