Basic characteristics of the C. cyanescens karyotype
The basic karyotype structure for C. cyanescens (composed of 2n = 20, XYp, with meta-submetacentric chromosomes) is in concordance with previous karyotype data reported for Coprophanaeus species
[26, 31, 32]. However, this is the first study to identify a B chromosome in this species as well as in the Phanaeini tribe. In contrast to the small size of the B chromosome observed in C. cyanescens, the B chromosomes were medium- or large-sized in the other Scarabaeidae species
[21, 22, 36]. In Onthophagus vacca, the presence of one medium-sized B chromosome was observed with the presence of heterochromatin in its centromeric region, whereas Onthophagus similis and O. gazella showed respectively medium- and small-sized B chromosomes; however, there was no information about the heterochromatic pattern. Large heterochromatic B chromosomes, ranging in number from three to nine, were detected in all the specimens studied for Bubas bubalus. Individuals carrying one heterochromatic B chromosome in two populations of Dichotomius geminatus, corresponding to an average prevalence rate of 20.93% and 25.00% in each of the populations, were observed
The frequency with which B chromosomes are detected in natural populations varies widely between populations. B chromosomes can be present in high frequencies based on the degree to which a species can tolerate the extra chromosome and their power of accumulation
. It is difficult to determine the factors that are involved in the low frequency of B chromosomes in the population studied, and several mechanisms may be involved, including selection, random transmission, and historical factors.
Among Coleoptera species, the studies reporting the presence of B chromosomes have generally focused on the presence or absence of this element and have not considered their frequency in the population or their molecular content
[18, 21, 23, 36]. The presence of B chromosomes was reported in representatives of the Cetoniinae and Scarabaeinae, subfamilies of Scarabaeidae
[21, 22]. The evolution of the Scarabaeinae karyotype appears to have occurred under diverse mechanisms of chromosomal rearrangements
, which could have contributed to the origin of the B chromosome in this group.
Molecular cytogenetic mapping of C. cyanescens
The hybridization of the C
t-1 DNA to the pericentromeric regions extending up to the long arms of C. cyanescens chromosomes is in agreement with the heterochromatin distribution pattern observed in this species
. Although heterochomatin analyses were not conducted in the present work, the accumulation of repeated DNAs in the pericentromeric region of the B suggests also the compartimentalization of heterochromatin in the same region. The formation of the heterochromatic chromocenters in the Phanaeini species
[38, 39] indicates that this mechanism of heterochromatin amplification may be involved in the formation of diphasic chromosomes, including the large pericentromeric block of the B chromosome.
The distribution of C
t-1 DNA in the A complement and the B chromosome suggests an intraspecific origin of the extra element and the occurrence of homogenization mechanisms in the heterochromatic regions between the B and A elements. Generally, B chromosomes of more recent origin are enriched in repetitive DNA sequences when compared with the genome from which they originated
[1, 23]. This enrichment is indicative of a massive amplification of repetitive sequences over a relatively short time-scale; and, it has also been suggested that repetitive sequences amplification may be a mechanism through which a chromosome fragment (as a neo-B chromosome) may become stabilized and selected
[1, 23]. This does not appear to be the case for C. cyanescens, indicating that the B chromosome may not have been recently established in this species. Although the data obtained indicates an intraspecific origin of the B chromosome, it was not possible to identify which chromosomal A element was involved in the process. However, the chromosomes carrying the 5S and 18S RNA genes are probably not involved in this process, as the B element does not contain rRNA gene sequences.
The cytogenetic mapping of the LOA-like non-LTR retrotransposon mostly to the pericentromeric regions, including those of the B chromosome, indicates the exchange of genetic material between the A and B chromosomes, implying that the B chromosome has coexisted with the A chromosomes during the period of transposition. However, it is not possible to reject the hypothesis that the B chromosome originated from a segment without LOA-like that was received later, by transposition. According to a previous report
, B chromosomes can accumulate DNA from various sources, including transposable elements, and may affect the structure of the genome by ectopic recombination. A study in Drosophila melanogaster identified 25 transposon-mediated rearrangements by ectopic recombination in the region flanking the white locus
. The B chromosomes could act as a refuge for TEs, which in turn would generate structural variability in the whole genome. The hybridization that occurred in homologous regions, such as the pericentromeric regions, is another indication of recombination between the A complement and the B chromosome, and this recombination event could be explained by the chromocenter formation during the beginning of meiosis
The present study is the first report on the cytogenetic mapping of a transposable element in coleopteran chromosomes. The LOA non-LTR retrotransposon was first isolated from the genome of Drosophila silvestris, a species that is endemic to the Hawaiian Islands
. These elements belong to evolutionarily younger clades of non-LTR retrotransposons
, contain very few known elements, and have mostly been identified in Drosophila, Aedes and Ciona genomes
The distribution of LOA-like elements in the chromosomes reinforces an evolutionary relationship between the A complement and the B chromosome at least in the pericentromeric area. Recent work involving the centromere-enriched retrotransposons indicates that these elements preferentially insert into the centromeric regions
. The LOA-like elements may have been maintained in the genome of C. cyanescens due to a possible functional role they play in the maintenance of the pericentromeric regions. The absence of LOA-like elements in the sex chromosomes suggests that sex differentiation occurs before the distribution of this transposable element into the genome. Subsequently, the suppression of recombination could have produced the differences observed in the distribution of TEs between the A complement and the sex chromosomes. These results suggest that LOA-like element could have been involved in the maintenance of the pericentromeric regions and might contribute to the origin of the B chromosome.