Five-specimen cytogenetic study finds a 22-chromosome karyotype in Nysius graminicola

A cytogenetic study of five male Nysius graminicola found that the seed-predating true bug has 22 chromosomes and carries detectable 18S ribosomal DNA loci on its X and Y chromosomes. The animal study provides the first karyotype description for the species and expands the known chromosome-number diversity of the genus Nysius.

Nysius is a widely distributed genus within Lygaeidae and includes more than 100 described species and subspecies, with additional species remaining unrecognized. Its members live in ruderal habitats, can be extremely abundant, and sometimes become agricultural pests. The genus is also taxonomically complex because closely related species have strikingly similar external features. DNA sequencing of standard gene regions has therefore been proposed as one means of facilitating species identification.

Before this study, cytogenetic data were available for 24 Nysius species, approximately 25% of the known species in the genus. Most had a diploid chromosome number of 14, with 12 autosomes and XY sex chromosomes, as well as a pair of very small microchromosomes. Nysius tennellus was the only previously reported exception, with 22 chromosomes. The authors proposed that the 22-chromosome arrangement may have arisen through fragmentation of autosomes from the 14-chromosome state, which they regarded as ancestral for most of the genus.

For the present investigation, the researchers collected five male N. graminicola on cereals in a flood meadow 20 km northeast of Voronezh, Russia, on 15 August 2023. They fixed the insects in a 3:1 mixture of alcohol and acetic acid, prepared slides from their testes, and examined male meiosis from prophase and metaphase I through metaphase II. The work combined routine karyological analysis using Schiff-Giemsa staining, silver staining for nucleolus organizer regions, and fluorescence in situ hybridization with an 18S rDNA probe from the firebug Pyrrhocoris apterus. The evidence is limited to five males from one locality, although the study provides the first cytogenetic examination of this species.

The observed karyotype was 2n = 22, with 20 autosomes and XY sex chromosomes, and the authors denoted its meioformula as n = 9AA+mm+X+Y. At metaphase I, the cells contained 10 autosomal bivalents, including the small microchromosome pair, while the X and Y chromosomes lay separately. During metaphase II, each daughter cell contained 11 elements, comprising 10 autosomes divided into chromatids and a pseudobivalent XY. The sex chromosomes segregated equationally in the first meiotic division and reductionally in the second, a pattern described as inverted or post-reductional meiosis.

The chromosome preparations also showed a radial arrangement at metaphase I and metaphase II, with the sex chromosomes and microchromosomes located centrally within a ring formed by the autosomal bivalents. The 18S rDNA probe produced signals on both sex chromosomes during meiosis, with the signal on the Y chromosome appearing larger and brighter. Silver staining separately revealed nucleolar remnants associated with both sex chromosomes in interphase and prophase cells, supporting the presence of 18S rDNA at these locations. These observations establish the detected probe sites; they do not show that every ribosomal RNA gene or all ribosomal DNA in N. graminicola is confined to the sex chromosomes.

Earlier fluorescence in situ hybridization had mapped rDNA sites in N. cymoides and N. helveticus, two species with the modal 14-chromosome karyotype, to their largest pair of autosomes. This pattern led the researchers to hypothesize that N. graminicola might also possess an autosomal 18S rDNA site. That expectation was not supported by the new observations, because the detectable 18S probe signals were instead associated with the X and Y chromosomes. The comparison does not demonstrate that all rDNA clusters moved from autosomes to sex chromosomes, nor does it establish the precise evolutionary history of the loci.

Chromosomal structure offers a possible clue to the higher chromosome count. Species with the 14-chromosome arrangement possess a pair of very large chromosomes, whereas N. graminicola and N. tennellus lack that conspicuous pair and instead have chromosomes arranged in a decreasing size series. This supports the authors’ hypothesis of autosomal fragmentation, although the study does not directly test the rearrangement history. The 22-chromosome karyotype is also one of two modal arrangements reported in Lygaeidae, including its subfamily Orsillinae.

The authors suggest that transposable elements may have contributed to the dispersal of 18S rDNA loci because chromosomal rearrangements alone appear insufficient to explain the observed distribution. Such elements have been associated with rRNA-gene movement in other true bugs and insects. They also note that movement of rDNA clusters between autosomes and sex chromosomes may be evolutionarily significant through genetic differentiation and speciation, but the present observations do not demonstrate either effect in Nysius. The study therefore establishes a distinct chromosome arrangement and sex-chromosome-associated 18S rDNA sites in N. graminicola, while leaving the processes responsible for their evolution and their distribution across the wider genus unresolved.

Subject of Research: Cytogenetic characterization of the chromosome number and 18S rDNA locations in Nysius graminicola

Article Title: Karyotype diversity in the genus Nysius Dallas, 1852 ( Hemiptera , Heteroptera , Lygaeidae ) is much greater than you might think

Article References: Golub, N. V., Anokhin, B. A., & Kuznetsova, V. G. (2023). Karyotype diversity in the genus Nysius Dallas, 1852 (Hemiptera, Heteroptera, Lygaeidae) is much greater than you might think. Comparative Cytogenetics, 17, 287-293. https://doi.org/10.3897/compcytogen.17.116628

Image Credits: AI Generated

DOI: 10.3897/compcytogen.17.116628

Keywords: Nysius graminicola, karyotype diversity, chromosome number, 18S rDNA, fluorescence in situ hybridization, sex chromosomes, cytogenetics, true bugs, Lygaeidae, Karyotype, diversity, genus