Conserved satellite DNA motif and lack of interstitial telomeric sites in highly rearranged African Nothobranchius killifish karyotypes.

Using African annual killifishes of the genus Nothobranchius from temporary savannah pools with rapid karyotype and sex chromosome evolution, we analysed the chromosomal distribution of telomeric (TTAGGG)n repeat and Nfu-SatC satellite DNA (satDNA; isolated from Nothobranchius furzeri) in 15 species across the Nothobranchius killifish phylogeny, and with Fundulosoma thierryi as an out-group. Our fluorescence in situ hybridization experiments revealed that all analysed taxa share the presence of Nfu-SatC repeat but with diverse organization and distribution on chromosomes. Nfu-SatC landscape was similar in conspecific populations of Nothobranchius guentheri and Nothobranchius melanospilus but slightly-to-moderately differed between populations of Nothobranchius pienaari, and between closely related Nothobranchius kuhntae and Nothobranchius orthonotus. Inter-individual variability in Nfu-SatC patterns was found in N. orthonotus and Nothobranchius krysanovi. We revealed mostly no sex-linked patterns of studied repetitive DNA distribution. Only in Nothobranchius brieni, possessing multiple sex chromosomes, Nfu-SatC repeat occupied a substantial portion of the neo-Y chromosome, similarly as formerly found in the XY sex chromosome system of turquoise killifish N. furzeri and its sister species Nothobranchius kadleci-representatives not closely related to N. brieni. All studied species further shared patterns of expected telomeric repeats at the ends of all chromosomes and no additional interstitial telomeric sites. In summary, we revealed (i) the presence of conserved satDNA class in Nothobranchius clades (a rare pattern among ray-finned fishes); (ii) independent trajectories of Nothobranchius sex chromosome differentiation, with recurrent and convergent accumulation of Nfu-SatC on the Y chromosome in some species; and (iii) genus-wide shared tendency to loss of telomeric repeats during interchromosomal rearrangements. Collectively, our findings advance our understanding of genome structure, mechanisms of karyotype reshuffling, and sex chromosome differentiation in Nothobranchius killifishes from the genus-wide perspective.

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

Homomorphic sex chromosomes and their turnover are common in teleosts. We investigated the evolution of nascent sex chromosomes in several populations of two sister species of African annual killifishes, Nothobranchius furzeri and N. kadleci, focusing on their under-studied repetitive landscape. We combined bioinformatic analyses of the repeatome with molecular cytogenetic techniques, including comparative genomic hybridization, fluorescence in situ hybridization with satellite sequences, ribosomal RNA genes (rDNA) and bacterial artificial chromosomes (BACs), and immunostaining of SYCP3 and MLH1 proteins to mark lateral elements of synaptonemal complexes and recombination sites, respectively. Both species share the same heteromorphic XY sex chromosome system, which thus evolved prior to their divergence. This was corroborated by sequence analysis of a putative master sex determining (MSD) gene gdf6Y in both species. Based on their divergence, differentiation of the XY sex chromosome pair started approximately 2 million years ago. In all populations, the gdf6Y gene mapped within a region rich in satellite DNA on the Y chromosome long arms. Despite their heteromorphism, X and Y chromosomes mostly pair regularly in meiosis, implying synaptic adjustment. In N. kadleci, Y-linked paracentric inversions like those previously reported in N. furzeri were detected. An inversion involving the MSD gene may suppress occasional recombination in the region, which we otherwise evidenced in the N. furzeri population MZCS-121 of the Limpopo clade lacking this inversion. Y chromosome centromeric repeats were reduced compared with the X chromosome and autosomes, which points to a role of relaxed meiotic drive in shaping the Y chromosome repeat landscape. We speculate that the recombination rate between sex chromosomes was reduced due to heterochiasmy. The observed differences between the repeat accumulations on the X and Y chromosomes probably result from high repeat turnover and may not relate closely to the divergence inferred from earlier SNP analyses.

