Embryology and cytogenetics of Eupatorium pauciflorum and E-intermedium (Compositae)

The embryology of Eupatorium pauciflorum indicates diplospory with autonomo us endosperm development. The embryo sac is of the polygonum type and the p olar nuclei mostly fuse before anthesis. The occurrence of precocious embry o and endosperm development in unopened florets, and the total absence of g erminated pollen grains on exposed stigmas, as well as the absence of polle n tubes in the ovules, indicate agamospermy to be obligate and embryo and e ndosperm development autonomous. The study of microsporogenesis revealed th e total absence of pollen production in consequence of microsporocyte degen eration before the onset of meiosis, which resulted in absolute male steril ity. E. pauciflorum was demonstrated to be an autotriploid with a basic set of 10 chromosomes, each represented three times. Embryological studies sho wed E. intermedium to undergo reductive meiosis with tetrad formation durin g megasporogenesis, followed by monosporic embryo sac development of the po lygonum type. The polar nuclei fuse before anthesis. The egg cell invariabl y attains anthesis still undivided, without precocious embryony. Meiosis of microsporogenesis results in the regular formation of 10 bivalents and the subsequent stages of microsporogenesis are normal. Stigmatic loads indicat e the regular occurrence of pollination with viable, functional grains. Kar yotypic studies revealed a complement of 20 chromosomes separable into 10 p airs. It is concluded that E, pauciflorum, as represented by the material s tudied, is apomictic while E. intermedium is sexual.