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Embryology and cytogenetics of apomictic hexaploid Eupatorium odoratum L. (Compositae)

James R. Coleman

Departamento de Botânica, Instituto de Biociências, Letras e Ciências Exatas, UNESP, 15055 São José do Rio Preto, SP, Brasil

ABSTRACT

The embryology of Eupatorium odoratum indicates it to have Antennaria type diplospory with autonomous development of the endosperm and pronounced precocious embryony. The embryo sac is of the Polygonum type and the polar nuclei do not fuse. Both polar nuclei apparently contribute to the formation of the endosperm. It is estimated that about 20% of the ovules reach anthesis with the egg undivided and available for sexual reproduction. Pollen production is extremely reduced as a consequence of the degeneration of the sporogenous tissue prior to meiosis. Microsporocytes at diakinesis showed univalents, bivalents and trivalents, although in some cells pairing was highly reduced. E. odoratum is demonstrated to be an autohexaploid with a basic set of ten chromosomes, each represented six times. It is suggested that apomictic hexaploid E. odoratum originated from hybridization between facultatively apomictic triploid populations which were previously isolated and adaptively differentiated, in accordance with Stebbins secondary contact hypothesis.

Keywords: Embryology; Cytogenetics; Hexaploid; Eupatorium odoratum L.

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