Abstract
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.
Highlights
We present here a detailed karyotypic analysis of Dictyoloma vandellianum, the only representative of the genus (Groppo 2010) formerly placed in the Rutaceae subfamily Dictyolomatoideae but recognized for the subfamily Cneoroideae
In relation to Aurantieae, the karyotype of Dictyoloma vandellianum differed in the following aspects: 1. It had acro/ telocentric chromosomes that are absent in Aurantoideae species (Guerra 1993; Costa Silva et al 2019); 2
RDNA sites, Dictyoloma vandelianum had no other CMA+ band, whereas most Aurantieae species have several other CMA+ bands associated with different repetitive sequences (Barros e Silva et al 2010; Deng et al 2019; He et al 2020); 3
Summary
The Rutaceae is a highly diversified and nearly cosmopolitan plant family comprising 150–162 genera and 1500–2096 species (Groppo et al 2012). The systematic treatment of Engler (1931) split Rutaceae into seven subfamilies, based mainly on fruit and floral characters, molecular phylogenetic studies suggest the exclusion of the monogeneric subfamily Rhabdodendroideae and the inclusion of some genera from other families (Harrisonia (Simaroubaceae), Cneorum (Cneoraceae), and Ptaeroxylon (Ptaeroxylaceae)) to ensure its monophyletic status (Chase et al 1999; Groppo et al 2008; Morton & Telmer 2014). Only two subfamilies of Rutaceae are recognized (Groppo et al 2012): Cneoroideae, comprising only eight genera (Appelhans et al 2011), including Cneorum and Dictyoloma; and Rutoideae, which congregates four former Englerian subfamilies (Rutoideae, Aurantioideae, Toddalioideae, and Flindersioideae). The karyology and cytotaxonomy of Aurantieae were wellexplored by both classical (Guerra 2009) and molecular methods (Wu et al 2018; Costa Silva et al 2019; He et al 2020; Mendes et al 2020), the ancestral or base chromosome number of the family is still a matter of debate (Shan et al 2006)
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