Chromosome numbers and heterochromatin variation in introgressed and non-introgressed populations of Epidendrum (Orchidaceae: Epidendroideae): interspecific transfers of heterochromatin lead to divergent variable karyotypes in the parental populations.

Abstract

• 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, 2n = 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.