Taxonomic Diversity Not Associated with Gross Karyotype Differentiation: The Case of Bighead Carps, Genus Hypophthalmichthys (Teleostei, Cypriniformes, Xenocyprididae)

Abstract

The bighead carps of the genus Hypophthalmichthys (H. molitrix and H. nobilis) are important aquaculture species. They were subjected to extensive multidisciplinary research, but with cytogenetics confined to conventional protocols only. Here, we employed Giemsa-/C-/CMA3- stainings and chromosomal mapping of multigene families and telomeric repeats. Both species shared (i) a diploid chromosome number 2n = 48 and the karyotype structure, (ii) low amount of constitutive heterochromatin, (iii) the absence of interstitial telomeric sites (ITSs), (iv) a single pair of 5S rDNA loci adjacent to one major rDNA cluster, and (v) a single pair of co-localized U1/U2 snDNA tandem repeats. Both species, on the other hand, differed in (i) the presence/absence of remarkable interstitial block of constitutive heterochromatin on the largest acrocentric pair 11 and (ii) the number of major (CMA3-positive) rDNA sites. Additionally, we applied here, for the first time, the conventional cytogenetics in H. harmandi, a species considered extinct in the wild and/or extensively cross-hybridized with H. molitrix. Its 2n and karyotype description match those found in the previous two species, while silver staining showed differences in distribution of major rDNA. The bighead carps thus represent another case of taxonomic diversity not associated with gross karyotype differentiation, where 2n and karyotype structure cannot help in distinguishing between genomes of closely related species. On the other hand, we demonstrated that two cytogenetic characters (distribution of constitutive heterochromatin and major rDNA) may be useful for diagnosis of pure species. The universality of these markers must be further verified by analyzing other pure populations of bighead carps.

Measurement of Chromosomal Arms and FISH Reveal Complex Genome Architecture and Standardized Karyotype of Model Fish, Genus Carassius

“…The arms of the FISH images were measured on chromosomes that bear positive rDNA loci because the FISH protocol involves denaturation step after which chromosomal structure is disrupted due to high temperature. The rDNA positive loci are mostly accumulated at the pericentromeric chromosomal region in Carassius and another cyprinid fishes [ 32 , 40 , 41 ] and thus the identification of the centromere and measurement of the arms are precise on rDNA positive chromosomes. The i value of rDNA positive chromosomes (arrows on Figure 4 ) were assigned to the closest i value of the standardized karyotype from Table 2 .…”

Present and Future Salmonid Cytogenetics

“…Such uniform stasis has been discovered in groups as diverse as plants [106][107][108], amphibians [109,110] and birds [111]. Among teleost fishes, multiple groups display such apparent karyotype stasis persisting in significantly long stages of lineage divergences, e.g., pikes of the genus Esox [112,113], fishes of the family Leuciscidae ( [88,[114][115][116][117] and references therein), Gobionidae [34], Xenocyprinidae [118] and especially many percomorph groups [119][120][121][122][123][124]. The underlying evolutionary mechanisms for this mode of karyotype (non) differentiation have not been identified so far but they may be at least partially linked with the functional arrangement of chromatin within the interphase nucleus and the degree of tolerance to its change [125,126].…”

Sex chromosome differentiation via changes in the Y chromosome repeat landscape in African annual killifishes Nothobranchius furzeri and N. kadleci

“…The remaining sequence represents non-transcribed spacer (NTS) as confirmed by 96% sequence similarity of the entire 470 bp long fragment with the 5S rDNA sequences from N. furzeri genome (e.g sequence ID: EU780558.1.). Regarding 18S rDNA, for a majority of experiments we used optimized 18S rDNA probe previously prepared from Hypophthalmichthys molitrix [54]. For a subset of experiments, we prepared 18S rDNA probe from genomic DNA (gDNA) of Nothobranchius guentheri .…”