Studies have shown that Acacia Mill. sensu lato consists of seven genera (Acacia sensu stricto, Acaciella Britton & Rose, Mariosousa Seigler & Ebinger, Senegalia Raf., and Vachellia Wight & Arnott) but does not constitute a monophyletic group. The Senegalia sensu lato clade, which corresponds to the genera Acaciella, Mariosousa, and Senegalia sensu stricto, likewise does not constitute a monophyletic group. The pollen morphology of a total of 22 species of Senegalia that occur in Brazil, and that of representatives of the genera Acaciella, Parasenegalia, Mariosousa, and Vachellia, is analyzed here. Pollen material was obtained from exsiccata deposited at some herbaria of Brazil. The pollen was acetolyzed and analyzed using both light and scanning electron microscopy. The dispersal units (polyads) were generally medium to large sized. All of the Senegalia species studied had apolar pollen grains organized in polyads comprising 16 pollen grains. The number of pollen grains in polyads is a synapomorphic character for recognizing those genera. Acaciella polyads have eight pollen grains; Parasenegalia and Mariosousa polyads have 16 pollen grains, and Vachellia polyads have more than 32 pollen grains that are irregularly arranged. The sexine and nexine had identical thickness in most of the species analyzed. Exine ornamentation varied from rugulate to rugulate-perforated to psilate-perforated.

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