Can ploidy levels explain the variation of Herbertia lahue (Iridaceae)?

Abstract

Polyploidy is often related with phenotypic variation, as observed in Herbertia lahue, a geophyte species. This study examined the H. lahue polyploid series and departure in cytogenetic, morphometric, and pollen data. Diploids (2n=2x=14) present bimodal karyotype with two long and five short chromosome pairs, while hexaploids (2n=6x=42) and octoploids (2n=8x=56) present a gradual decrease in chromosome size. All cytotypes have CMA+/DAPI- bands co-localized with 18S rDNA sites in the satellite region (no DAPI+ bands in any cytotype). Unlike diploids and octoploids, 5S rDNA interstitial sites in hexaploids are not in a syntenic position with 18S rDNA sites. Genome size is effective as an indirect predictor of the cytotypes since 2C-values increased according to ploidy level. The reduction in the number of the rDNA sites in polyploids associated with their lower 1Cx-values compared to diploids may suggest a genome downsizing process. Morphometric analysis revealed significant differences among cytotypes, and discriminant analysis identified three morphometric groupings corresponding to the cytotypes. The phenotypic variation observed in pollen grains, bulbs, and ovary characters suggested the gigas effect. Concluding, remarkable differentiation was observed at both genomic and phenotypic characters in all the cytotypes analyzed, suggesting a possible ongoing speciation process in H. lahue.

Figure 1 -. Map with the locations of Herbertia lahue populations sampled in this study. Circles indicate diploid populations, asterisks indicate hexaploid populations and squares indicate octoploid populations.

Figure 2 -. Mitotic metaphases of each cytotype of Herbertia lahue and its respective ideogram. (A) and (B) Diploid H. lahue; (C) and (D) Hexaploid H. lahue; (E) and (F) Octoploid H. lahue. Bars = 5 μm.

Figure 3 -. Mitotic metaphases of Herbertia lahue with CMA/DAPI chromosome banding (CMA⁺ bands yellow) and hybridized with 18S (green) and 5S (red) rDNA probes. (A) and (B) Diploid H. lahue; (C) and (D) Hexaploid H. lahue; (E) and (F) Octoploid H. lahue. Arrows indicate 18S rDNA sites co-located with CMA bands. Bar = 5 μm.

Figure 4 -. Effect of ploidy level on equatorial axis and polar axis measurements, and on the quantity of pollen grains per anther in Herbertia lahue cytotypes. ^{a, b} Letters indicate differences and mean values marked with the same letter are not significantly different at P < 0.05, by Tukey’s test.

Figure 5 -. Effect of ploidy level on the morphometry of characters examined in Herbertia lahue cytotypes. ^{a, b, c} Letters indicate differences and mean values marked with the same letter are not significantly different at P < 0.05, by Tukey’s test.

Figure 6-. Discriminant analysis performed with 18 morphological characters examined in Herbertia lahue cytotypes.