ON THE REPRODUCTIVE BIOLOGY OF THE INVASIVE ALIEN SPECIES ERIGERON ANNUUS (ASTERACEAE) IN THE BULGARIAN FLORA

Bulgarian accessions of the invasive alien species Erigeron annuus have been studied embryologically. The aim was to establish the mode of reproduction and reveal the features of the reproductive system which facilitate the successful dispersal and invasive behaviour of the species. In the male generative sphere the meiosis and microsporogenesis pass with various disturbances that lead to formation of morphologically unequal pollen. In the female generative sphere gametophytic apomixis - diplospory, from mitotic Antennaria- and meiotic Taraxacum-type - as well as residual sexuality were registered. Consequently, the embryo develops partenogenetically from an unreduced egg cell and the endosperm is autonomously formed or the embryo and endosperm develop after double fertilization. This reproductive system presupposes for both adaptation to different environmental conditions and good colonizing abilities, and together with the high seed set and efficient seed-dispersal mechanism, contribute to the invasive behaviour of the species.