Genomic and meiotic analysis of Mokara orchids

Abstract

Mokara orchids are the special group of artificially created trigeneric hybrids involving members of Vanda, Arachnis, and Ascocentrum, the genomic symbols of which are often designated as V, A, and As, respectively. Eight Mokara clonal cultivars grown commercially in Singapore were selected for genomic and meiotic investigation. They were classified into three categories based on their diverse origins. Three cultivars derived from 2x-Arachnis x 2x-(Vanda x Ascocentrum) were found to be diploid (2n = 38), and their genomic constitutions were likely to be A + 1/2 (As + V). Their microsporocytes exhibited low frequencies of bivalents, which could have involved A-As and/or A-V chromosome pairs. All four cultivars derived from 2x-(Arachnis x Vanda) x 2x-(Ascocentrum x Vanda) were triploid with an inferred genomic content of A + V + 1/2 (As + V). Up to 19 bivalents were detected in the microsporocytes. These were most likely formed between related pairs of V-As and V-V chromosomes and possibly A-As. The only cultivar obtained from 2x-Mokara x 2x-(Ascocentrum x Vanda) was also triploid, but its genomic makeup was likely to be A + 1/2 (As + V) + 1/2 (As + V). Large numbers of bivalents occurred, probably representing mainly As-As, As-V, and V-V chromosome pairs. However, the integrity of individual chromosomes could not be assumed because prior intergenomic recombination could have occurred in parents. Triploid cultivars of both genomic types were more vigorous and generally more horticulturally desirable for cut-flower production than their diploid counterparts.