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Despite the importance of polyploidy for the evolution of Opuntia, few researches have been focused to identify the ploidy of cultivars used in plant breeding programmes. We aimed to provide chromosome numbers, ploidy and heterochromatin band patterns of seven species and 24 cultivars from Opuntia, and investigate the relationship between phenotypic characteristics and ploidy in these species. All cultivars analyzed of Opuntia atropes, O. cochenillifera, and one cultivar of O. undulata are diploids, O. larreyi, one cultivar of O. stricta and two cultivars of O. undulata are tetraploids and all cultivars of O. ficus-indica, one cultivar of O. stricta and four cultivars of Opuntia sp. are octoploids. CMA/DAPI banding revealed the occurrence of terminal bands in all cultivars and pericentromeric bands in six cultivars. Polyploid species of Opuntia had superior mean values than diploid ones in relation to the cladode width, cladode length, cladode area, cladode thickness and plant height. Diploids had superior mean values than polyploids in relation to the number of cladodes. Polyploidy may be a useful tool for obtaining Opuntia cultivars with quantitative characteristics of interest. [ABSTRACT FROM AUTHOR]

Background and Aims The extent to which genome size and chromosome numbers evolve in concert is little understood, particularly after polyploidy (whole-genome duplication), when a genome returns to a diploid-like condition (diploidization). We study this phenomenon in 46 species of allotetraploid Nicotiana section Suaveolentes (Solanaceae), which formed <6 million years ago and radiated in the arid centre of Australia. Methods We analysed newly assessed genome sizes and chromosome numbers within the context of a restriction site-associated nuclear DNA (RADseq) phylogenetic framework. Key Results RADseq generated a well-supported phylogenetic tree, in which multiple accessions from each species formed unique genetic clusters. Chromosome numbers and genome sizes vary from n = 2x = 15 to 24 and 2.7 to 5.8 pg/1C nucleus, respectively. Decreases in both genome size and chromosome number occur, although neither consistently nor in parallel. Species with the lowest chromosome numbers (n = 15–18) do not possess the smallest genome sizes and, although N. heterantha has retained the ancestral chromosome complement, n = 2x = 24, it nonetheless has the smallest genome size, even smaller than that of the modern representatives of ancestral diploids. Conclusions The results indicate that decreases in genome size and chromosome number occur in parallel down to a chromosome number threshold, n = 20, below which genome size increases, a phenomenon potentially explained by decreasing rates of recombination over fewer chromosomes. We hypothesize that, more generally in plants, major decreases in genome size post-polyploidization take place while chromosome numbers are still high because in these stages elimination of retrotransposons and other repetitive elements is more efficient. Once such major genome size change has been accomplished, then dysploid chromosome reductions take place to reorganize these smaller genomes, producing species with small genomes and low chromosome numbers such as those observed in many annual angiosperms, including Arabidopsis. [ABSTRACT FROM AUTHOR]

Some groups in Epidendrum have stable chromosome numbers, whereas the species of Epidendrum subgenus Amphiglottium exhibit high levels of chromosome number variability and other genomic alterations. Polyploidy and dysploidy have both been invoked to explain chromosome number variation in the subgenus, but little is known about what is driving the high frequency of chromosome modification within some species in this group. To test whether hybridization and introgression are involved, we use fluorochrome staining to evaluate the chromosome numbers and heterochromatin bands in E. subgenus Amphiglottium , including individual parental genotypes from introgressed and non-introgressed populations. Two main types of heterochromatin were observed forming CMA and DAPI bands in Epidendrum spp. B chromosomes were identified in some individuals. In non-introgressed populations, E. secundum has three cytotypes, 2 n = 28, 56 and 84, found in different parts of its geographical range and a small number of DAPI bands. In introgressed populations of E. secundum , there was dysploid variation among individuals that were morphologically typical parental plants, and in one introgressed population a high number of DAPI bands was observed in what appeared on morphological grounds to be pure parental individuals. Our data corroborate the hypothesis that hybridization and introgression play a role in generating chromosome variation (number and form) and heterochromatin diversification in E. subgenus Amphiglottium. [ABSTRACT FROM AUTHOR]

Here we study a challenging group of karyotypically highly variable Epidendrum spp. using phylogenetic methods to help understand how hybridization/introgression contributes to karyotype evolution. We hypothesize that species with great chromosome number variation are a result of past hybridization/introgression. Conflicting topologies in trees constructed using separate plastid and nuclear datasets suggest past hybridization events that occurred most probably at least 3.7 Mya. A basic number x = 14 and substantial karyotype change followed by species divergence are suggested. Descending dysploidy and polyploidy were the most frequent changes estimated across the phylogenetic tree of the group. Two species, Epidendrum secundum and E. xanthinum , have probably experienced unidirectional gene flow involving their ancestors (the pollen recipients) and ancestors of E. puniceoluteum and E. denticulatum/E. flammeum , respectively, the pollen donors. However, it is not possible to say whether hybridization participated in the origin of E. secundum and E. xanthinum or merely contributed to their genomic divergence and karyotype change through introgression as has been observed in modern hybrid zones in Epidendrum. This pattern of introgression causing karyotype disruption and divergence could help explain the enigma of some highly diverse genera, such as Epidendrum. Further studies using a wider sampling of the genus could test if gene flow and karyotype variability are associated with the increase of speciation rates. [ABSTRACT FROM AUTHOR]

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