RESUMO
Cuscuta is a cytogenetically diverse genus, with karyotypes varying 18-fold in chromosome number and 127-fold in genome size. Each of its four subgenera also presents particular chromosomal features, such as bimodal karyotypes in Pachystigma. We used low coverage sequencing of the Cuscuta nitida genome (subgenus Pachystigma), as well as chromosome banding and molecular cytogenetics of three subgenus representatives, to understand the origin of bimodal karyotypes. All three species, C. nitida, C. africana (2n = 28) and C. angulata (2n = 30), showed heterochromatic bands mainly in the largest chromosome pairs. Eighteen satellite DNAs were identified in C. nitida genome, two showing similarity to mobile elements. The most abundant were present at the largest pairs, as well as the highly abundant ribosomal DNAs. The most abundant Ty1/Copia and Ty3/Gypsy elements were also highly enriched in the largest pairs, except for the Ty3/Gypsy CRM, which also labelled the pericentromeric regions of the smallest chromosomes. This accumulation of repetitive DNA in the larger pairs indicates that these sequences are largely responsible for the formation of bimodal karyotypes in the subgenus Pachystigma. The repetitive DNA fraction is directly linked to karyotype evolution in Cuscuta.
Assuntos
Cuscuta , Cuscuta/genética , DNA Ribossômico/genética , Evolução Molecular , Cariótipo , FilogeniaRESUMO
Interspecific hybridization and genome duplication to form allopolyploids are major evolutionary events in angiosperms. In the parasitic genus Cuscuta (Convolvulaceae), molecular data suggested the existence of species of hybrid origin. One of them, C. veatchii, has been proposed as a hybrid between C. denticulata and C. nevadensis, both included in sect. Denticulatae. To test this hypothesis, a cytogenetic analysis was performed with CMA/DAPI staining and fluorescent in situ hybridization using 5S and 35S rDNA and genomic probes. Chromosomes of C. denticulata were small with a well-defined centromeric region, whereas C. nevadensis had larger, densely stained chromosomes, and less CMA+ heterochromatic bands. Cuscuta veatchii had 2n = 60 chromosomes, about 30 of them similar to those of C. denticulata and the remaining to C. nevadensis. GISH analysis confirmed the presence of both subgenomes in the allotetraploid C. veatchii. However, the number of rDNA sites and the haploid karyotype length in C. veatchii were not additive. The diploid parentals had already diverged in their chromosomes structure, whereas the reduction in the number of rDNA sites more probably occurred after hybridization. As phylogenetic data suggested a recent divergence of the progenitors, these species should have a high rate of karyotype evolution.
Assuntos
Evolução Biológica , Cuscuta/genética , Genoma de Planta , Ploidias , Arizona , DNA de Plantas/genética , Hibridização in Situ Fluorescente , Cariotipagem , MéxicoRESUMO
Cuscuta is a widely distributed genus of holoparasitic plants. Holocentric chromosomes have been reported only in species of one of its subgenera (Cuscuta subg. Cuscuta). In this work, a representative of this subgenus, Cuscuta approximata, was investigated looking for its mitotic and meiotic chromosome behaviour and the heterochromatin distribution. The mitotic chromosomes showed neither primary constriction nor Rabl orientation whereas the meiotic ones exhibited the typical quadripartite structure characteristic of holocentrics, supporting the assumption of holocentric chromosomes as a synapomorphy of Cuscuta subg. Cuscuta. Chromosomes and interphase nuclei displayed many heterochromatic blocks that stained deeply with hematoxylin, 4',6-diamidino-2-phenylindole (DAPI), or after C banding. The banded karyotype showed terminal or subterminal bands in all chromosomes and central bands in some of them. The single pair of 45S rDNA sites was observed at the end of the largest chromosome pair, close to a DAPI band and a 5S rDNA site. Two other 5S rDNA site pairs were found, both closely associated with DAPI bands. The noteworthy giant nuclei of glandular cells of petals and ovary wall exhibited large chromocentres typical of polytenic nuclei. The chromosomal location of heterochromatin and rDNA sites and the structure of the endoreplicated nuclei of C. approximata seemed to be similar to those known in monocentric nuclei, suggesting that centromeric organization has little or no effect on chromatin organization.