Molecular structure, origin and function of B chromosomes in yellow-necked wood mouse Apodemus flavicollis(Mammalia, Rodentia)
Molekularna struktura, poreklo i funkcija B hromozoma kod žutogrlog šumskog miša Apodemus flavicollis (Mammalia, Rodentia)
Abstract:
B chromosomes (Bs) are supernumerary chromosomes in a standard karyotype, unnecessary for normal growth and development. Bs are present in all main groups of multicellular eukaryotes, except birds, and extremely variable considering origin, morphology, size, number, molecular structure, and phenotypic effects. 3% of all analyzed species have B chromosomes. The Bs do not follow rules of Mendelian segregation law and do not recombine with members of the basic A chromosome complement. The main questions considered in contemporary studies of Bs are surrounding their origin, mechanisms of formation and maintenance. Knowledge of their molecular structure, transcriptional activity, and phenotypic effects, will allow a deeper understanding of their biological importance. Within mammals, the most frequent carriers of Bs are rodents. The genus Apodemus includes 20 species, six of which carry Bs, in addition to A chromosome set. In this study, B chromosomes were analyzed in specimens of the yellow-necked mouse, Apodemus flavicollis. Carriers usually have one or two Bs, and the highest recorded number in this species is eight B chromosomes. In different frequencies, supernumeraries are present in all studied populations of A. flavicollis in the territory of the Republic of Serbia. Numerous population studies of this species are indicating that B carriers have a higher fitness compared to individuals without B chromosomes. The aim of this study was to define sequence, origin and transcriptional activity of selected B regions in species A. flavicollis. Specimens were collected from different localities in the Republic of Serbia, Russian Federation and the Republic of Belarus. B chromosomes have been isolated by microdissection from the karyotype of three individuals and amplified in the polymerase chain reaction (PCR). B-specific fluorescent probes were synthesized. Estimation of homology between B-specific probes and chromosomes of A set had been obtained by fluorescent in situ hybridization (FISH). Homology was shown between the B chromosome and: a) pericentromeric region (PR) of sex chromosomes b) subtelomeric region of the two pairs of small chromosomes and c) lower homology with the distal region of the Y chromosome. Despite the distinct geographical origin of samples and different number of Bs in karyotypes, hybridized probes showed the same affinity to Bs. This indicates the same origin and/or structure of all Bs in A. flavicollis. This study has demonstrated that C-kit gene does not contribute to the composition of supernumeraries in A. flavicollis, contrary to the studies of Bs in numerous mammalian species. Comparing sequences of Bs and referent genome of the species Mus musculus, 39 genes in 22 chromosomal regions that originate from 16 chromosomes of the referent genome have been identified. With the exception of one region present on B chromosome from Russian Federation, that contained one additional chromosomal region not present on the samples from Serbian populations, results demonstrate that all analyzed Bs are composed of the same A-chromosomes’ regions. B chromosomes are rich in microtubule and cell cycle protein-coding genes. Other present categories include genes that are coding nucleotide-binding, membrane, and metal-binding proteins, as well as a high amount of repetitive elements. Transposable elements were tandemly arranged in some cases. Genes present on B chromosomes are at different stages of degradation. In contrast to the view accepted for many decades that Bs are genetically inert elements, this study demonstrates that pseudogenes on Bs are transcribed. The study also demonstrates that Bs-genes, or standard-genome genes in the presence of Bs, show higher transcriptional levels in young females. Furthermore, transcription of Cenpe gene is linked with the number of Bs, in the manner that samples with one B show the highest transcription level. B chromosomes in A. flavicollis probably originate from the pericentromeric region of the X chromosome which is positioned near pseudoautosomal region, they are featured with stable genetic structure throughout the whole areal of the species and are transcriptionally active depending on the sex, age and number of Bs.
Keywords:
B chromosomes; Genetics; Microdissection; Ssequencing; RT-PCR; Apodemus flavicollis; FISHSource:
University of Belgrade, Faculty of Biology, 2019, 1-90Funding / projects:
- Genetic and phenetic diversity in natural populations across different environments - contribution of B chromosome polymorphism (RS-MESTD-Basic Research (BR or ON)-173003)