B Chromosomes in Free-Living Flatworms of the Genus Macrostomum (Platyhelminthes, Macrostomorpha)

B chromosomes (Bs) or supernumerary chromosomes are extra chromosomes in the species karyotype that can vary in its copy number. Bs are widespread in eukaryotes. Usually, the Bs of specimens collected from natural populations are the object of the B chromosome studies. We applied another approach analyzing the Bs in animals maintained under the laboratory conditions as lines and cultures. In this study, three species of the Macrostomum genus that underwent a recent whole-genome duplication (WGD) were involved. In laboratory lines of M. lignano and M. janickei, the frequency of Bs was less than 1%, while in the laboratory culture of M. mirumnovem, it was nearer 30%. Their number in specimens of the culture varied from 1 to 14. Mosaicism on Bs was discovered in parts of these animals. We analyzed the distribution of Bs among the worms of the laboratory cultures during long-term cultivation, the transmission rates of Bs in the progeny obtained from crosses of worms with different numbers of Bs, and from self-fertilized isolated worms. The DNA content of the Bs in M. mirumnovem was analyzed with the chromosomal in situ suppression (CISS) hybridization of microdissected DNA probes derived from A chromosomes (As). Bs mainly consisted of repetitive DNA. The cytogenetic analysis also revealed the divergence and high variation in large metacentric chromosomes (LMs) containing numerous regions enriched for repeats. The possible mechanisms of the appearance and evolution of Bs and LMs in species of the Macrostomum genus were also discussed.

B Chromosomes in Free-Living Flatworms of the Genus Macrostomum (Platyhelminthes, Macrostomorpha)

B chromosomes (Bs) or supernumerary chromosomes are extra chromosomes in the species karyotype that can vary in their copy number. Bs are widespread. Usually, the Bs of specimens collected from natural populations are involved in studies. We applied another approach analyzing the Bs in animals of laboratory cultures. In this study, three species of the Macrostomum genus that underwent a recent whole-genome duplication (WGD) were involved. In laboratory lines of M. lignano and M. janickei, the frequency of Bs was less than 1%, while in the laboratory culture of M. mirumnovem, it was nearer 30%. Their number in specimens of the culture varied from 1 to 14. Mosaicism on Bs was discovered in parts of these animals. We analyzed the distribution of Bs among the worms of the laboratory cultures during long-term cultivation, the transmission rates of Bs in the progeny obtained from crosses of worms with different numbers of Bs, and from self-fertilized isolated worms. The DNA content of the Bs in M. mirumnovem was analyzed with the chromosomal in situ suppression (CISS) hybridization of microdissected DNA probes derived from A chromosomes (As). Bs mainly consisted of repetitive DNA. The cytogenetic analysis also revealed the divergence and high variation in large metacentric chromosomes (LMs) containing numerous regions enriched for repeats. The possible mechanisms of the appearance and evolution of Bs and LMs in species of the Macrostomum genus were also discussed.

The B Chromosomes of Prochilodus lineatus (Teleostei, Characiformes) Are Highly Enriched in Satellite DNAs

B or supernumerary chromosomes are dispensable elements that are widely present in numerous eukaryotes. Due to their non-recombining nature, there is an evident tendency for repetitive DNA accumulation in these elements. Thus, satellite DNA plays an important role in the evolution and diversification of B chromosomes and can provide clues regarding their origin. The characiform Prochilodus lineatus was one of the first discovered fish species bearing B chromosomes, with all populations analyzed so far showing one to nine micro-B chromosomes and exhibiting at least three morphological variants (Ba, Bsm, and Bm). To date, a single satellite DNA is known to be located on the B chromosomes of this species, but no information regarding the differentiation of the proposed B-types is available. Here, we characterized the satellitome of P. lineatus and mapped 35 satellite DNAs against the chromosomes of P. lineatus, of which six were equally located on all B-types and this indicates a similar genomic content. In addition, we describe, for the first time, an entire population without B chromosomes.

Cytomolecular investigations using repetitive DNA probes contribute to the identification and characterization of Characidium sp. aff. C. vidali (Teleostei: Characiformes)

Characidium sp. aff. C. vidali is a species found in coastal streams in southeastern Brazil, which has karyotypic explanatory elements as the occurrence of microstructural variations, keeping the chromosomal macrostructure of the genus. The objective of this study was to apply cytomolecular tools in the chromosomes of Characidium sp. aff. C. vidali to identify characteristics in their karyotype contributing to cytogenetic definition of this species, adding information about the evolution of the chromosomal structure of the group. The species showed 2n = 50 chromosomes and from 1 to 4 additional B microchromosomes. FISH technique showed histone H3 and H4 genes in the short arm of pair 10, and microsatellites (CA)15, (CG)15, (GA)15 and (TTA)10 clustered in the subtelomeric portions of all A chromosomes, with total accumulation by supernumerary. The telomeric probe marked terminal regions of all chromosomes, in addition to the interstitial portion of four pairs, called ITS sites, with these markings being duplicated in two pairs, hence the double-ITS classification. C-banding revealed that supernumerary chromosomes are completely heterochromatic, that ITS sites are C-banding positive, but double-ITS sites are C-banding negative. So, throughout the evolution to Characidium, genomic events are occurring and restructuring chromosomes in populations.

