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

Cuscuta is a cytogenetically diverse genus, with karyotypes varying 18-fold in chromosome number and 89-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.

Heteropolygonatum hainanense (Asparagaceae), a new species endemic to Hainan (China)

Heteropolygonatum Tamura & Ogisu (1997: 950; Asparagaceae, Nolinoideae) differs from Polygonatum Miller (1754: without pagination) by two distinctive features: the perigone in Heteropolygonatum is imbricate whereas it is valvate in Polygonatum (Floden 2014b, Chen & Tamura 2000, Tamura et al. 1997a, b). The basic chromosome number in Heteropolygonatum is x =16 with distinctly bimodal karyotypes vs. the lower, although variable, basic chromosome number in Polygonatum is x = 9–15 (Tamura & Xu 2001, Yamashita & Tamura 2001, Chen & Tamura 2000, Tamura et al. 1997a, 1997b). Recently, several additional species have been added to this small genus, based on cytological data or morphological characters unique to Heteropolygonatum in tribe Polygonateae (Chao et al. 2013, Floden 2014b, Xiao et al. 2017). This genus now includes 12 species, with the addition of the one described here.