Abstract
Astragalus section Hymenostegis is one of the important characteristic elements of thorn-cushion formations in the Irano-Turanian floristic region. In this paper, we examined the chromosome number of 17 species (15 new reports) and provide estimates of genome size for 62 individuals belonging to 38 taxa of A. sect. Hymenostegis, some species outside this section, plus two Oxytropis species. Based on chromosome counts 11 species were found to be diploid (2n = 16), four species tetraploid (2n = 32) and two taxa hexaploid (2n = 48). From genome size measurements on silica-gel dried material, three ploidy levels (2x, 4x and 6x) were inferred, with a majority of species being diploid. The 2C values reach from 2.07 pg in diploid Astragalus zohrabi to 7.16 pg in hexaploid A. rubrostriatus. We found indications that species might occur with different cytotypes. A phylogenetic framework using nrDNA ITS sequences was constructed to understand the evolution of ploidy changes and genome sizes. It showed that genome size values among the studied taxa differ only slightly within ploidy levels and are nearly constant within most species and groups of closely related taxa within the genus Astragalus. The results of this study show that there is a rather strong correlation between genome sizes and chromosome numbers in sect. Hymenostegis. The resolution of the ITS-based phylogenetic tree is too low to infer evolutionary or environmental correlations of genome size differences. Polyploidization seems to contribute to the high species number in Astragalus, however, in sect. Hymenostegis it is not the main driver of speciation.
Highlights
Hymenostegis was established as x = 8 and the examined species belong to three ploidy levels: 11 taxa are diploid (2n = 2x = 16), four are tetraploid (2n = 4x = 32) and two were found to be hexaploid (2n = 6x = 48)
Within our own analyzed species, we found a strong correlation between ploidy level and genome size
In A. melanostictus we found 2n = 4x = 32 chromosomes in the individual that was used for chromosome counts, while the two individuals used for genome-size estimations resulted (i) in highly disparate 2C values (1.54 vs. 2.21 pg) that (ii) fit only for the latter value to the genome-size classes defined above
Summary
A phylogenetic framework using nrDNA ITS sequences was constructed to understand the evolution of ploidy changes and genome sizes It showed that genome size values among the studied taxa differ only slightly within ploidy levels and are nearly constant within most species and groups of closely related taxa within the genus Astragalus. Polyploidization seems to contribute to the high species number in Astragalus, in sect Hymenostegis it is not the main driver of speciation. Relationships within Astragalus were studied for decades by molecular phylogenetic analyses [12,13,14,15,16,17,18] still some sectional and species-level relationships in Astragalus remain unanswered [14,15,16,17,18] and explanations for the high species number of the genus are lacking.
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