Allopolyploid origin and genome differentiation of the parasitic speciesCuscuta veatchii(Convolvulaceae) revealed by genomic in situ hybridization

Abstract

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.

Polyploidy Index and Its Implications for the Evolution of Polyploids

“…Genome sequencing has provided rich evidence that polyploidy contributed substantially to the diversification of land plants (Jiao et al, 2011; Soltis et al, 2015; Kellogg, 2016; Farhat et al, 2019; Huang and Zhu, 2019; Ibiapino et al, 2019; Sessa, 2019) and crop domestication (Salman-Minkov et al, 2016; Van Drunen and Husband, 2019). Despite near-parity of occurrence of auto- and allopolyploids among extant plants, surprisingly, and interestingly, genomic analysis showed that maize ( Zea mays ), bread wheat ( Tritium aestivum ), canola ( Brassica napus ), and the common ancestor of grasses have allopolyploid origins, with some of them as young as thousands of years (canola, ∼7,500 years, and bread wheat, ∼10,000 years) (Chalhoub et al, 2014; International-Wheat-Genome-Sequencing-Consortium, 2014; Wang et al, 2015a), while others are tens of millions of years old [maize, formed ∼26 million years ago (mya) (Schnable et al, 2009; Wang et al, 2015b), and the common ancestor of grasses, ∼98 mya] (Murat et al, 2015; Wang et al, 2015b).…”

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

“…66 sites) seems to confirm the correlation between genome size and number of rDNA sites (Prokopowich et al, 2003;Vallès et al, 2013). However, C. nevadensis, with 2n = 30 and much smaller chromosomes, had 16 rDNA sites (Ibiapino et al, 2019); therefore, this relationship is not clear for Cuscuta species.…”

“…In C. indecora and C. monogyna not all 5S and 35S rDNA sites were clearly differentiated with CMA, either because the sites were too small or because they presented a variable GC content. In three other species of Cuscuta subgenus Grammica investigated with sequential CMA/DAPI and FISH staining (Ibiapino et al, 2019), only the 35S rDNA sites were CMA + , indicating a less variable composition of their rDNA repeats.…”

“…The only Cuscuta species investigated by chromosome banding and FISH using 5S and 35S rDNA probes were C. approximata Bab. of subgenus Cuscuta (Guerra and García, 2004), and the three species of subgenus Grammica section Denticulatae (Ibiapino et al, 2019), which exhibited a variable number of CMA and DAPI bands and rDNA sites.…”

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

“…Subgenus Grammica presents the largest variation in chromosome number and size. This subgenus has at least five cases of interspecific hybridization which can contribute to this chromosome number variation (Fogelberg, 1938; Pazy and Plitmann, 1994; García, 2001; García and Castroviejo, 2003; McNeal et al ., 2007; Ibiapino et al ., 2019; García et al ., 2019). A preliminary study of two species of Pachystigma revealed bimodal karyotypes and extensive heterochromatic blocks in the larger chromosomes, suggesting the influence of repetitive DNA in the emergence of bimodality in this subgenus (García et al ., 2019).…”