Intense proliferation of rDNA sites and heterochromatic bands in two distantly related Cuscuta species (Convolvulaceae) with very large genomes and symmetric karyotypes

Abstract

The genome size varies widely among angiosperms but only a few clades present huge variation at a low phylogenetic level. Among diploid species of the genus Cuscuta the genome size increased enormously in at least two independent lineages: in species of subgenus Monogynella and in at least one species (C. indecora) of the subgenus Grammica. Curiously, the independent events lead to similar karyotypes, with 2n = 30 mostly metacentric chromosomes. In this paper we compared the patterns of heterochromatic bands and rDNA sites of C. indecora and C. monogyna, aiming to evaluate the role of these repetitive fractions in these karyotypes. We found out that the large genomes of these species were incremented by a huge number of small heterochromatic CMA + and DAPI + bands and 5S and 35 rDNA sites, most of them clearly colocalized with CMA + bands. Silver nitrate impregnation revealed that the maximum number of nucleoli per nucleus was low in both species, suggesting that some of these sites may be inactive. Noteworthy, the tandem repeats did not generate large bands or sites but rather dozens of small blocks dispersed throughout the chromosomes, apparently contributing to conserve the original karyotype symmetry.

Karyotype asymmetry in Cuscuta L. subgenus Pachystigma reflects its repeat DNA composition

“…The latter studies revealed a numerical variation of these bands, with species having few bands, like C. denticulata Engelm. with only one pair of evident CMA + /DAPIbands, to species like C. monogyna Vahl, with approximately 90 CMA + /DAPIbands and DAPI + /CMAbands in pachytene (Ibiapino et al, 2019(Ibiapino et al, , 2020. Despite this variation, our previous unpublished data showed that karyotypes with less bands are more frequent, even in polyploid species.…”

“…In the species with a higher number of CMA + /DAPI - or DAPI + /CMA - bands, they are localised in interstitial regions. There is no evidence that this banding pattern is different for each subgenus and there may be similar patterns between different subgenera (Ibiapino et al ., 2020). In the three species of the subgenus Pachystigma analysed here, the multiple CMA/DAPI bands were mainly present at the largest pairs.…”

“…Pachystigma . Indeed, repetitive DNA in Cuscuta is involved in the expansion of the genome, causing an increase in chromosomes, such as in C. monogyna and C. indecora (Ibiapino et al ., 2020; Neumann et al ., 2021). In these two cases, however, chromosomes increased proportionally in size, maintaining karyotype symmetry, and resulting in similar karyotypes, although C. monogyna and C. indecora belong to different subgenera ( Monogynella and Grammica , respectively).…”

“…Chromosome number and size, as well as the number of rDNA sites in C. nitida, were within the range of variation already reported for species of the genus Cuscuta. So far, most Cuscuta species have shown few rDNA sites, varying from two to 36 sites of 5S rDNA and from two to 30 sites of 35S rDNA (Fogelberg, 1938;García, 2001;García and Castroviejo, 2003;Guerra and García, 2004;McNeal et al, 2007;Ibiapino et al, 2019Ibiapino et al, , 2020García et al, 2019). Thus, our results do not suggest neither hybridization nor polyploidy as the cause of bimodality in Pachystigma.…”

“…The genus Cuscuta shows high cytogenetic variation in chromosome number (2n = 8 to 2n = 150), chromosome size (1.66 µm to 21.60 µm), and genome size (1C = 0.39 Gbp to 1C = 34.73 Gbp). The genus also presents symmetric to bimodal karyotypes, as well as monocentric and holocentric chromosomes (García and Castroviejo, 2003;Guerra and García, 2004;McNeal et al, 2007;Ibiapino et al, 2019Ibiapino et al, , 2020García et al, 2019;Oliveira et al, 2020;Neumann et al, 2020). Each Cuscuta subgenus seems to have different karyotypic features.…”