Abstract
New chromosome observations are reported for 30 species and varieties from 4 genera of Ambrosieae: Ambrosia (including Franseria), Diceria, Hymenoclea and Iva. Neither polyploidy nor aneuploidy is known in the genera Dicorea, Hymenoclea or Xanthium. Aneuplcid reduction appears to have played a role in the genome evolution of several species of Iva and Ambrosia. Polyploid species occur in both Iva and Ambrosia and polyploid series exist for at least 5 species or species aggregates of the latter. All available evidence indicates that the primitive chromosome number for the tribe is x = 18, differentiation and speciation having occurred at this level, which is here termed diploid. The group, however, must ultimately have been of polyploid origin from forms with x = 9.
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