# Chromosome Number and Genome Size Evolution in _Brasolia_ and _Sobralia_ (Sobralieaee, Orchidaceae)
## 3 Discussion
### Phylogenetic Reconstruction
In tribe Sobralieae, we could confirm the importance of descending dysploidy with chromosome number reduction from \(2n=48\) (\(n=24\)) to \(2n=46\), \(44\), \(42\) in _Sobralia_ and _Brasolia_ genera. However, based on the homogeneous chromosome morphology presented in the species analyzed here, it was impossible to infer the fission-fusion points or chromosome loss, causing the chromosome number reduction, making mandatory the use of in situ hybridization techniques. The frequent occurrence of dysploidy in Sobralieae is in accordance with the diploidisation period following whole-genome duplication. Polyploidy is a central evolutionary mechanism in plants, but its occurrence is not random or constant throughout plant evolution [21; 22; 23]. Otherwise, it is suggested that polyploidy pulses occurred with high frequency at some geological periods as the Cretaceous-Paleogene transition (K-Pg; 66 Mya), Paleogene (Paleocene-Eocene transition; 56 Mya), and Neogene (at Late Miocene; 11 - 5 Mya), configuring periods of major stress (reviewed by [24]). Such concentration of polyploidy events in periods of geological history is confirmed by a high concentration of polyploid basal nodes in the Angiosperms phylogeny, representing some large extant plant families [23; 24; 25; 26], which could be true for Orchidaceae with an estimated age of 90 million years ago (Mya; [27; 28; 29; 30]). Furthermore, the suggested base chromosome number in Orchidaceae is x = 7 [8] or x = 10 [31], which suggests that most extant orchid species are polyploids. Such an assumption could be applied to the tribe Sobralieae (x = 24). The tribe Sobralieae is placed on the base of the subfamily Epidemorideae (44.50-60.27 Mya; [30]), the largest Orchidaceae subfamily [32]. Sobralieae possibly diverged at 45 Mya on the Neotropics, with _Brasolia_, _Eleantthus_, and _Sobralia_ diverging more recently, at ca. 7-10 Mya [27; 28; 30]. Based on the previously assumed base chromosome number of Orchidaceae, the tribe Sobralieae (x = 24) should be considered a hexalploid followed by descending dysploidy (x = 7 -> 14 -> 28 -> 26 -> 24) or tetraploid followed by ascending dysploidy (x = 10 -> 20 -> 22 -> 24). The genomic redundancy generated by polyploidy were \"accommodated\" in a diploid genome by the diplodisation period with descending dysploidy as one of the main chromosomal rearrangements observed during diplodisation [33]. The identification of descending dysploidy as the main mechanism of karyotype evolution in tribe Sobralieae is in accordance with the diplodisation period following polyploid pulses, possibly placed on the ancestral node of Orchidaceae. Since _Brasolia_ and _Sobralia_ are subject to the same karyotype evolution mechanism and because chromosome number has no phylogenetic signal, it is hard to use the karyotype to differentiate these groups. However, along the descending dysploidy, the elimination of repetitive sequences is another frequent event during the diplodisation period [34], affecting the genome size, which presents a phylogenetic signal.