The role of cytogenetic variation in Akodon cursor species complex speciation (Rodentia: Sigmodontinae)

Chromosome polymorphism in populations of Akodon cursor complex (124 specimens) and the karyotypes of other Akodon species (92 specimens) from Brazil were analyzed. Five species were analyzed: Akodon cursor with 2n = 14, and Akodon aff. cursor with 2n = 16, Akodon montensis with 2n = 24, Akodon paranaensis with 2n = 44 and Akodon serrensis with 2n = 46. Chromosome polymorphism was observed in A. cursor and A. aff. cursor showing pericentric inversions, and also in A. paranaensis and A. montensis presenting supernumerary chromosomes. Pericentric inversion polymorphism affecting two autosomes pairs in A. cursor karyotype was found in Hardy-Weinberg equilibrium. Akodon with 2n = 16 occurred from Bahia to Rio Grande do Norte states and A. cursor with 2n = 14 from Bahia to Paraná states. Molecular analyses showed high genetic distance estimates between such set of samples, suggesting that karyomorphic type 2n = 16 is fixed in the northern part of the distribution of Akodon cursor complex. Captive hybrids males between Akodon [2n = 14] x [2n = 16] are apparently sterile, suggesting these populations may represent two full species.

Host-specific subtelomere: Genomic architecture of pathogen emergence in asexual filamentous fungi

Several asexual species of filamentous fungal pathogens contain supernumerary chromosomes carrying secondary metabolite (SM) or pathogenicity genes. Supernumerary chromosomes have been shown in in vitro experiments to transfer from pathogenic isolates to non-pathogenic ones and between isolates whose fusion can result in vegetative or heterokaryon incompatibility (HET). However, much is still unknown about the acquisition and maintenance of SM/pathogenicity gene clusters in the adaptation of these asexual pathogens to their hosts. We investigated several asexual fungal pathogens for genomic elements involved in maintaining telomeres for supernumerary and core chromosomes during vegetative reproduction. We found that in vegetative species or lineages with a nearly complete telomere-to-telomere genome assembly (e.g. Fusarium equiseti and five formae speciales of the F. oxysporum species complex), core and super-numerary chromosomes were flanked by highly similar subtelomeric sequences on the 3’ side and by their reverse complements on the 5’ side. This subtelomere sequence structure was preserved in isolates from the same species or from polyphyletic lineages in the same forma specialis (f.sp.) of the F. oxysporum species complex. Moreover, between some isolates within F. oxysporum f.sp. lycopersici, the mean rate of single nucleotide polymorphisms (SNPs) in a supernumerary chromosome was at least 300 times lower than those in core chromosomes. And a large number of HET domain genes were located in SM/pathogenicity gene clusters, with a potential role in maintaining these gene clusters during vegetative reproduction.

Meiotic drive of female-inherited supernumerary chromosomes in a pathogenic fungus

Meiosis is a key cellular process of sexual reproduction that includes pairing of homologous sequences. In many species however, meiosis can also involve the segregation of supernumerary chromosomes, which can lack a homolog. How these unpaired chromosomes undergo meiosis is largely unknown. In this study we investigated chromosome segregation during meiosis in the haploid fungus Zymoseptoria tritici that possesses a large complement of supernumerary chromosomes. We used isogenic whole chromosome deletion strains to compare meiotic transmission of chromosomes when paired and unpaired. Unpaired chromosomes inherited from the male parent as well as paired supernumerary chromosomes in general showed Mendelian inheritance. In contrast, unpaired chromosomes inherited from the female parent showed non-Mendelian inheritance but were amplified and transmitted to all meiotic products. We concluded that the supernumerary chromosomes of Z. tritici show a meiotic drive and propose an additional feedback mechanism during meiosis, which initiates amplification of unpaired female-inherited chromosomes.

Rex Retroelements and Teleost Genomes: An Overview

Repetitive DNA is an intriguing portion of the genome still not completely discovered and shows a high variability in terms of sequence, genomic organization, and evolutionary mode. On the basis of the genomic organization, it includes satellite DNAs, which are organized as long arrays of head-to-tail linked repeats, and transposable elements, which are dispersed throughout the genome. These repeated elements represent a considerable fraction of vertebrate genomes contributing significantly in species evolution. In this review, we focus our attention on Rex1, Rex3 and Rex6, three elements specific of teleost genomes. We report an overview of data available on these retroelements highlighting their significative impact in chromatin and heterochromatin organization, in the differentiation of sex chromosomes, in the formation of supernumerary chromosomes, and in karyotype evolution in teleosts.