Abstract
Since the density of simple sequence repeats (SSRs) may vary between different chromosomes of the same species in eukaryotic genomes, we screened SSRs of the whole genome of the yellow necked mouse, Apodemus flavicollis, in order to reveal SSR profiles specific for animals carrying B chromosomes. We found that the 2200 bp band was amplified by primer (CAG)4AC to a highly increased level in samples with B chromosomes. This quantitative difference (B-marker) between animals with (+B) and without (0B) B chromosomes was used to screen 20 populations (387 animals). The presence/absence of Bs was confirmed in 96.5% of 342 non mosaic individuals, which recommends this method for noninvasive B-presence detection. A group of 45 animals with mosaic and micro B (μB) karyotypes was considered separately and showed 55.6% of overall congruence between karyotyping and molecular screening results. Relative quantification by qPCR of two different targeted sequences from B-marker indicated that these B-specific fragments are multiplied on B chromosomes. It also confirms our assumption that different types of Bs with variable molecular composition may exist in the same individual and between individuals of this species. Our results substantiate the origin of Bs from the standard chromosomal complement. The B-marker showed 98% sequence identity with the serine/threonine protein kinase VRK1 gene, similarly to findings reported for Bs from phylogenetically highly distant mammalian species. Evolutionarily conserved protein-coding genes found in Bs, including this one in A. flavicollis, could suggest a common evolutionary pathway.
Highlights
IntroductionB-Chromosomes in Apodemus flavicollis and new ones are regularly discovered
An extremely heterogeneous group of supernumerary chromosomes, nonessential for organism viability, exists in approximately 15% of cytogenetically examined eukaryotic species [1]PLOS ONE | DOI:10.1371/journal.pone.0160946 August 23, 2016B-Chromosomes in Apodemus flavicollis and new ones are regularly discovered
In quest of a distinctive B marker among repetitive sequences, frequently reported in B chromosomes of various species [13,26,27,28,29,30,31,32], clear quantitative difference was observed in a 2200 bp electrophoretic band (Fig 2) between +B and 0B samples
Summary
B-Chromosomes in Apodemus flavicollis and new ones are regularly discovered. After more than a century of intensive research, Bs continue to be one of the most enigmatic components of the genome In spite of their disposability and contrary to other accessory karyotype elements, Bs have the special ability of maintenance in certain species through numerous generations. They are variable in size, number, morphology, phenotypic effects and ways of transmission, have irregular modes of inheritance, pairing incapacity with As during meiosis and, being dispensable, they follow their own evolutionary path [3]. Learning about their biology could help us to understand better the evolution of genomes and gene regulation under varying amounts of selection pressure [4,5,6]
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