Polyploidy in Ambrosia dumosa (Compositae: Ambrosieae)

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Abstract
Ambrosia dumosa (A. Gray ex Torr.) Payne is one of the most abundant shrubs of the Sonoran Desert. Allopatric and sympatric populations of this species frequently exhibit distinctive morphological features, particularly with regard to leaf-segment width, but also in blooming time and ecology. Chromosome numbers ofn = 18, 36, and 54 were frequently encountered in our preliminary investigations ofA. dumosa. Fifty-six chromosome number determinations have now accumulated for this taxon, and these additional counts confirm the existence of three major chromosome races withinA. dumosa. Diploids generally occur throughout the range of the species; hexaploids tend to be confined to the deserts of California; and tetraploids occur through most of the range of the species, with the exception of southern Baja California and Owens Valley, California. External morphology was found to be correlated with chromosome number only locally, but even in many local areas, morphological intergradation occurs between different chromosome races. It is therefore considered impractical to attempt a formal taxonomy which would recognize these chromosomal races.

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