The molecular organisation of a B chromosome tandem repeat sequence from Brachycome dichromosomatica

B Chromosomes in Genus Sorghum (Poaceae)

B chromosomes (Bs) are supernumerary dispensable genomic elements that have been reported in several thousand eukaryotic species. Since their discovery, Bs have been subjected to countless studies aiming at the clarification of their origin, composition, and influence on the carriers. Despite these efforts, we still have very limited knowledge of the processes that led to the emergence of Bs, the mechanisms of their transmission, and the effects of Bs on the hosts. In the last decade, sophisticated molecular methods, including next-generation sequencing, have provided powerful tool to help answer some of these questions, but not many species have received much attention yet. In this review, we summarize the currently available information about Bs in the genus Sorghum, which has so far been on the periphery of scientific interest. We present an overview of the occurrence and characteristics of Bs in various Sorghum species, discuss the possible mechanisms involved in their maintenance and elimination, and outline hypotheses of the origin of Bs in this genus.

Evolution of Plant B Chromosome Enriched Sequences

B chromosomes are supernumerary chromosomes found in addition to the normal standard chromosomes (A chromosomes). B chromosomes are well known to accumulate several distinct types of repeated DNA elements. Although the evolution of B chromosomes has been the subject of numerous studies, the mechanisms of accumulation and evolution of repetitive sequences are not fully understood. Recently, new genomic approaches have shed light on the origin and accumulation of different classes of repetitive sequences in the process of B chromosome formation and evolution. Here we discuss the impact of repetitive sequences accumulation on the evolution of plant B chromosomes.

High chromosomal variation in wild horn fly Haematobiairritans (Linnaeus) (Diptera, Muscidae) populations

The horn fly, Haematobiairritans is an obligate haematophagous cosmopolitan insect pest. The first reports of attacks on livestock by Haematobiairritans in Argentina and Uruguay occurred in 1991, and since 1993 it is considered an economically important pest. Knowledge on the genetic characteristics of the horn fly increases our understanding of the phenotypes resistant to insecticides that repeatedly develop in these insects. The karyotype of Haematobiairritans, as previously described using flies from an inbred colony, shows a chromosome complement of 2n=10 without heterochromosomes (sex chromosomes). In this study, we analyze for the first time the chromosome structure and variation of four wild populations of Haematobiairritans recently established in the Southern Cone of South America, collected in Argentina and Uruguay. In these wild type populations, we confirmed and characterized the previously published "standard" karyotype of 2n=10 without sex chromosomes; however, surprisingly a supernumerary element, called B-chromosome, was found in about half of mitotic preparations. The existence of statistically significant karyotypic diversity was demonstrated through the application of orcein staining, C-banding and H-banding. This study represents the first discovery and characterization of horn fly karyotypes with 2n=11 (2n=10+B). All spermatocytes analyzed showed 5 chromosome bivalents, and therefore, 2n=10 without an extra chromosome. Study of mitotic divisions showed that some chromosomal rearrangements affecting karyotype structure are maintained as polymorphisms, and multiple correspondence analyses demonstrated that genetic variation was not associated with geographic distribution. Because it was never observed during male meiosis, we hypothesize that B-chromosome is preferentially transmitted by females and that it might be related to sex determination.

Plant B chromosomes

B chromosomes are dispensable elements of the genome that do not recombine with the A chromosomes of the regular complement and that follow their own evolutionary pathway. Here, we survey current knowledge on the DNA/chromatin composition, origin, and drive mechanisms of B chromosomes and discuss the potential research applications of supernumerary chromosomes.

The B chromosomes of the African cichlid fish Haplochromis obliquidens harbour 18S rRNA gene copies

BACKGROUND

Diverse plant and animal species have B chromosomes, also known as accessory, extra or supernumerary chromosomes. Despite being widely distributed among different taxa, the genomic nature and genetic behavior of B chromosomes are still poorly understood.

RESULTS

In this study we describe the occurrence of B chromosomes in the African cichlid fish Haplochromis obliquidens. One or two large B chromosome(s) occurring in 39.6% of the analyzed individuals (both male and female) were identified. To better characterize the karyotype and assess the nature of the B chromosomes, fluorescence in situ hybridization (FISH) was performed using probes for telomeric DNA repeats, 18S and 5S rRNA genes, SATA centromeric satellites, and bacterial artificial chromosomes (BACs) enriched in repeated DNA sequences. The B chromosomes are enriched in repeated DNAs, especially non-active 18S rRNA gene-like sequences.

