By Krit Raemakers, owner of Plant Cytometry Services
Expert in flow cytometry and plant breeding
The complex genetics of the orchid
Orchids are among the most species-rich plant families on earth, with an estimated 25,000 species. Yet only a small part of this colourful family eventually finds its way into living rooms, greenhouses and garden centres. The path from wild species to commercial plant is anything but easy.
For many years, ornamental plant cultivation has been working with breeding, whereby different species are crossed with each other to develop new, attractive varieties. But for orchids, this brings unique challenges. The use of genetically diverse species can lead to enormous differences in genome size – sometimes a tenfold difference between the obtained “novelty” and the original species. Almost all species are endopolyploid, i.e. naturally have nuclei with a higher ploidy level. This makes the genetic puzzle even more complicated.
Why orchids require a different approach
In many plants, the size of the genome – and often also the number of chromosomes (ploidy) – is determined using flow cytometric techniques. This works well for most species, but orchids are a different story. Many orchids are extremely endopolyploid: a natural process in which some cells duplicate their DNA multiple times, without the cell dividing. As a result, it can happen that in one leaf of a diploid plant only nuclei are present with a higher ploidy level of up to 128x. In other words: even though the plant is diploid in principle, most or even all nuclei measured in the leaf have a higher ploidy level.
This makes it difficult to make statements about the actual genome size/ploidy using a standard measurement. The solution lies in measuring young pods or pollinia (pollen clumps). If present, underground meristems of plants can also be used (see figure D).
A rare phenomenon: partial endoreduplication
Wild orchids have a special phenomenon that is not found anywhere else in the plant world: partial endoreduplication (PE). In this case, only part of the genome is duplicated, instead of the whole thing. The amount varies per species and ranges from 12% to 90%.
As a result, a plant with PE (see figure B) has ploidy levels that are different from those in a plant without PE (see figure A). As if that were not special enough, plants sometimes have a considerable number of cells that skip the first round of PE (see figure C). The picture below shows what this looks like.
Literature:
Diversity in genome size and GC content shows adaptive potential in orchids and is closely linked to partial endoreplication, plant life-history traits and climatic conditions. Pavel Trávníček, Martin Čertner, Jan Ponert, Zuzana Chumová, Jana Jersáková, Jan Suda. New phytologist, 2019. 224: 4 https://doi.org/10.1111/nph.15996