Abstract
This study represents the first report on the embryological characteristics of triploid male-sterile dandelion Taraxacum belorussicum (section Palustria) from Poland. While this taxon is considered to be a male-sterile species, we found that the investigated individuals produced pollen. Irregular tetrads, triads and diads with microspores of unequal size were observed in the pollen loculi as a result of disturbed meiotic division, while anthers’ tapetum did not show structural disorders. Possible reasons for the plasticity in the expression of male sterility, as well as the role of pollen in apomicts, are discussed. Flowers of the examined individuals contained well-developed nectaries. The course of embryological processes in the ovules indicated an apomictic mode of reproduction in T. belorussicum. We observed meiotic diplospory of the Taraxacum type, in which first meiotic division starts but results in nuclear restitution, while undisturbed second meiotic division gives rise to a dyad of unreduced megaspores (diplodyad). After three mitotic divisions of the chalazal megaspore, a seven-celled unreduced female gametophyte developed. The features of ovule anatomy and characteristics of a mature female gametophyte corresponded to these described in sexually reproducing dandelions.
Highlights
Taraxacum Wigg. belonging to the subfamily Cichorioideae of the Asteraceae is a large cosmopolitan genus comprising approximately 3000 recognized species grouped into about 60 sections (Battjes et al 1992; Kirschner et al 2015)
It is well known that the seeds of apomictic dandelions develop completely independent of the pollen, as a result of meiotic diplospory, parthenogenesis, and autonomous endosperm formation (Gustafsson 1946; Richards 1973)
The inflorescence of T. belorussicum consists of only ligulate florets, which is typical of the Cichorioideae
Summary
Taraxacum Wigg. (dandelion) belonging to the subfamily Cichorioideae of the Asteraceae is a large cosmopolitan genus comprising approximately 3000 recognized species grouped into about 60 sections (Battjes et al 1992; Kirschner et al 2015). In the sexual pathway of reproduction, regular meiosis and double fertilization are indispensable for seeds formation, whereas these processes are omitted during seeds’ production in agamospermous dandelions, resulting in offspring that are genetically identical to the mother plant For this reason, the genus Taraxacum is often used as a model system in research on molecular mechanisms underlying apomixis (Vijverberg et al 2010 and references therein), and for comparative embryological analysis of the sexual and apomictic reproductive modes (Małecka 1971a, 1973; Van Baarlen et al 2000, 2002; Musiał and Koscinska-Pajak 2013; Musiał et al 2013b, 2015; Płachno et al 2014, 2015). Crossing experiments and genetic analyses revealed that in Taraxacum these basic elements of apomictic developmental pathway are controlled by three dominant loci, and that diplospory, as well as parthenogenesis, are inherited independently, while the inheritance of autonomous endosperm formation is still not fully settled (Van Dijk et al 1999, 2003; Van Dijk and BakxSchotmann 2004; Vijverberg et al 2004, 2010)
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