CONCLUSION

Our results suggest that the B chromosome could have originated from rDNA bearing subtelo/acrocentric A chromosomes through formation of an isochromosome, or by accumulation of repeated DNAs and rRNA gene-like sequences in a small proto-B chromosome derived from the A complement.

The giant B chromosome of the cyprinid fish Alburnus alburnus harbours a retrotransposon-derived repetitive DNA sequence

The cyprinid fish Alburnus alburnus possesses one of the largest supernumerary chromosomes in all vertebrates. In the present study, amplified fragment length polymorphism analyses (AFLP) and fluorescence in-situ hybridization (FISH) were performed in order to characterize these extraordinary chromosomes in detail. Sequence analysis of the B chromosome-specific DNA revealed a strong homology to a Drosophila Gypsy/Ty3 retrotransposon and also to a medaka (Oryzias latipes) one. The sequence is highly abundant on the B chromosome but undetectable in the normal A chromosome complement. It is also absent from the B chromosome of the closely related species, Rutilus rutilus, suggesting a specific spreading of the mobile element during evolution of the giant supernumerary chromosome within A. alburnus. Meitotic chromosomes were in-situ hybridized with the B chromosome-specific probe, documenting that the additional chromosome behaves as an autopaired ring chromosome in diakineses. Our results suggest that the supernumerary chromosome of A. alburnus is not derived from the normal chromosome complement but has evolved independently.

The pericentromeric heterochromatin of the grass Zingeria biebersteiniana (2n = 4) is composed of Zbcen1-type tandem repeats that are intermingled with accumulated dispersedly organized sequences

DNA reassociation and hydroxyapatite chromatography were used to isolate high-copy DNA of the grass Zingeria biebersteiniana (2n = 4). In situ hybridization demonstrated that the DNA isolated was enriched for pericentromere-specific repetitive sequences. One abundant pericentromere-specific component is the differentially methylated tandem-repeat family Zbcen1. Other sequences isolated, Zb46 and Zb47A, are dispersed and display similarity to parts of the gypsy- and copia-like retrotransposable elements of other grasses. In situ hybridization with the copia-like sequence Zb47A resulted in dispersed labelling along the chromosome arms, with a significant signal accumulation in the pericentromeric region of all chromosomes. It is concluded that the pericentromeric heterochromatin of Z. biebersteiniana is composed of members of the Zbcen1 tandem repeat family and that these tandem arrays are intermingled with accumulated putative copia-like retrotransposon sequences. An observed Rab1 interphase orientation suggests that the length of the chromosomes rather than the genome size is the determining factor of the Rab1 phenomenon.

Structure and evolution of supernumerary chromosomes in the Pacific giant salamander, Dicamptodon tenebrosus

We examined the genetic make-up and plausible origins of the supernumerary (B) chromosomes of the Pacific giant salamander, Dicamptodon tenebrosus, from the Pacific Northwest of North America. These salamanders have variable numbers of B chromosomes, from 0 to 10 per individual. Salamanders from the most southerly and northerly regions of the species' range have lower average numbers of B chromosomes than salamanders in the middle of the range. To assess how the supernumerary chromosomes originated in D. tenebrosus, B chromosome DNA was isolated by microdissection and amplified by degenerate oligonucleotide-primed PCR. The B chromosome DNA hybridized similarly to genomic DNA from individuals of D. tenebrosus and the related species D. copei and D. ensatus, thus demonstrating that the supernumerary chromosomes were derived from the normal chromosome complement. Unique hybridization bands in both D. copei and D. tenebrosus suggest that the shared sequences have evolved independently.

B-chromosome evolution

B chromosomes are extra chromosomes to the standard complement that occur in many organisms. They can originate in a number of ways including derivation from autosomes and sex chromosomes in intra- and interspecies crosses. Their subsequent molecular evolution resembles that of univalent sex chromosomes, which involves gene silencing, heterochromatinization and the accumulation of repetitive DNA and transposons. B-chromosome frequencies in populations result from a balance between their transmission rates and their effects on host fitness. Their long-term evolution is considered to be the outcome of selection on the host genome to eliminate B chromosomes or suppress their effects and on the B chromosome's ability to escape through the generation of new variants. Because B chromosomes interact with the standard chromosomes, they can play an important role in genome evolution and may be useful for studying molecular evolutionary processes.