Genome and karyotype relationships in the genus Dendrobium (Orchidaceae)

Abstract

Investigations on sexual compatability, karyotype analysis, and genome relationships were made with species in the genus Dendrobium of the family Orchidaceae. A hybridization study was made utilizing 38 species of 10 taxonomic sections in 48 combinations. A total of 783 pollinations was made with 164 fruits harvested, of which 113 produced viable crosses. Five intrasectional and 20 intersectional combinations resulted in viable seedlings. The Eugenanthe x Eugenanthe combination showed little compatability among the species within the section and showed equal or more compatability with the Ceratobium, Phalaenanthe, and Pedilonum sections. Species in the Ceratobium section crossed as readily with species in the Phalaenanthe section as within the Ceratobium section. The percentage of non-aborted embryos was determined for each fruit harvested. The separation of all the species into the classical taxonomic sections of the genus was not possible on the basis of their crossability or percentage of non-aborted embryos. Chromosome numbers of 33 species in 11 sections were determined, of which 31 were 2n=38 and 2 were 2n=40. Five of these had not previously been reported. Detailed examinations of chromosome morphology were made of 23 species in 11 sections. The mean chromosome size was as variable within the sections as between the sections. The sections could not be distinguished by the average chromosome length of their constituents. No relationship was found between chromosome size and geographical and climatological distribution in the species studied. A mean S% and F% for each species and the average for each section was calculated. Individual sections could not be distinguished on the basis of S% and F% although a few individual species could be detected by these values in conjunction with other morphological characteristics of the karyotypes. The evolution of the karyotypes was not reflected in the external morphological specializations of the sections. Meiosis in four intrasectional Ceratobium hybrids consistently showed 19 bivalents and the products of meiosis were normal tetrads with 19 chromosomes distributed to each microspore. Five intersectional Phalaenanthe x Ceratobium hybrids displayed an average of 16.80 bivalents and 4.40 univalents. Microspore division exhibited an average of 93.2% tetrads and 3.9% dyads, with tetrads and dyads with microcytes also observed. Meiosis in a Phalaenanthe x Latourea hybrid showed 2.06 bivalents and 33.88 univalents with the products of meiosis being tetrads, dyads, and tetrads and dyads with microcytes. Heterochromatic and heteromorphic bivalents were observed in the intersectional hybrids. The results indicated that the genomes within the Ceratobium are closely homologous; the genomes of Ceratobium and Phalaenanthe are closely related; and the genomes of Latourea are more closely related to Ceratobium than Phalaenanthe.