Egg Apparatus in Sexual and Apomictic Species of Taraxacum: Structural and Immunocytochemical Aspects of Synergid Cells

Abstract

The paper reports a comparative study of the female gametophyte and especially synergid structure in sexual and apomictic dandelions. We analyzed diploid sexually reproducing Taraxacum linearisquameum (2n = 2x = 16) and two triploids, T. alatum and T. udum (2n = 3x =24), with autonomous embryo and endosperm development. There were no observed differences in the organization of the mature megagametophyte between the examined species. Both meiotically reduced and diplosporous embryo sacs showed typical polarity of the egg apparatus cells, together with development of a filiform apparatus in the synergids, but immunocytochemical analyses indicated that microtubules form longitudinal brush-like bundles adjacent to the filiform apparatus in the synergids of the sexual T. linearisquameum. This arrangement of cytoskeletal elements is similar to the configuration described in other amphimictic plants. The synergids of the apomictic T. alatum and T. udum show a uncharacteristic and relatively weak cytoskeleton with no brush-like bundles. We discuss the role of synergids in autonomous apomicts.K Ke ey y w wo or rd ds s: : Apomixis, female gametophyte, synergid cell, filiform apparatus, microtubular cytoskeleton, Taraxacum.ACTA BIOLOGICA CRACOVIENSIA Series Botanica 55

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“…Together with the Poaceae and the Rosaceae, it is one of the three angiosperm families in which apomixis is relatively common (Richards, 2003). In addition, special modifications of the integumentary tissue and the differentiation of periendothelial nutrient tissue have been described in mature ovules of several Asteraceae, including species of Bellis, Chondrilla, Galinsoga, Helianthus, Hieracium, Onopordum, Pilosella, Solidago, and Taraxacum (Cooper and Brink, 1949;Engell and Petersen, 1977;Koltunow et al, 1998;Musiał and Kościńska-Pająk, 2013;Kolczyk et al, 2014Kolczyk et al, , 2016Płachno et al, 2017). In the case of Taraxacum, cells have extremely thickened walls with a spongy structure in the peri-endothelial zone , while in Hieracium and Pilosella the process of gelatinization takes place in this zone; it is associated with the formation of mucus-filled lysogenic cavities (Koltunow et al, 1998;Płachno et al, 2017).…”

Exploring the sister cells of embryo sac: developmental and functional attributes

“…Interestingly, Arabidopsis can reverse polytubey block when the egg cell or central cell remain unfertilized, allowing the second pollen tube to recover the early fertilization failure (Beale et al, 2012;Kasahara et al, 2012;Maruyama et al, 2015). The synergids in apomictics and non-apomictics are also being examined to further elucidate their role in syngamy as well as double fertilization (Musiał and Kościńska-Pająk, 2013). Synergids show some anomalies and subcellular structures that are not characteristic of amphimictics (Lawit et al, 2004).…”