Diploidy, polyploidy, and agamospermy among species of Eupatorium (Compositae)

Abstract

Diploids of eight species of Eupatorium, triploids of three species, and tetraploids of four species are reported here for the first time. Meiosisin microsporocytes of triploids and tetraploids either failed to occur resulting in failure of pollen production or less frequently resulted in either incomplete chromosomal pairing or complete asynapsis; the latter two events lead to the production of grossly malformed, abortive pollen with unevenly deposited walls. Megasporogenesis examined in two polyploid plants led to the formation of eight-nucleate, unreduced embryo sacs which formed endosperm and embryos without pollination, indicating that the sporophytes develop asexually through agamospermy. Evidence is given that this may be a facultative characteristic. Megasporogenesis, studied in diploids of four species, produced normal embryo sacs with no indication of agamospermous development. Diploid plants of 11 of 12 species were tested and found to be self-incompatible.Since diploids could readily be distinguished from polyploids by pollen production (diploids produced normal pollen, polyploids produced none or malformed pollen), distributions of a large number of diploid, triploid, and tetraploid individuals could be determined from examining anthers of herbarium specimens. Distribution maps were made using these data as well as those from chromosome counts. Diploid members of species having both diploid, triploid, and (or) tetraploid members consistently had smaller distributional ranges than polyploids of the same species.