Chromosome variations and diversity of Epidendrum ibaguense Lindl. (Orchidaceae) on the Tepequém's Tepuy, Roraima, Brazil.

Studies addressing chromosome variations have elucidated many points regarding the taxonomy of the Orchidaceae. Epidendrum L. besides being one the largest orchid genera, present remarkable morphological, and inter- and intraspecific chromosome variations. Thus, based on a previous report on flower color variation in individuals of E. ibaguense (magenta, pink, white, and red), our aim was to determine its chromosome number and test whether this trait is associated with flower color variation in natural populations on the Tepequém's Tepuy, Roraima. Root apices were pre-treated with 8-hydroxyquinoline at 4°C for 24 h and subsequently submitted to conventional cytogenetic procedures. Slides with the best spreading and contraction of chromosomes were photographed under light microscopy. Chromosome number was determined by counting at least 10 mitotic metaphase cells per individual. The types of interphase nuclei were determined for 30 nuclei per individual. E. ibaguense presented intra- and interpopulation variation in chromosome number, with 2n = 58, 72, and 76. The chromosome number 2n = 58 was most commonly found in individuals with magenta, pink, and white flowers, while the remaining two chromosome numbers occurred mostly in red-flowered individuals. The types of interphase nuclei were associated with the chromosome number. Individuals with 2n = 58 presented a predominance of semi-reticulated nuclei, while in those with 2n = 72 and 76 the nuclei were predominantly non-reticulated. The dominance of disploidy in E. ibaguense suggests that this cytotype provides this species with a territorial advantage and a higher reproductive success, possibly contradicting the polyploid hypothesis. Our results suggest that chromosome number may not represent a reproductive barrier in genus Epidendrum.

Environmental variations drive polyploid evolution in neotropical Eugenia species (Myrtaceae).

Polyploidy is one of the most important mechanisms of speciation and diversification in plant evolution. Polyploidy results in genetic variation among individuals of the same species and even between populations, and may be responsible for differences in environmental tolerance between populations of the same species. This study determined chromosome numbers of Eugenia L. (Myrtaceae, x = 11) for 26 populations of 14 species by conventional cytogenetic techniques. Nine species (13 populations) were diploid (2n = 2x = 22), but diploid and/or polyploid cytotypes were found in the other five species (13 populations), with 2n = 33, 2n = 44, and 2n = 55. Data on chromosome number/ploidy level for other Eugenia species/populations were collected from the literature and included in this cytogeographic analysis. For each collection point (32 species and 62 populations), environmental variables were recorded using georeferencing techniques through the DIVA-GIS v.7.5 program. Environmental variables such as temperature, altitude, rainfall, solar radiation, soil type, and vegetation were analyzed with the R program, using Mann-Whitney and chi-square tests, principal component analysis, and graphic analyses, such as scatterplots, boxplots, and barplot. Polyploid and diploid populations had different spatial distribution patterns and were found in areas subjected to different environmental conditions. Polyploid individuals were collected from locations with more adverse environmental conditions, usually at higher elevations than the diploid individuals. Polyploidy allows species to occur at locations with varying environmental conditions. As diploidy and polyploidy occur under different environmental conditions, species with cytotypes exhibit wide environmental tolerance.