Evolution of Agamospermous Races in Boehmeria and Eupatorium

Abstract Recent progress in research on the evolution of agamospermy in Boehmeria (Urticaceae) and Eupatorium (Asteraceae) is reviewed. Specific questions addressed are: (1) Is there any trend in distributions and habitat preferences of sexual and agamospermous races? (2) Is hybridity or a high level of heterozygosity a prerequisite for the evolution of agamospermous races? (3) Do agamospermous plants produce genetically variable offsprings through hybridization with sexual relatives? and (4) Do agamospermous plants have the theoretically predicted reproductive advantage over sexuals? From the results presented, the following conclusions can be drawn. (1) Sexual and agamospermous relatives are usually ecologically differentiated; sexuals tend to require extreme habitats, while agamosperms prefer disturbed places. (2) Hybridity or a high level of heterozygosity is not a prerequisite to the evolution of agamospermous races. (3) Agamospermous races produce genetically variable offspring through hybridization with sexual relatives and subsequent segregation. (4) Agamospermous plants have a reproductive advantage often greater than twofold; part of this advantage is attributed to polyploidy. The question of why agamospermous races do not replace sexuals remains open. Male recovery in agamosperms through hybridization and/or the disadvantage of polyploidy associated with agamospermy might work as mechanism(s) enabling sexuals to resist colonization of reproductively superior agamosperms under some extreme ecological conditions.