No abstract

The representative sampling confirmed previous data on cytotype distribution, i.e. the spatial aggregation of mixed-ploidy populations in south-west Moravia and Lower Austria and the predominance of ploidy-uniform populations in other parts of the area investigated. Recurrent origin of polyploids from diploid progenitors via unreduced gametes and their successful establishment are considered the key factors promoting intrapopulational ploidy mixture ('primary hybrid zones'). As an alternative to the generally accepted theory of cytotype co-existence based on the development of different means of inter-ploidy reproductive isolation, it is suggested that a long-term ploidy mixture can also be maintained in free-mating populations provided that the polyploids originate with a sufficient frequency. In addition, the prevalence (or subdominance) of the triploid cytotype in several mixed-ploidy populations represents the first evidence of such a phenomenon in plant systems with exclusively sexual reproduction.

A morphological survey and assessment of genetic diversity using amplified fragment length polymorphism (AFLP) were applied to study the variability of two agamospermous species of Hieracium section Cernua R. Uechtr. characterized by disjunct, subalpine distributions: H. silesiacum (Western Carpathians and Eastern Sudetes) and H. vierhapperi (Western Carpathians and Eastern Alps). Contrasting patterns were revealed. In H. silesiacum, no genetic or morphological differences were found between its remote populations from the Sudetes and the Tatry mountains, suggesting a recent dispersal. In contrast, distinct morphological differentiation of population in the Nízke Tatry mountains was confirmed by high genetic differentiation; the analysis indicated independent origin of this population and gave grounds for taxonomic separation and description of a new species, H. austrotatricum. In H. vierhapperi, genetic differentiation and slight morphological difference (colour of achenes) was observed between disjunct populations from the Carpathians and the Alps. The data suggest long-term isolation of these populations resulting in pronounced independent history. High number of markers monomorphic for all populations supports the existing hypothesis of common sexual ancestors from H. section Cernua involved in the origin of these polyploid taxa. No or only minor morphological differentiation within the taxa indicates high stability of apomictic populations of Hieracium.

Members of the genera Hieracium and Pilosella are model plants that are used to study the mechanisms of apomixis. In order to have a proper understanding of apomixis, knowledge about the relationship between the maternal tissue and the gametophyte is needed. In the genus Pilosella, previous authors have described the specific process of the “liquefaction” of the integument cells that surround the embryo sac. However, these observations were based on data only at the light microscopy level. The main aim of our paper was to investigate the changes in the integument cells at the ultrastructural level in Pilosella officinarum and Hieracium alpinum. We found that the integument peri-endothelial zone in both species consisted of mucilage cells. The mucilage was deposited as a thick layer between the plasma membrane and the cell wall. The mucilage pushed the protoplast to the centre of the cell, and cytoplasmic bridges connected the protoplast to the plasmodesmata through the mucilage layers. Moreover, an elongation of the plasmodesmata was observed in the mucilage cells. The protoplasts had an irregular shape and were finally degenerated. After the cell wall breakdown of the mucilage cells, lysigenous cavities that were filled with mucilage were formed.Electronic supplementary materialThe online version of this article (doi:10.1007/s00709-017-1120-1) contains supplementary material, which is available to authorized users.