Incidence and evolutionary relevance of autotriploid cytotypes in a relict member of the genus Daphne (Thymelaeaceae)

Abstract

Odd ploidy-level cytotypes in sexually reproducing species are considered a dead end due to absent or reduced fertility. If sterility is only partial, however, their contribution to the population gene pool can be augmented by longevity and clonal growth. To test this, we investigated the cytotype origin and spatial pattern, and pollen viability in three relict shrub species of the genus Daphne (Thymelaeaceae Juss.) in central Europe. Daphne cneorum subsp. cneorum is a widespread European species that has a broad ecological amplitude, whereas D. cneorum subsp. arbusculoides and D. arbuscula are narrow endemics of the western Pannonian Plain and the Western Carpathians, respectively. Our study confirmed that all three taxa are diploid. However, of more than a thousand analysed individuals of D. cneorum subsp. cneorum, five in four different populations were triploid. Our data indicate that these triploids most likely originate from recurrent autopolyploidization events caused by the fusion of reduced and unreduced gametes. High pollen viability was observed in all three taxa and in both diploid and triploid cytotypes, ranging from 65 to 100 %. Our study highlights the significant role of odd ploidy-level cytotypes in interploidy gene flow, calling for more research into their reproduction, genetic variability, and overall fitness. Interestingly, while the endemic D. arbuscula differs from D. cneorum based on genetic and genome size data, D. cneorum subsp. arbusculoides was indistinguishable from D. cneorum subsp. cneorum. However, our study reveals that the subspecies differ in the number of flowers per inflorescence. This is the first comprehensive cytogeographic study of this intriguing genus at a regional scale, and in spite of its karyological stability, it contributes to our understanding of genomic evolution in plant species with a wide ecological amplitude.

Keywords: Carpathians; Daphne; ITS; Pannonian Basin; endemics; genome size stasis; pollen fertility; polyploidy; relicts; triploids.

Figure 1.

Studied taxa and their habitats: (A) Daphne arbuscula; (B and C) calcareous cliffs in Muránska Planina Mts., Western Carpathians (SK-DA 7 and SK-DA 8); (D) D. cneorum subsp. cneorum—diploid cytotype; Sokol, Veľká Fatra Mts., Western Carpathians (SK-DC 6); (E) D. cneorum subsp. cneorum—triploid cytotype; Záhorská nížina Lowland, Pannonian Basin (SK-DC 1); (F) calcareous relict pine forest, Karlschutt, Hochschwab Mts., Eastern Alps Mts. (AT-DC 11); (G) steppic grasslands on gypsum, Sfăraș-Jebucu, Transylvanian Plateau, Romania (RO-DC 19); (H) siliceous sands, Záhorská nížina Lowland, Pannonian Basin (SK-DC 1); (I) steppic calcareous grasslands, Gánt, Transdanubian Mts., Pannonian Basin (HU-DC 17); (J) subalpine calcareous rocks, Turnu peak, Piatra Craiului Mts., Southern Carpathians (RO-DC 25); (K) D. cneorum subsp. arbusculoides (HU-DCA 2); (L) grassland and forest margins on the siliceous substrate, Őrség region, Pannonian Basin (HU-DCA 2). Photos A–D, F, H, I, K, L credits to Z. Gajdošová; E credit to J. Kučera, G credit to B.-I. Hurdu; J credit to A. Indreica.

Figure 2.

Map of sampled populations from the analysed Daphne taxa: DA—D. arbuscula; DC—D. cneorum subsp. cneorum; DCA—D. cneorum subsp. arbusculoides. Country codes: AT—Austria, HU—Hungary, RO—Romania, SK—Slovakia, SLO—Slovenia. Taxa and cytotypes are indicated by different symbols. Population codes follow Supporting Information—Table S1.

Figure 3.

Overall variability in parametric analyses in Daphne arbuscula, D. cneorum subsp. cneorum diploids (2n = 2x) and triploids (2n = 3x) and D. cneorum subsp. arbusculoides: (A) relative monoploid genome size [a.u.]; (B) absolute genome size [pg]; (C) pollen viability [%]; (D) number of flowers per inflorescences. GLMM-estimated mean values (dots) and their 95 % confidence intervals (error bars) are displayed. The estimates labelled with the same lowercase letters were not significantly different according to Tukey’s pairwise comparison, and vice-versa.

Figure 4.

Bayesian inference as recovered by MrBayes: (A) phylogeny based on sequences of the ITS1-5.8S-ITS2 region of nuclear ribosomal DNA; (B) phylogeny based on sequences of the ndhF-rpl32 region of chloroplast DNA. A combination of sequences generated within this study and those retrieved from GenBank were used in both phylogenies. Bayesian posterior probabilities are indicated near the branches. Population codes of samples generated in this study follow Supporting Information—Table S1.