Abstract
The genus Trifolium L. is characterized by basic chromosome numbers 8, 7, 6, and 5. We conducted a genus-wide study of ribosomal DNA (rDNA) structure variability in diploids and polyploids to gain insight into evolutionary history. We used fluorescent in situ hybridization to newly investigate rDNA variation by number and position in 30 Trifolium species. Evolutionary history among species was examined using 85 available sequences of internal transcribed spacer 1 (ITS1) of 35S rDNA. In diploid species with ancestral basic chromosome number (x = 8), one pair of 5S and 26S rDNA in separate or adjacent positions on a pair of chromosomes was prevalent. Genomes of species with reduced basic chromosome numbers were characterized by increased number of signals determined on one pair of chromosomes or all chromosomes. Increased number of signals was observed also in diploids Trifolium alpestre and Trifolium microcephalum and in polyploids. Sequence alignment revealed ITS1 sequences with mostly single nucleotide polymorphisms, and ITS1 diversity was greater in diploids with reduced basic chromosome numbers compared to diploids with ancestral basic chromosome number (x = 8) and polyploids. Our results suggest the presence of one 5S rDNA site and one 26S rDNA site as an ancestral state.
Highlights
A part of the third-largest plant family, Fabaceae, the genus Trifolium includes ca. 255 species [1,2,3,4] with cosmopolitan distribution throughout a large range of biotopes characterized by different temperature and climate conditions
We show a rough correlation between polymorphisms in internal transcribed spacer 1 (ITS1) sequences and mean 26S ribosomal DNA (rDNA) sites per chromosome in all analysed species in each section (Figure 4)
The Trifolium ancestral karyotype contains a single pair of 5S and 26S rDNA sites
Summary
A part of the third-largest plant family, Fabaceae, the genus Trifolium (clovers) includes ca. 255 species [1,2,3,4] with cosmopolitan distribution throughout a large range of biotopes characterized by different temperature and climate conditions. A part of the third-largest plant family, Fabaceae, the genus Trifolium (clovers) includes ca. More than half of its species originated in the Mediterranean region [6,7], which still has the largest number of endemic species [8,9]. Changes in chromosome number played a role in the evolution of the genus Trifolium [10]. Chromosome evolution in Trifolium has been studied since the 1980s [1,12,13]. 31 species with reduced basic chromosome numbers have been identified (x = 7, 6, 5), 11 of which exhibit both reduced and ancestral diploid or polyploid counts, while two of these species exhibit two different reduced counts [3]
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