Cytogenetics of nine species of mediterranean blennies and additional evidence for an unusual multiple sex-chromosome system in Parablennius tentacularis (Perciformes, Blenniidae)

Multiple sex chromosomes in teleost fishes from a cytogenetic perspective: state of the art and future challenges

Despite decades of cytogenetic and genomic research of dynamic sex chromosome evolution in teleost fishes, multiple sex chromosomes have been largely neglected. In this review, we compiled available data on teleost multiple sex chromosomes, identified major trends in their evolution and suggest further trajectories in their investigation. In a compiled dataset of 440 verified records of fish sex chromosomes, we counted 75 multiple sex chromosome systems with 60 estimated independent origins. We showed that male-heterogametic systems created by Y-autosome fusion predominate and that multiple sex chromosomes are over-represented in the order Perciformes. We documented a striking difference in patterns of differentiation of sex chromosomes between male and female heterogamety and hypothesize that faster W sex chromosome differentiation may constrain sex chromosome turnover in female-heterogametic systems. We also found no significant association between the mechanism of multiple sex chromosome formation and percentage of uni-armed chromosomes in teleost karyotypes. Last but not least, we hypothesized that interaction between fish populations, which differ in their sex chromosomes, can drive the evolution of multiple sex chromosomes in fishes. This underlines the importance of broader inter-population sampling in studies of fish sex chromosomes. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.

Preferential accumulation of sex and Bs chromosomes in biarmed karyotypes by meiotic drive and rates of chromosomal changes in fishes

Mechanisms of accumulation based on typical centromeric drive or of chromosomes carrying pericentric inversions are adjusted to the general karyotype differentiation in the principal Actinopterygii orders. Here, we show that meiotic drive in fish is also supported by preferential establishment of sex chromosome systems and B chromosomes in orders with predominantly bi-brachial chromosomes. The mosaic of trends acting at an infra-familiar level in fish could be explained as the interaction of the directional process of meiotic drive as background, modulated on a smaller scale by adaptive factors or specific karyotypic properties of each group, as proposed for the orthoselection model.

Twenty years of physical mapping of major ribosomal RNA genes across the teleosts: A review of research

Molecular cytogenetic data on the number and position of 45S ribosomal RNA genes (rDNA; located in nucleolus organizing regions, NORs) detected by FISH in 330 species of 77 families and 22 orders of bony fishes (Teleostei) and, additionally, 11 species of basal ray-finned fishes are compiled and analyzed. The portion of species with single rDNA sites in the sample amounts to 72%. The percentage of species with multiple NORs decreases with increasing numbers of rDNA loci per genome, i.e. scarcely 3% of species carry 4 or more rDNA-bearing chromosome pairs. 43% of all rDNA sites analyzed occur terminally on the short arms of chromosomes or constitute them. In general, terminal rDNA sites account for 87% of all examined cases. Interspecific variation in the location of single rDNA sites among related taxa, polymorphisms of multiple NORs in some groups of teleosts and analytical outcomes on the subject are reviewed.

Turnover of sex chromosomes and speciation in fishes

Closely related species of fishes often have different sex chromosome systems. Such rapid turnover of sex chromosomes can occur by several mechanisms, including fusions between an existing sex chromosome and an autosome. These fusions can result in a multiple sex chromosome system, where a species has both an ancestral and a neo-sex chromosome. Although this type of multiple sex chromosome system has been found in many fishes, little is known about the mechanisms that select for the formation of neo-sex chromosomes, or the role of neo-sex chromosomes in phenotypic evolution and speciation. The identification of closely related, sympatric species pairs in which one species has a multiple sex chromosome system and the other has a simple sex chromosome system provides an opportunity to study sex chromosome turnover. Recently, we found that a population of threespine stickleback (Gasterosteus aculeatus) from Japan has an X₁X₂Y multiple sex chromosome system resulting from a fusion between the ancestral Y chromosome and an autosome, while a sympatric threespine stickleback population has a simple XY sex chromosome system. Furthermore, we demonstrated that the neo-X chromosome (X₂) plays an important role in phenotypic divergence and reproductive isolation between these sympatric stickleback species pairs. Here, we review multiple sex chromosome systems in fishes, as well as recent advances in our understanding of the evolutionary role of sex chromosome turnover in stickleback speciation.

