Distribution of Sexual and Agamospermous Populations of Eupatorium (Compositae) in Asia

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<jats:p><jats:bold>Abstract</jats:bold> Examinations of chromosome number voucher specimens show that sexual diploid and agamospermous polyploid plants of <jats:italic>Eupatorium</jats:italic> in Asia can be distinguished by morphology, fertility, and stainability of pollen grains. Using these criteria, reproductive systems (sexual vs. agamospermous) were estimated for 558 herbarium specimens of <jats:italic>Eupatorium</jats:italic> in East Asia. Of 22 taxa examined, six included both sexual and agamospermous specimens; those of one taxon were all agamospermous; and those of 15 taxa were all sexual. This result shows that 21 taxa are not agamospermous microspecies but are differentiated at the diploid level. Sexual populations of most taxa had restricted geographical distributions. <jats:italic>Eupatorium chinense var. chinense</jats:italic> and var. <jats:italic>oppositifolium</jats:italic> consisted of both sexual and agamospermous populations. Sexual populations of these two varieties were allopatric and distinct in external morphology, suggesting differentiation at the species level. The specimens of <jats:italic>E. heterophyllum</jats:italic> s. str. were all agamospermous, while those of <jats:italic>E. mairel</jats:italic>, often treated as synonymous with <jats:italic>E. heterophyllum</jats:italic>, were sexual.</jats:p><jats:p>In comparing East Asian and North American <jats:italic>Eupatorium</jats:italic> for the distributional patterns of sexual and agamospermous populations, three differences can be pointed out: (1) Agamospermous plants of autopolyploid origin have evolved in eight species and are widespread in North America; while most agamospermous plants in East Asia may be of allopolyploid origin, have relatively restricted ranges, and are less frequent than the diploid plants with the exception of in <jats:italic>E. chinense var. oppositifolium</jats:italic>, (2) The number of sexual species with wide range is greater in North America than in East Asia. (3) The number of sexual species with restricted ranges is greater in East Asia than in North America. The results obtained suggest that speciation among the plants of <jats:italic>Eupatorium</jats:italic> has occurred more recently in East Asia than in North America.</jats:p>

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