Intrachromosomal karyotype asymmetry in Orchidaceae

Abstract

The asymmetry indexes have helped cytotaxonomists to interpret and classify plant karyotypes for species delimitation efforts. However, there is no consensus about the best method to calculate the intrachromosomal asymmetry. The present study aimed to compare different intrachromosomal asymmetry indexes in order to indicate which are more efficient for the estimation of asymmetry in different groups of orchids. Besides, we aimed to compare our results with the Orchidaceae phylogenetic proposal to test the hypothesis of Stebbins (1971). Through a literature review, karyotypes were selected and analyzed comparatively with ideal karyotypes in a cluster analysis. All karyotypes showed some level of interchromosomal asymmetry, ranging from slightly asymmetric to moderately asymmetric. The five tested intrachromosomal asymmetry indexes indicated Sarcoglottis grandiflora as the species with the most symmetrical karyotype and Christensonella pachyphylla with the most asymmetrical karyotype. In the cluster analysis, the largest number of species were grouped with the intermediary ideal karyotypes B or C. Considering our results, we recommend the combined use of at least two indexes, especially Ask% or A1 with Syi, for cytotaxonomic analysis in groups of orchids. In an evolutionary perspective, our results support Stebbins’ hypothesis that asymmetric karyotypes derive from a symmetric karyotypes.

Ploidy Level, Karyotype, and Genome Size of Bletilla Species (Orchidaceae) From China

“…At least five spread metaphases with clear and separate chromosomes were examined for chromosome number and karyotypic asymmetry. Karyotype features included chromosome number (2n), HCL, and Stebbins karyotype (SK) (Medeiros-Neto et al, 2017;Stebbins, 1971), and the asymmetric indexes were selected as follows: interchromosomal asymmetry indexes A 2 , intrachromosomal asymmetry indexes A 1 (Zarco, 1986), Syi (Venora et al, 2002), and asymmetry coefficient of karyotypes AsK% (Arano, 1963). The arm ratio (r = length of the long arm/length of the short arm) was used to classify the chromosomes as metacentric (m, r = 1.0 to 1.7), submetacentric (sm, r = 1.7 to 3.0), subtelocentric (st, r = 3.0 to 7.0), and asrocentric (t, r = 7.0 to 1) (Levan et al, 1964).…”

Chromosome Analysis of Dendrobium stockelbuschii and Dendrobium sylvanum Orchids

“…The ideograms were analyzed using Microsoft Office Visio. The chromosomal asymmetry index value is calculated using the formula [10]. : average of the chromosome length in a karyotype.…”

Induction of Polyploidy Using Colchicine in Flower Buds from Phalaenopsis Hybrids

“…The results of these calculations explain that the A2 value is small or close to zero in both control and treatment. According to Neto et al [21], the A2 value tends to approach zero in some species of Epidendroideae and Orchidoideae subfamilies. These results indicate that the size dispersion in one karyotype is not too large or small.…”