Chromosome homeologies and high variation in heterochromatin distribution between Citrus L. and Poncirus Raf. as evidenced by comparative cytogenetic mapping

Abstract

The genus Citrus is well-known for its economic importance and complex taxonomy. Only three to six Citrus taxa are considered true biological species, among them is Citrus medica, the citron. Previous studies based on chromomycin A3 (CMA)/4',6-diamidino-2-phenylindole staining showed that the citron has a homomorphic karyotype, consisting of four distinct chromosome types according to the CMA(+) heterochromatin distribution. Based on the pattern of CMA(+) bands and the hybridization of 25 bacterial artificial chromosomes (BACs) from a genomic library of Poncirus trifoliata, a closely related species to the genus Citrus, we were able to identify each chromosome pair and build a comparative cytogenetic map for C. medica. The data showed a high degree of sequence conservation between these genera, enabling heterologous hybridization of BACs, and the establishment of chromosomal homeologies. It was thus possible to visualize changes in the position of some BACs in relation to CMA(+) bands. Since no breakdown of synteny was observed between these species, expansions and contractions in repetitive DNA sequences seem to be the major driving force of chromosomal evolution since the separation of these two genera.

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

“…However, no indication of paracentric inversion was found in meiosis of Citrus × Poncirus hybrids [Agarwal, 1987b]. Compared BAC-FISH maps [Mendes et al, 2011] and genetic maps [Ollitrault et al, 2012] between Citrus and Poncirus species also suggested conservation of marker orders. Therefore, the possibility of repeated transposition events placing these nomad sites close together de novo in other chromosome pairs should be considered.…”

“…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].…”

“…However, some meiotic investigation of citrus species and Citrus × Poncirus hybrids revealed a quite normal chromosome pairing [Agarwal, 1987a, b;Guerra, 2009]. A comparative cytogenetic mapping between P. trifoliata and C. medica , based on BAC-FISH, also revealed a conservative karyotype [Mendes et al, 2011], in spite of the large difference in the CMA band patterns of these species. Indeed, most variations in CMA bands observed in Citrus and related genera are probably due to dispersion plus amplification or deletion of tandemly repeated sequences [Barros e Silva et al, 2010] rather than to large chromosome rearrangements.…”

Chromosome homologies between Citrus and Poncirus—the comparative cytogenetic map of mandarin (Citrus reticulata)

“…Most of the Citrus cultivars were classified as heteromorphic in respect to their banding pattern (homologue chromosomes showed distinct heterochromatin distributions) and a minority as homomorphic (Cornélio et al 2003;Yamamoto and Tominaga 2003;Moraes et al 2007a (Cornélio et al 2003;Yamamoto and Tominaga 2003;Carvalho et al 2005;Moraes et al 2007a, b;Guerra 2009; Barros e Silva et al 2010;Mendes et al 2011). The heteromorphic accessions are considered of hybrid origin, whereas homomorphic accessions might be pure or true species (Guerra 2009).…”

Comparative Analyses in<b><i> Lotus</i></b>: The Cytogenetic Map of <b><i>Lotus uliginosus </i></b>Schkuhr

“…Because these clones showed unique signals without blocking DNA in L. japonicus , it is probable that the dispersed hybridization pattern had been caused due to the presence of moderately repetitive sequences from these genomic regions in L. uliginosus that are present in lower copy number in L. japonicus . The difference in the hybridization pattern of the same BAC between species was also observed in a comparative mapping in Citrus [Mendes et al, 2011]. In Daucus , 2 BACs from carrot could not be mapped in wild species [Iovene et al, 2011].…”