Orchid Chromosomes Reveal Hidden Genetic Differences

Orchids may look alike on the surface, but their chromosomes contain a much more intricate record of evolutionary history. A comparative cytogenetic study of four members of the orchid subtribe Orchidinae has mapped ribosomal DNA across the chromosomes of Ophrys tenthredinifera, Serapias vomeracea, Himantoglossum hircinum and Anacamptis morio. The work identifies chromosome-number stability, a new triploid form of the green-winged orchid and striking differences in the organization of two families of ribosomal genes. These features help researchers distinguish closely related plants, reconstruct genome changes and understand how terrestrial orchids diversified across Europe and the Mediterranean.

The investigators used fluorescence in situ hybridization, or FISH, to locate DNA sequences directly on plant chromosomes. Red probes recognized the 18S-5.8S-25S ribosomal DNA family, while green probes detected 5S ribosomal DNA. These repeated sequences are valuable cytogenetic markers because their number and chromosomal position can vary among closely related taxa. The researchers complemented FISH with Feulgen staining, Giemsa C-banding, silver staining for nucleolar organizer regions, DAPI staining and CMA fluorescence banding. Chromosomes were observed in mitotic cells from immature ovaries, allowing the team to construct karyotypes and compare both physical structures and gene locations.

Ophrys tenthredinifera, a bee-orchid species represented in the study by the subspecies neglecta and grandiflora, was remarkably uniform. Every population examined had a diploid chromosome complement of 2n = 2x = 36. Both subspecies shared the same basic structure: 32 metacentric and four submetacentric chromosomes, including four pairs with satellites on their short arms. Their asymmetry measurements were also similar, indicating a highly symmetrical and stable karyotype. Because the chromosome number and morphology showed little differentiation, the results did not support recognizing the two subspecies as separate taxa.

Fluorescent mapping nevertheless revealed fine-scale differences in where ribosomal sequences were located. The O. tenthredinifera genome carried three 18S-5.8S-25S rDNA sites and two pairs, or four sites, of 5S rDNA. Silver staining detected nucleolar organizer regions associated with the satellite-bearing chromosomes and up to three nucleoli in interphase nuclei. CMA staining also highlighted the NOR-bearing pair, while C-banding showed centromeric heterochromatin and a telomeric band on one chromosome pair. Together, these observations linked the major ribosomal loci to secondary constrictions and provided a more detailed description of genome organization than chromosome counts alone.

The tongue orchid Serapias vomeracea also had a diploid complement of 36 chromosomes, but its ribosomal DNA map differed from that of O. tenthredinifera. FISH detected six 18S-5.8S-25S rDNA sites distributed across three chromosome pairs, along with four 5S rDNA sites on two pairs. One pair of 5S sites produced a much stronger signal than the other. The researchers interpreted this imbalance as possible evidence of recent chromosome restructuring, a subtle genomic distinction that may be useful in future comparisons with other members of the genus.

Himantoglossum hircinum, the lizard orchid, likewise had 36 chromosomes, arranged as 28 metacentrics and eight submetacentrics. Secondary constrictions occurred on the short arms of chromosome pairs 4, 5 and 7, and C-banding detected small amounts of constitutive heterochromatin in centromeric and telomeric regions. Up to four nucleoli were visible in interphase nuclei. FISH located four 18S-5.8S-25S rDNA sites and four 5S rDNA sites. These results distinguish the species from Barlia robertiana, which has been treated both as a separate genus and as a subgenus within Himantoglossum, and has a different arrangement of the two ribosomal families.

The most consequential finding came from Anacamptis morio, commonly called the green-winged orchid. The study confirmed the familiar diploid form with 2n = 2x = 36 chromosomes, a complement of 30 metacentric and six submetacentric chromosomes. The diploid carried six 18S-5.8S-25S sites and two 5S sites. More unusually, FISH detected an individual with 54 chromosomes, corresponding to a triploid complement of 2n = 3x = 54. This triploid possessed nine major ribosomal sites and three 5S sites, exactly 50 percent more of each than the diploid. To the authors’ knowledge, it was the first reported case of autotriploidy in A. morio.

Finding a triploid is significant because polyploidy can preserve genetic variation, alter fertility, create reproductive barriers and accelerate plant evolution. In this case, the proportional increase in chromosome number and ribosomal loci is consistent with three complete chromosome sets rather than an irregular gain or loss of individual chromosomes. The discovery also shows why counts taken from limited samples can miss biologically important variation within a species. A population may appear chromosomally uniform in one region or generation while rare polyploid individuals occur elsewhere, potentially representing a distinct pathway in the orchid’s reproductive and evolutionary history.

The study demonstrates how chromosome-level evidence can resolve questions that remain difficult for conventional taxonomy. A stable count of 36 chromosomes does not mean that all orchids are genetically equivalent, because the location, number and activity of repeated DNA can differ even when chromosome totals match. By contrast, strongly divergent patterns can sometimes expose relationships hidden by superficial similarity. The authors argue that continued use of FISH, alternative banding methods and molecular analysis should help clarify groups involving Anacamptis, Himantoglossum and Barlia, while also guiding work on polyploid orchids such as A. pyramidalis and the Neotinea commutata complex.

For conservation-minded botanists, the work also illustrates why a chromosome survey can complement field inventories. Ophrys includes many narrowly distributed species, and populations separated by geography or ecological conditions may not be cytogenetically equivalent. The comparison of O. tenthredinifera subspecies, for example, found only limited karyological divergence despite their distinct geographic ranges, while the triploid A. morio demonstrates a form of numerical variation within a recognized species. By documenting the distribution of ribosomal DNA and heterochromatin alongside chromosome counts, the study provides a practical framework for future sampling. It turns chromosomes into a detailed map of genomic history, showing that evolutionary differences may be recorded not only in the number of chromosomes but also in the molecular addresses where the genome’s most conspicuous repeated genes are stored.

Subject of Research: Comparative chromosome and ribosomal DNA mapping in Orchidinae orchids

Article Title: Comparative chromosome studies in species of subtribe Orchidinae ( Orchidaceae )

Article References: Turco, A., Albano, A., Medagli, P., Wagensommer, R. P., & D'Emerico, S. (2021). Comparative chromosome studies in species of subtribe Orchidinae (Orchidaceae). Comparative Cytogenetics, 15(4), 507-525. https://doi.org/10.3897/compcytogen.v15.i4.75990

Image Credits: AI Generated

DOI: 10.3897/compcytogen.v15.i4.75990

Keywords: orchids, cytogenetics, chromosomes, fluorescence in situ hybridization, ribosomal DNA, polyploidy, autotriploidy, genome organization, Orchidinae, plant evolution, Comparative, chromosome