Abstract
Within the agamic Pilosella complex, apomixis (asexual reproduction through seed) involves apospory, parthenogenesis, and autonomous endosperm development. Observations of reproductive biology in P. brzovecensis throughout four growing seasons in the garden have shown that both tetraploid and pentaploid plants of this species do not produce viable seeds and reproduce exclusively vegetatively by underground stolons. The reasons for the seed development failure were unknown, therefore our research focused on the analysis of reproductive events in the ovules of this taxon. We found that apospory was initiated in the ovules of both cytotypes. Multiple aposporous initial (AI) cells differentiated in close proximity to the megaspore mother cell (MMC) and suppressed megasporogenesis at the stage of early prophase I. However, none of the AI cells was able to further develop into a multi-nucleate aposporous embryo sac (AES) due to the inhibition of mitotic divisions. It was unusual that callose was accumulated in the walls of AI cells and its synthesis was most likely associated with a response to the dysfunction of these cells. Callose is regarded as the isolating factor and its surprising deposition in the ovules of P. brzovecensis may signal disruption of reproductive processes that cause premature termination of the aposporous development pathway and ultimately lead to ovule sterility. The results of our embryological analysis may be the basis for undertaking advanced molecular studies aimed at fully understanding of the causes of female sterility in P. brzovecensis.
Highlights
In Europe, Hieracium L. (Hieracium s. lat.) comprises two genera, Hieracium s. str. and Pilosella Vaill
The LOSS OF APOMEIOSIS (LOA) locus stimulates the differentiation of somatic aposporous initial (AI) cells close to sexually programmed cells and regulates mitotic development of unreduced aposporous embryo sac (AES), the LOSS OF PARTHENOGENESIS (LOP) locus is required for both autonomous embryo and endosperm development, and the AutE locus, which is genetically separable from LOA and parthenogenesis, controls autonomous endosperm formation (Catanach et al 2006; Henderson et al 2017; Koltunow et al 2011b; Ogawa et al 2013)
AI cells as well as young AESs may apparently compete for space and nutrients inside the ovules of P. brzovecensis, this competition does not appear to be a major cause of complete female sterility in light of previous observations of reproductive processes in other Pilosella apomicts
Summary
In Europe, Hieracium L. (Hieracium s. lat.) comprises two genera, Hieracium s. str. and Pilosella Vaill. Linear megaspore tetrads in the ovules of amphimictic Hieracium and Pilosella species. Both genera comprise polyploid complexes with a basic chromosome number x = 9. Str., diploids, triploids, tetraploids and very rare pentaploids are known (Musiał and Szeląg 2015, 2019). Pilosella shows a broad range of ploidy levels, i.e. from diploid to octoploid; its species are often represented by two or more cytotypes (Rotreklová et al 2002, 2005; Schuhwerk and Lippert 1997). Pilosella are rare and occur mainly in refugial regions of southern Europe (Merxmüller 1975) such as the Balkan Peninsula where diploid populations are consistently sought (Szeląg and Ilnicki 2011; Szeląg et al 2007).
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