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Cytogenetics can be used as a tool to study the evolution of polyploidy and taxonomy. Here we focus on aquatic African pipids, dwarf clawed frogs (Hymenochirus). Our study reveals that dwarf clawed frogs, present for decades in captivity, are best referred to as Hymenochirus sp. instead of the commonly used name ' H. boettgeri ' or sometimes ' H. curtipes '. We present the first karyotype from a morphologically identified specimen of H. boettgeri with a known locality in the north-western Congo, which is tetraploid with 2n = 36. The captive Hymenochirus species has been found diploid in previous studies with different reported chromosome numbers; here we reveal 2n = 20A + 1B chromosomes. Our findings suggest that the tetraploid H. boettgeri karyotype evolved through fusion of two biarmed chromosomes and subsequent allotetraploidization, and is functionally diploid, similar to the origin of tetraploid clawed frogs in the subgenus Xenopus. We observed the stable presence of a single B chromosome in both sexes of our individuals from the captive population of Hymenochirus sp. However, additional investigation is necessary to clarify whether there is variation in the number of A and B chromosomes among populations, individuals, and/or tissues. Further research is also needed to understand the evolution and taxonomy of the genus Hymenochirus. [ABSTRACT FROM AUTHOR]

Fishes of the genus Carassius are useful experimental vertebrate models for the study of evolutionary biology and cytogenetics. Carassius demonstrates diverse biological characteristics, such as variation in ploidy levels and chromosome numbers, and presence of microchromosomes. Those Carassius polyploids with ≥ 150 chromosomes have microchromosomes, but the origin of microchromosomes, especially in European populations, is unknown. We used cytogenetics to study evolution of tandem repeats (U1 and U2 small nuclear DNAs and H3 histone) and microchromosomes in Carassius from the Czech Republic. We tested the hypotheses whether the number of tandem repeats was affected by polyploidization or divergence between species and what mechanism drives evolution of microchromosomes. Tandem repeats were found in tetraploid and hexaploid Carassius gibelio , and tetraploid Carassius auratus and Carassius carassius in conserved numbers, with the exception of U1 small nuclear DNA in C. auratus. This conservation indicates reduction and/or loss in the number of copies per locus in hexaploids and may have occurred by divergence rather than polyploidization. To study the evolution of microchromosomes, we used the whole microchromosome painting probe from hexaploid C. gibelio and hybridized it to tetraploid and hexaploid C. gibelio , and tetraploid C. auratus and C. carassius. Our results revealed variation in the number of microchromosomes in hexaploids and indicated that the evolution of the Carassius karyotype is governed by macrochromosome fissions followed by segmental duplication in pericentromeric areas. These are potential mechanisms responsible for the presence of microchromosomes in Carassius hexaploids. Differential efficacy of one or both of these mechanisms in different tetraploids could ensure variability in chromosome number in polyploids in general. [ABSTRACT FROM AUTHOR]

Allopolyploid genomes are divided into compartments called subgenomes that are derived from lower ploidy ancestors. In African clawed frogs of the subgenus Xenopus (genus Xenopus), allotetraploid species have two subgenomes (L and S) with morphologically distinct homoeologous chromosomes. In allotetraploid species of the sister subgenus Silurana , independently evolved subgenomes also exist, but their cytogenetics has not been investigated in detail. We used a diverse suite of cytogenetic and molecular FISH techniques on an allotetraploid species in Silurana — Xenopus calcaratus —to explore evolutionary dynamics of chromosome morphology and rearrangements. We find that the subgenomes of X. calcaratus have distinctive characteristics, with a more conserved a-subgenome resembling the closely related genome of the diploid species X. tropicalis , and a more rapidly evolving b-subgenome having more pronounced changes in chromosome structure, including diverged heterochromatic blocks, repetitive sequences, and deletion of a nucleolar secondary constriction. Based on these cytogenetic differences, we propose a chromosome nomenclature for X. calcaratus that may apply to other allotetraploids in subgenus Silurana , depending on as yet unresolved details of their evolutionary origins. These findings highlight the potential for large-scale asymmetry in subgenome evolution following allopolyploidization. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]