The research team, working with living material from the Active Germplasm Bank of Embrapa Eastern Amazon in Belém, Pará, Brazil, examined five species: Oenocarpus bacaba, O. bataua, O. distichus, O. mapora, and O. minor. These are the palms that matter most to Amazonian communities, valued for food, tools, utensils, and construction materials. Seeds from multiple accessions of each species were germinated under controlled conditions, and researchers analyzed root tip cells from the resulting seedlings. The work was authorized by Brazilian federal institutions, and voucher specimens were deposited in the IAN Herbarium in Belém, ensuring the study can be verified and built upon by future researchers.

The headline finding is a striking constancy. Every single species, regardless of how different the palms look in the forest, carries exactly 36 chromosomes in its somatic cells, written as 2n = 36. Only one of these species, O. bataua, had ever been counted before, and the new work confirmed that earlier report. For the other four, these are the first chromosome numbers ever published. In a family as vast as the palms, with roughly 2,400 species across 190 genera, such baseline data is remarkably scarce. The subfamily that contains Oenocarpus, called Arecoideae, is actually the least cytogenetically explored of all five palm subfamilies, with at least five tribes lacking chromosome counts entirely.

But the story does not end with the count. When the researchers constructed detailed karyotypes, organized portraits of each species’ full chromosome set, they discovered meaningful differences hiding beneath the identical numbers. The karyotype formulas varied: O. bacaba showed two metacentric, eleven submetacentric, and five acrocentric chromosomes, while O. bataua displayed eight metacentric and ten submetacentric chromosomes with no acrocentrics at all. O. distichus, O. mapora, and O. minor each had their own distinct formulas. The position of secondary constrictions, the pinched regions on chromosomes that mark the locations of nucleolus organizer regions carrying ribosomal DNA genes, also differed from species to species, appearing on different chromosome pairs in each one.

These differences matter because they point to a mechanism. The researchers infer that structural chromosome rearrangements, particularly translocations and pericentric inversions, accumulated during the evolution of the genus, reshuffling chromosome arms while leaving the total count untouched. Transposable elements, the mobile DNA sequences that can jump around genomes, may have contributed as well. This is evolution working at the architectural level: the same number of pieces, arranged in different ways. According to the classic framework of karyotype evolution developed by G. Ledyard Stebbins, such rearrangements are important drivers of speciation because they increase asymmetry within chromosome sets and help differentiate closely related lineages from one another.

The asymmetry analysis reinforced this picture. Using two independent classification methods, one from Stebbins and one from Zarco, the team quantified how uneven the chromosomes were within and between each species’ complement. O. bataua emerged as the most symmetrical, classified in category 2b with the lowest values for both intrachromosomal and interchromosomal asymmetry. Three species, O. distichus, O. mapora, and O. minor, clustered together in category 3b, forming a visibly similar group on a scatterplot of the two asymmetry indices. O. bacaba stood apart as the most asymmetrical of all five, landing in category 3c with the highest values on both measures. Whether that asymmetry signals a more derived karyotype or simply a different evolutionary trajectory remains an open question for future work.

The team also looked at how DNA is packaged when cells are not dividing, examining the resting state known as interphase. All five species displayed semi-reticulate interphase nuclei, characterized by strongly stained chromatin clumps called chromocenters with irregular edges. This matters more than it might sound, because in the palm family, interphase nucleus type has proven remarkably constant among related species, sometimes even at the level of whole tribes. The semi-reticulate pattern is considered typical of species with medium-sized chromosomes, roughly three to five micrometers long, which is exactly what the researchers measured. The finding slots Oenocarpus neatly into established patterns of chromatin organization across the Arecaceae.

Genome size told its own story, and a more restrained one. Using flow cytometry, a technique that lasers individual nuclei to measure their DNA content, the team estimated the 2C nuclear DNA amount for each species. The values ranged narrowly from 6.46 picograms in O. bataua to 6.96 picograms in O. minor, and statistical analysis found no significant differences among the five. That uniformity is notable given the wild variation across the palm family as a whole. Other arecoid palms show 2C values from under 4 picograms in the coconut palm to more than 27 picograms in Pinanga subintegra, a spread of more than fourteenfold among diploid palms. Within genera, however, DNA amounts rarely vary much, and Oenocarpus fits that rule perfectly.

One discrepancy adds a layer of scientific intrigue. The earlier study that counted O. bataua’s chromosomes also estimated its DNA content, using an older technique called Feulgen microdensitometry, and arrived at a value more than one picogram higher than the flow cytometry result reported now. The two methods estimate DNA differently, and while results often correlate, discrepancies between them have been documented in other plants without a settled explanation. The authors suggest that reanalyzing the original specimens with flow cytometry, or sampling more genotypes across the species’ range, would clarify whether the gap reflects methodology or genuine intraspecific variation, a phenomenon documented in the coconut palm. Genotype origin and technique both appear capable of nudging genome size estimates in palms.

For the Amazonian communities who depend on these palms, and for the plant breeders working to improve them, the study delivers practical groundwork. Chromosome numbers, karyotype formulas, and genome sizes are foundational data for characterizing germplasm banks and managing breeding programs, and they anchor cytotaxonomic classifications that help distinguish species and guide conservation. The constancy of 2n = 36 across the genus, matching counts in related members of the tribe Euterpeae, confirms Oenocarpus as a conservative lineage in chromosome number while revealing a dynamic history of structural rearrangement beneath the surface. The next chapter, the authors note, involves understanding the mechanisms driving karyotype differentiation in more detail and testing the phylogenetic relationships proposed for the genus. For now, five of the Amazon’s most useful palms finally have their cellular identities on record, and their chromosomes tell a story of stability on the outside and restless change within.

Subject of Research: Cytogenetics and nuclear DNA content of five Oenocarpus palm species

Article Title: Interphase nuclei, karyotypes and nuclear DNA amounts in five species of Oenocarpus ( Arecaceae )

Article References: Oliveira, N. P. D., Gesteira, G. D. S., Padilha de Oliveira, M. D. S., & Davide, L. C. (2024). Interphase nuclei, karyotypes and nuclear DNA amounts in five species of Oenocarpus (Arecaceae). Comparative Cytogenetics, 18, 59-72. https://doi.org/10.3897/compcytogen.18.117597

Image Credits: AI Generated

DOI: 10.3897/compcytogen.18.117597

Keywords: Oenocarpus, Arecaceae, cytogenetics, karyotype, flow cytometry, chromosome number, genome size, Amazonia, speciation, chromosome rearrangement, interphase nuclei, Arecoideae