Chromosomal complements of some Atlantic Blennioidei and Gobioidei species (Perciformes)

A remarkable degree of chromosomal conservatism (2n=48, FN=48) has been identified in several families of Perciformes. However, some families exhibit greater karyotypic diversity, although there is still scant information on the Atlantic species. In addition to a review of karyotypic data available for representatives of the suborders Blennioidei and Gobioidei, we have performed chromosomal analyses on Atlantic species of the families Blenniidae, Ophioblennius trinitatis Miranda-Ribeiro, 1919 (2n=46; FN=64) and Scartella cristata (Linnaeus, 1758)(2n=48; FN=50), Labrisomidae, Labrisomus nuchipinnis (Quoy & Gaimard, 1824)(2n=48; FN=50) and Gobiidae, Bathygobius soporator (Valenciennes, 1837)(2n=48; FN=56). Besides variations in chromosome number and karyotype formulas, Ag-NOR sites, albeit unique, were located in different positions and/or chromosome pairs for the species analyzed. On the other hand, the heterochromatic pattern was more conservative, distributed predominantly in the centromeric/pericentromeric regions of the four species. Data already available for Gobiidae, Blenniidae and Labrisomidae show greater intra- and interspecific karyotypic diversification when compared to other groups of Perciformes, where higher uniformity is found for various chromosome characteristics. Evolutionary dynamism displayed by these two families is likely associated with population fractionation resulting from unique biological characteristics, such as lower mobility and/or specific environmental requirements.

Cytogenetic characterization of three Balistoidea fish species from the Atlantic with inferences on chromosomal evolution in the families Monacanthidae and Balistidae

The Tetraodontiformes are the most derived group of teleostean fish. Among other apomorphies, they are characterized by a high degree of fusions or significant bone loss in the head and body. In the early phylogenetic proposals presented for this order, the families Balistidae and Monacanthidae have been unanimously considered to be closely related. Although they have moderate species diversity, they are scarcely known in cytogenetic aspect and chromosomal pattern comparisons between these groups have yet to be established. The species Cantherhines macrocerus (Hollard,1853), Cantherhines pullus (Ranzani, 1842) (Monacanthidae) and Melichthys niger (Bloch, 1786) (Balistidae) were cytogenetically analyzed using conventional (Ag-impregnation, C-banding, CMA3- and DAPI-fluorescence) and molecular (FISH with an 18S rDNA probe) cytogenetic protocols. The karyotypes of all three species were very similar possessing diploid chromosome numbers 2n = 40 and composed exclusively of acrocentric chromosomes. Single NOR-bearing pair as well as positive heterochromatic blocks at pericentromeric regions were identified in the karyotypes of the three species studied. NOR-bearing sites were positively labeled after Ag-impregnation, C-banding, CMA3-fluorescence and FISH with an 18S rDNA probe but were negative after DAPI-fluorescence. Such remarkable shared conspicuous chromosomal characters corroborate either close phylogenetic relationship of these families, previously established by morphological and molecular data, or rather conservative nature of karyotype differentiation processes. The later hypothesis, however, appears less probable due to centric or in tandem fusions documented for another Balistoidea species.

Multiple sex chromosome system of X1X1X2X2/X1X2)Y type in lutjanid fish, Lutjanus quinquelineatus (Perciformes)

The karyotype and other chromosomal markers as revealed by C-banding and Ag-staining were studied in Lutjanus quinquelineatus and L. kasmira (Lutjanidae, Perciformes). While in latter species, the karyotype was invariably composed of 48 acrocentric chromosomes in both sexes, in L. quinquelineatus the female karyotype had exclusively 48 acrocentric chromosomes (2n = 48) but that of the male consisted of one large metacentric and 46 acrocentric chromosomes (2n = 47). The chromosomes in the first meiotic division in males showed 22 bivalents and one trivalent, which was formed by an end-to-end association and a chiasmatic association. Multiple sex chromosome system of X(1)X(1)X(2)X(2)/X(1)X(2)Y type resulting from single Robertsonian fusion between the original Y chromosome and an autosome was hypothesized to produce neo-Y sex chromosome. The multiple sex chromosome system of L. quinquelineatus appears to be at the early stage of the differentiation. The positive C-banded heterochromatin was situated exclusively in centromeric regions of all chromosomes in both species. Similarly, nucleolus organizer region sites were identified in the pericentromeric region of one middle-sized pair of chromosomes in both species. The cellular DNA contents were the same (3.3 pg) between the sexes and among this species and related species.