Determination of the chromosome base number of a taxon is fundamental to understanding karyotypic variation and its implications for the evolution of that group. This usually requires careful evaluation of cytological literature and robust phylogenetic support. The base number for the family Rutaceae (x = 9 or x = 18) has long been the subject of debate. Here, we analyzed the banding pattern, rDNA sites, and genome size of Dictyoloma vandellianum, subfamily Cneoroideae, the sister group of the remaining Rutaceae, and revised critical points about the chromosome base number of the family. We found that this species has n = 9, which differs from the n = 18 possessed by other cytologically known Cneoroideae species. Thus, n = 9 occurs in the main clades of Rutaceae and is the most probable base number of the family. The hypothesis of x = 18 as the base number is no longer sustainable, although n = 18 is very common in Rutaceae. Moreover, the fluorescent banding pattern and the relatively large genome size (1C = 1.3 pg) of D. vandellianum suggest that its chromosomal organization is highly divergent from Aurantieae, the only large Rutaceae clade where species with n = 9 are greatly dominant.

<i>Alstroemeria</i> (Alstroemeriaceae) displays a conserved and highly asymmetric karyotype, where most rDNA sites can be properly recognized by the size and morphology of the chromosomes. We analyzed the intraspecific variation of rDNA sites in <i>A. longistaminea </i>and compared with their distribution in other species (<i>A. caryophyllaea</i> and <i>A. piauhyensis</i>) and a representative of the sister genus, Bomarea edulis. All three <i>Alstroemeria</i> species presented 2n = 16 and one to six B chromosomes were found in some individuals of <i>A. longistaminea</i>. There was a set of 12 conserved rDNA sites (four 5S and eight 35S) and up to 11 variable sites. B chromosomes were almost entirely covered by 35S signals, coupled with tiny 5S sites. Noteworthy, most rDNA sites found in <i>A. caryophyllaea</i> and <i>A. piauhyensis</i> were localized in chromosome positions similar to those in <i>A. longistaminea</i>, suggesting the existence of conserved hotspots for rDNA accumulation. Some of these hotspots were absent in Chilean <i>Alstromeria</i> as well in <i>B. edulis</i>. We propose that insertions of rDNA sequences on chromosomes do not occur randomly but rather on preferential sites or hotspots for insertions. The maintenance of these arrays, however, may be favored/constrained by different factors, resulting in stable or polymorphic sites.

The condensation patterns (CPs) of prophase chromosomes represent poorly explored and little understood karyotype features. Three distinct chromosome domains are observed in conventionally stained prophases of most angiosperms with small chromosomes: heterochromatin and early condensing euchromatin (ECEu), which are observed as early condensing regions (ECRs), and late condensing euchromatin or late condensing regions (LCRs). All three prophase domains have been often reported in the Annonaceae species. To gain a better understanding of these regions, we investigated the CPs, heterochromatic bands, and rDNA sites in seven Annona species with 2n = 14, 28, and 54 and Xylopia  frutescens with 2n = 16. Besides, histone H4K5 acetylation, telomeric sites, and DNA methylation were analysed in some of these species. LCRs corresponded to a small hyperacetylated, and hypomethylated fraction of the metaphase chromosomes. During interphase, the chromocentres displayed variable proportions of heterochromatin and ECEu. The LCRs and ECRs were conserved even between disploid and polyploid species, whereas rDNA sites and heterochromatic bands varied in number and location. Our data suggest that chromatin compartmentalization in ECR and LCR regions may represent the simplest functional organization of the small chromosomes of Annonaceae, while the remaining characters are less relevant.

Ameroglossum is composed of shrubs endemic to inselbergs in north-eastern Brazil, currently circumscribed in Linderniaceae. Chromosomal counts for this family are few, but quite variable, ranging from 2n = 14 to 60. We investigated the chromosomal numbers of 14 species of Linderniaceae with emphasis on Ameroglossum and analysed the distribution of heterochromatin and 5S and 35S rDNA sites for most species. We found 2n = 60 for the species of Ameroglossum (except Ameroglossum genaroanum with 2n = 64), Catimbaua and Isabelcristinia, 2n = 50 for Cubitanthus alatus and 2n = 28 for Torenia thouarsii and Vandellia diffusa. All of them had small, similar chromosomes and 5S and 35S rDNA sites overlapping with CMA+ bands. The species with 2n = 50–64 showed a single pair of 5S and 35S rDNA sites, V. diffusa had two pairs of 5S and one pair of 35S rDNA sites and T. thouarsii had two pairs of each. Furthermore, other CMA+ and DAPI+ bands have been seen in almost all species. The karyotypical similarities among the species of Ameroglossum, Catimbaua, Cubitanthus and Isabelcristinia suggest a close relationship between representatives of Linderniaceae typical of inselbergs.