The evolution of CMA bands in Citrus and related genera

Abstract

Most species of Citrus and related genera display a similar karyotype with 2n = 18 and a variable number of terminal heterochromatic blocks positively stained with chromomycin A(3) (CMA(+) bands). Some of these blocks are 45S rDNA sites, whereas others may correspond to the main GC-rich satellite DNA found in several Citrus species. In the present work, the distribution of the 45S rDNA and the main satellite DNA isolated from C. sinensis (CsSat) were investigated by in situ hybridization in seven species of Citrus, two species of closely related genera (Fortunella obovata and Poncirus trifoliata) and four species of the subfamily Aurantioideae, which were less related to Citrus (Atalantia monophylla, Murraya paniculata, Severinia buxifolia, and Triphasia trifolia). In Citrus, Fortunella, and Poncirus, most CMA(+) bands colocalized only with CsSat sites, whereas others colocalized only with rDNA sites. However, some of these species displayed a few CMA(+) bands that colocalized with sites of both probes and other CMA(+) bands that did not colocalized with any of the probes. On the other hand, in the four species less related to Citrus, no CsSat signal was found on chromosomes. On Southern blot, the CsSat probe hybridized with genomic DNA from Citrus, Fortunella, and Poncirus at high stringency only, while under the less stringent conditions, it also hybridized with distantly related species. Therefore, CsSat sequences are the principal component of the heterochromatic blocks of Citrus, Poncirus, and Fortunella, whereas CsSat-like sequences seem to be widespread in the subfamily Aurantioideae. These data further suggest that the variable number of terminal CMA(+) bands observed on chromosomes of Citrus and related genera are probably the consequence of amplification or reduction in the number of CsSat-like sequences distributed on chromosome termini, paralleled by mutation and homogenization events, as proposed by the library hypothesis.

Characterization of Eu- and Heterochromatin of <i>Citrus</i> with a Focus on the Condensation Behavior of 45S rDNA Chromatin

“…Blutorange and C. paradisi cv. Ruby Red were identical to those previously described for these species [Pedrosa et al, 2000;Cornélio et al, 2003;Moraes et al, 2007a;Barros e Silva et al, 2010]. The hybrid C. reticulata !…”

Linked 5S and 45S rDNA Sites Are Highly Conserved through the Subfamily Aurantioideae (Rutaceae)

“…Furthermore, Yamamoto et al [2009] reported a single pair of 5S rDNA sites in chromosomes of Citropsis gabunensis, Citrus reshni, Fortunella japonica, Clymenia polyandra and Swinglea glutinosa and in 3 pairs in Poncirus trifoliata , but they did not investigate the position of 45S rDNA. Likewise, the number of 45S rDNA sites was previously reported for several Citrus species [Kang et al, 2008;Marques et al, 2011] and some related genera [Barros e Silva et al, 2010]. However, simultaneous detection of 5S and 45S rDNA sites in the same cell was previously reported only for P. trifoliata and Citrus species of the subgenus Citrus [Pedrosa et al, 2000;Carvalho et al, 2005;Brasileiro-Vidal et al, 2007;Moraes et al, 2007a, b].…”

“…For example, if one of the 2 bands of a C chromosome undergoes a strong size reduction, the chromosome will look like a D type, or vice-versa [Moraes et al, 2007b]. Change from a D-type chromosome to a C or F type, for example, seems to have occurred several times in the subfamily Aurantioideae [Barros e Silva et al, 2010;Mendes et al, 2011]. In M. paniculata , a D chromosome pair was previously misinterpreted as a B pair because the occurrence of a secondary constriction in the CMA band [Guerra et al, 2000], but when observed with FISH, it became clear that the correct formula was 2C + 14D + 2F [Barros e Silva et al, 2010].…”

“…Change from a D-type chromosome to a C or F type, for example, seems to have occurred several times in the subfamily Aurantioideae [Barros e Silva et al, 2010;Mendes et al, 2011]. In M. paniculata , a D chromosome pair was previously misinterpreted as a B pair because the occurrence of a secondary constriction in the CMA band [Guerra et al, 2000], but when observed with FISH, it became clear that the correct formula was 2C + 14D + 2F [Barros e Silva et al, 2010]. Among Citrus species, variation in number and distribution of CMA bands and rDNA sites is related to the fact that most species are apomict hybrids [Moore, 2001;Guerra, 2009].…”

Karyotype analysis in Bignonieae (Bignoniaceae): chromosome numbers and heterochromatin

“…Despite the multitude of mechanisms capable of producing different patterns, variations in heterochromatin patterns have been used to confirm the taxonomic placements of numerous taxa. Citrus is a good example, as the seven chromosomes types described by Guerra (1993) and Cornélio et al (2003) have been widely used to distinguish between different specimens, cultivars, and hybrids -even species of related genera (Cornélio et al 2003, Carvalho et al 2005, Barros e Silva et al 2010.…”