Can obligate apomixis and more stable reproductive assurance explain the distributional successes of asexual triploids in Hieracium alpinum (Asteraceae)?

Abstract

Although reproductive assurance has been suggested to be one of the most important factors shaping the differential distributional patterns between sexuals and asexuals (geographic parthenogenesis), it has only rarely been studied in natural populations of vascular plants with autonomous apomixis. Moreover, there are almost no data concerning the putative relationship between the level of apomictic versus sexual plant reproduction on one hand, and reproductive assurance on the other. We assessed the level of sexual versus apomictic reproduction in diploid and triploid plants of Hieracium alpinum across its distributional range using flow cytometric analyses of seeds, and compared the level of potential and realized seed set, i.e. reproductive assurance, between the two cytotypes under field and greenhouse conditions. Flow cytometric screening of embryos and endosperms of more than 4,100 seeds showed that diploids produced solely diploid progeny sexually, while triploids produced triploid progeny by obligate apomixis. Potential fruit set was much the same in diploids and triploids from the field and the greenhouse experiment. While in the pollination‐limited environment in the greenhouse apomictic triploids had considerably higher realized fruit set than sexual diploids, there was no significant difference between cytotypes under natural conditions. In addition, sexuals varied to a significantly larger extent in realized fruit set than asexuals under both natural and greenhouse conditions. Our results indicate that triploid plants reproduce by obligate apomixis, assuring more stable and predictable fruit reproduction when compared to sexual diploids. This advantage could provide apomictic triploids with a superior colonisation ability, mirrored in a strong geographic parthenogenesis pattern observed in this species.

Breeding system of diploid sexuals within theRanunculus auricomuscomplex and its role in a geographical parthenogenesis scenario

“…Comprehensive population genetic studies on GP scenarios, however, either did not observe the respective genetic pattern (e.g., Cosendai et al., 2013) or found the colonization history and decline of genetic diversity in marginal sexual populations as less relevant for the GP pattern (e.g., Nardi et al., 2020). On the contrary, apomictic lineages can benefit from uniparental reproduction either via pollen‐independent (autonomous) apomixis (Mráz et al., 2018) or via pseudogamy and self‐compatibility (Hörandl, 2010) without negative effects of inbreeding. However, little is known about the effects of breeding systems, inbreeding, and fitness of sexual species in geographical parthenogenesis scenarios.…”

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.)…”

Greater reproductive assurance of asexual plant compared with sexual relative in a low‐density sympatric population: Experimental evidence for pollen limitation

“…Similarly, apomicts of Pilosella officinarum showed marginally higher fecundity when compared to sexuals (Sailer et al., 2020). By contrast, no differences in seed set were found between two reproductive forms in Antennaria parlinii (Asteraceae, O’Connell & Eckert, 1999) and Hieracium alpinum (Asteraceae, Mráz et al., 2019). In the two latter studies, allopatrically distributed asexual and sexual populations were studied and found high level of between‐population heterogeneity in seed sets within both apomicts and sexuals.…”

“…Therefore, nonautonomous seed formation, decreased seed sets and residual sexuality are likely the most important factors, why GP patterns are substantially rarer in pseudogamous apomicts when compared to autonomous apomicts (Hörandl et al., 2008). Surprisingly, given its ecological importance, the advantage of uniparental reproduction has rarely been quantified in autonomous apomicts (Mráz et al., 2019).…”

“…In the two latter studies, allopatrically distributed asexual and sexual populations were studied and found high level of between‐population heterogeneity in seed sets within both apomicts and sexuals. This variation was probably triggered by the considerable among‐population differences in environmental factors along huge latitudinal and longitudinal gradients (Mráz et al., 2019). Unfavourable weather and habitat conditions, which are frequent factors of seed abortion in angiosperms (Stephenson, 1981; Albert et al, 2001), were likely to have caused ca.…”