Evolutionary pattern of karyotypes and meiosis in pholcid spiders (Araneae: Pholcidae): implications for reconstructing chromosome evolution of araneomorph spiders.

BACKGROUND: Despite progress in genomic analysis of spiders, their chromosome evolution is not satisfactorily understood. Most information on spider chromosomes concerns the most diversified clade, entelegyne araneomorphs. Other clades are far less studied. Our study focused on haplogyne araneomorphs, which are remarkable for their unusual sex chromosome systems and for the co-evolution of sex chromosomes and nucleolus organizer regions (NORs); some haplogynes exhibit holokinetic chromosomes. To trace the karyotype evolution of haplogynes on the family level, we analysed the number and morphology of chromosomes, sex chromosomes, NORs, and meiosis in pholcids, which are among the most diverse haplogyne families. The evolution of spider NORs is largely unknown. RESULTS: Our study is based on an extensive set of species representing all major pholcid clades. Pholcids exhibit a low 2n and predominance of biarmed chromosomes, which are typical haplogyne features. Sex chromosomes and NOR patterns of pholcids are diversified. We revealed six sex chromosome systems in pholcids (X0, XY, X1X20, X1X2X30, X1X2Y, and X1X2X3X4Y). The number of NOR loci ranges from one to nine. In some clades, NORs are also found on sex chromosomes. CONCLUSIONS: The evolution of cytogenetic characters was largely derived from character mapping on a recently published molecular phylogeny of the family. Based on an extensive set of species and mapping of their characters, numerous conclusions regarding the karyotype evolution of pholcids and spiders can be drawn. Our results suggest frequent autosome-autosome and autosome-sex chromosome rearrangements during pholcid evolution. Such events have previously been attributed to the reproductive isolation of species. The peculiar X1X2Y system is probably ancestral for haplogynes. Chromosomes of the X1X2Y system differ considerably in their pattern of evolution. In some pholcid clades, the X1X2Y system has transformed into the X1X20 or XY systems, and subsequently into the X0 system. The X1X2X30 system of Smeringopus pallidus probably arose from the X1X20 system by an X chromosome fission. The X1X2X3X4Y system of Kambiwa probably evolved from the X1X2Y system by integration of a chromosome pair. Nucleolus organizer regions have frequently expanded on sex chromosomes, most probably by ectopic recombination. Our data suggest the involvement of sex chromosome-linked NORs in achiasmatic pairing.

Karyotype Evolution in Harvestmen of the Suborder Cyphophthalmi (Opiliones).

The morphologically uniform suborder Cyphophthalmi represents a basal group of harvestmen (Opiliones). As such, it plays an important role in the reconstruction of the karyotype evolution within this arachnid order. The cytogenetic analysis of 6 representatives of the suborder Cyphophthalmi, namely Miopsalis sp. (2n = 30; Stylocellidae), Austropurcellia arcticosa (Cantrell, 1980) (2n = 30; Pettalidae), Parapurcellia amatola de Bivort & Giribet, 2010 (2n = 32; Pettalidae), Paramiopsalis aff. ramulosus Juberthie, 1962 (2n = 28; Sironidae), Cyphophthalmus duricorius Joseph, 1868 (2n = 24; Sironidae), and Siro carpaticus Rafalski, 1956 (2n = 52; Sironidae) was performed. Fluorescence in situ hybridization with 18S rDNA probe was used to analyze the distribution of major ribosomal RNA genes in harvestmen. We confront the obtained cytogenetic data with current hypotheses on cyphophthalmid phylogeny to reconstruct their karyotype evolution. We conclude that the ancestral karyotype of harvestmen consisted of 2n = 30 elements with 1 chromosome pair bearing terminal rDNA clusters. The rDNA locus was multiplicated in the evolution of Cyphophthalmi. However, decreases as well as increases in the number of chromosomes have been detected in the karyotype evolution of Cyphophthalmi. Our data thus reveal unexpected diversity in cyphophthalmid karyotypes.