Reproductive pathways inHieracium s.s.(Asteraceae): strict sexuality in diploids and apomixis in polyploids

Abstract

The mode of reproduction in Hieracium s.s. is tightly associated with ploidy. While diploids produce seeds exclusively sexually, polyploids produce seeds by obligate or almost obligate apomixis. Strict apomixis can increase the reproductive assurance and this in turn can increase the colonization ability of apomicts. Nevertheless, our data clearly show that certain polyploid plants are still able to reproduce sexually and contribute to the formation of new cytotypes and genotypes. The finding of functional facultative apomicts is essential for future studies focused on evolution, inheritance and ecological significance of apomixis in this genus.

An unexpected new diploid Hieracium from Europe: Integrative taxonomic approach with a phylogeny of diploid Hieracium taxa

“…is expressed solely at the polyploid level. This pattern has recently been found in large‐scale flow cytometric studies of seed progeny of diploid and polyploid taxa (Mráz & Zdvořák, ; Mráz & al., ). In the same line of evidence, published (Mráz & Paule, ) and unpublished data (Pinc & al., unpub.)…”

“…Since H. telekianum reproduces apomictically (Mráz & Szeląg, ; Mráz & Zdvořák, ), we suppose that allopolyploidization has triggered an important shift from sexual to asexual reproduction in this species. At the current stage of knowledge, it is unclear, however, what is the exact role of hybridization and polyploidization in this process.…”

“…In contrast to prevailing polyploids, only ca. 25 diploid species are currently known in this genus (Chrtek & al., ; Mráz & Zdvořák, ). The majority of diploid taxa have often restricted ranges, which are concentrated in mountainous regions of SW and SE Europe (Merxmüller, ; Chrtek & al., ).…”

“…Importantly, ploidy level is tightly associated with the mode of reproduction in this genus (Mráz & Zdvořák, ). Hieracium polyploids reproduce strictly apomictically, although in seven plants from 685 analysed in total, a small amount of functional sexuality has been found using flow cytometric seed screening (Mráz & Zdvořák, ). A high level of obligate apomixis and relatively rare residual sexuality was revealed also cytoembryologically (Hand & al., ).…”

“…A high level of obligate apomixis and relatively rare residual sexuality was revealed also cytoembryologically (Hand & al., ). Apomictic seeds produced by Hieracium polyploids are formed from mitotically derived unreduced embryo sacs, which directly develop into embryo and endosperm without fertilization of egg cells and central nuclei (autonomous diplosporic apomixis of the Antennaria type, Bergman, ; Hand & al., ; Mráz & Zdvořák, ). On the contrary, diploids are strict outcrossers with a sporophytic self‐incompatibility system (Chrtek, ; Mráz, ) and produce seeds after double fertilization of reduced embryo sacs (e.g., Bergman, ; Gustafsson, ; Hand & al., ; Mráz & Zdvořák, ).…”