Embryology of Taraxacum atricapillum Sonck (section Borea) from Poland

Celem niniejszych badań było ustalenie liczby chromosomów oraz badania embriologiczne Taraxacum atricapillum Sonck (sekcja Borea). Przeprowadzona analiza kariologiczna wskazuje, że T. atricapillum jest gatunkiem triploidalnym (2n=3x=24).Jednokomorowa zalążnia T. atricapillum zawiera anatropowy, cienkośrodkowy zalążek z jednym integumentem. Tapetum integumentalne wykształca się w stadium komórki macierzystej megaspor. W starszych zalążkach wokół endotelium występuje strefa grubościennych komórek integumentalnych, która prawdopodobnie uczestniczy w odżywianiu dojrzałego gametofitu żeńskiego oraz prazarodka.W procesie rozmnażania T. atricapillum stwierdzono występowanie diplosporii typu Taraxacum, partenogenezy oraz autonomicznego formowania endospermy. Niezredukowane woreczki zalążkowe rozwijały się z chalazalnej komórki diplodiady na drodze trzech podziałów mitotycznych. W kwiatach zamkniętych koszyczków obserwowano zarodki globularne, dwie synergidy oraz komórki endospermy.Analiza pylników wykazała zaburzenia mikrosporogenezy, zwłaszcza pierwszego podziału mejotycznego oraz anomalie w strukturze tapetum pylnikowego, którego komórki ulegały nadmiernej wakuolizacji. Obserwowane zakłócenia były przyczyną degeneracji zawartości woreczków pyłkowych już w stadium tetrad mikrospor. Nietypowa struktura tapetum pylnikowego może wskazywać, że męska sterylność T. atricapillum może być warunkowana czynnikami cytoplazmatycznymi.The aim of this study was to determine the chromosome number as well as to conduct an embryological investigation of Taraxacum atricapillum Sonck (section Borea). The examined seedlings showed triploid (2n=3x=24) chromosome number. An unilocular ovary of T. atricapillum contains an anatropous, tenuinucellate and unitegmic ovule. The integumentary tapetum develops at the stage of megaspore mother cell. In older ovules, a zone of thick-walled cells occurs around the endothelium. Probably, this zone participates in the nutrition of the mature female gametophyte and the proembryo.Diplospory, parthenogenesis, and autonomous endosperm formation were observed during the reproduction of T. atricapillum. Unreduced embryo sacs developed from the chalazal cell of the diplodyad after three mitotic divisions. Globular embryos, two synergids, and endosperm cells were observed in flowers of the closed capitula.The analysis of anthers showed disturbances in microsporogenesis, especially during first meiotic division, as well as anomalies in anther tapetum structure, whose cells were strongly vacuolated. Observed disturbances caused degeneration within the pollen sacs content already at the tetrad stage. An unusual anther tapetum structure may indicate that the male sterility of T. atricapillum can be conditioned by cytoplasmic factors

Deposition of callose in young ovules of two Taraxacum species varying in the mode of reproduction

Although callose occurs during megasporogenesis in most flowering plants, the knowledge about its general function and the mechanisms by which the callose layer is formed in particular places is still not sufficient. The results of previous studies suggest a total lack of callose in the ovules of diplosporous plants in which meiosis is omitted or disturbed. This report is the first documentation of callose events in dandelions ovules. We demonstrated the pattern of callose deposition during the formation of megaspores through diplospory of Taraxacum type and during normal meiotic megasporogenesis in apomictic triploid Taraxacum atricapillum and amphimictic diploid Taraxacum linearisquameum. We found the presence of callose in the megasporocyte wall of both diplosporous and sexual dandelions. However, in a diplosporous dandelion, callose predominated at the micropylar pole of megaspore mother cell (MMC) which may be correlated with abnormal asynaptic meiosis and may indicate diplospory of the Taraxacum type. After meiotic division, callose is mainly deposited in the walls between megaspores in tetrads and in diplodyads. In subsequent stages, callose gradually disappears around the chalazal functional megaspore. However, some variations in the pattern of callose deposition within tetrad may reflect variable positioning of the functional megaspore (FM) observed in the ovules of T. linearisquameum