Chromosome mapping reveals varied rDNA patterns across the Schedonorus–Lolium grass complex

Chromosome mapping identified variation in the number, position and chromosomal association of 5S and 35S ribosomal DNA loci in five taxa from the Schedonorus–Lolium grass complex. The study directly characterized three previously unmapped taxa and integrated the new observations with earlier cytogenetic and DNA-sequence studies to examine possible evolutionary patterns across diploid and polyploid members of the group.

This was a chromosome-mapping study accompanied by a review of existing evidence, not an experiment designed to test biological function. Researchers examined Lolium perenne, two sources of L. multiflorum, Micropyropsis tuberosa and Festuca simensis. Seed for the five populations came from the Margot Forde Forage Germplasm Centre at AgResearch Grasslands in Palmerston North and PGG Wrightson Seeds in Christchurch, New Zealand.

Plants were germinated and grown in a glasshouse. Somatic chromosome preparations were obtained from meristematic tissue in actively growing root tips using a flame-drying technique, and good-quality preparations were selected by phase-contrast microscopy. Double-target fluorescence in situ hybridization used an 18S rDNA probe representing the 35S rDNA family and a 5S rDNA probe. Investigators analyzed at least five good-quality early-to-late metaphase cells from each plant for hybridization signals.

Diploid L. perenne, with 2n = 2x = 14 chromosomes, had six 35S signals representing three loci on three chromosome pairs. One locus was proximal on a short arm, while two were proximal on long arms. A single 5S locus on a short arm shared a chromosome with a 35S locus on the opposite arm. North European or Mediterranean L. multiflorum had the same pattern. The chromatin containing 35S rDNA was frequently decondensed and sometimes stretched in the flame-dried preparations.

The Moroccan population of L. multiflorum also had 2n = 2x = 14 chromosomes, but differed from the Eurasian population by having two rather than three 35S loci, each proximal on a long arm. A 5S locus was proximal on the short arm of one of the 35S-bearing chromosome pairs. The authors considered the Moroccan population potentially a distinct North African taxon with chromosomal affinities to Eurasian Lolium, although the chromosome observations did not establish its taxonomic status.

Diploid M. tuberosa, also with 2n = 2x = 14, had one 5S locus and one 35S locus on separate chromosome pairs, both proximal on short arms. This arrangement matched that previously reported for Festuca pratensis and was consistent with the authors’ interpretation of an ancestral diploid Schedonorus arrangement before the divergence of Lolium. That interpretation was based on comparison with phylogenetic evidence rather than a direct reconstruction of the ancestral chromosome.

Tetraploid F. simensis, with 2n = 4x = 28 chromosomes, had eight mapped signals on separate chromosomes. Its three pairs of 5S signals included one interstitial locus on a short arm and two terminal loci, one on a short arm and one on a long arm. Its single 35S locus was proximal on short arms, and the two rDNA types were not co-localized. The authors described this distribution as consistent with allopolyploidy and noted that the single 35S locus could reflect uniparental loss during diploidization. Similarity between the patterns in F. simensis and hexaploid F. gigantea led them to propose that F. simensis might have supplied two subgenomes of F. gigantea; this remained a possible relationship rather than demonstrated ancestry.

Across the reviewed diploid taxa, all eight studied Lolium species had exclusively proximal locations for both rDNA marker families and a single 5S locus. Their numbers of 35S loci ranged from two to five. The two marker types were syntenic in most taxa, but their relative positions differed among largely allogamous and largely autogamous groups. L. canariense was exceptional because its 5S and 35S loci were not syntenic, a pattern the authors said was compatible with its proposed position between the Festuca and Lolium lineages.

The review of polyploid Festuca found more varied locus arrangements. Reported 5S locus numbers ranged from two in the allotetraploids F. mairei and F. glaucescens to eight in decaploid F. letourneuxiana, while 35S locus numbers ranged from one in F. simensis to six in F. letourneuxiana. Terminal 35S loci were common, and F. simensis was the only species in the reviewed group reported to have an interstitial 5S locus. In the higher-ploid fescues, the authors proposed that additivity of 5S locus numbers, combined in some cases with loss or positional change of 35S loci, could help explain relationships among lineages. This did not imply additivity of both marker types.

Overall, the work extended the available rDNA mapping to seven of the ten known Lolium species and added F. simensis to the previously characterized polyploid fescues. Substantial variation in rDNA locus number and location occurred without obvious changes in the symmetrical, biarmed karyotypes characteristic of the complex. The authors proposed paracentric chromosome rearrangements or transposable-element activity as possible mechanisms, but neither was directly tested. Their evolutionary conclusions remain constrained by the small number of newly mapped populations, uneven taxon coverage among earlier studies and the inability of rDNA patterns alone to identify every ancestral relationship; the diploid progenitors of the allotetraploid species remain unidentified.

Subject of Research: Chromosomal distribution and evolutionary significance of 5S and 35S rDNA loci in the Schedonorus–Lolium grass complex

Article Title: Distribution patterns of rDNA loci in the Schedonorus – Lolium complex ( Poaceae )

Article References: Ansari, H. A., Ellison, N., Stewart, A. V., & Williams, W. M. (2022). Distribution patterns of rDNA loci in the Schedonorus-Lolium complex (Poaceae). Comparative Cytogenetics, 16(1), 39-54. https://doi.org/10.3897/compcytogen.v16.i1.79056

Image Credits: AI Generated

DOI: 10.3897/compcytogen.v16.i1.79056

Keywords: rDNA loci, chromosome mapping, fluorescence in situ hybridization, Schedonorus–Lolium complex, Lolium, Festuca, Micropyropsis tuberosa, polyploidy, karyotype evolution, cytogenetics, Distribution, patterns