Plesiomorphic and apomorphic pollen structure characteristics of anthemideae (Asteroideae: Asteraceae)

Anthemideae (Asteroideae: Asteraceae) pollen grains have basal columellae, a structural type called “anthemoid” in earlier publications. To survey structure variation in Anthemideae pollen, we examined freeze‐sectioned grains from 45 species within 23 representative genera using scanning electron microscopy (SEM). From resulting data and a literature review, we concluded that: 1) pollen of Anthemideae taxa is qualitatively identical except for Ursinia (grains essentially lack basal columellae) and the Artemisia group (branches of basal columellae are complex and interwoven); 2) the double tectum (a term introduced in this study) is a synapomorphy of Asteroideae and plesiomorphic in Anthemideae; 3) apomorphies of Anthemideae grains include large basal columellae, a thick foot layer, and absence of internal foramina; and 4) Anthemideae pollen is qualitatively different from similar pollen in Lactucoideae, a distinction we recognized by restricting “anthemoid” to Anthemideae grains. Ursinia grains have occasional basal columellae and features resembling rolled‐up columellae; we consider these vestiges of a reversal to the plesiomorphic condition. To assess quantitative structural variation, 2,200 image‐analysis measurements were taken from 73 SEM micrographs. Intrageneric variation was analyzed by standard deviation, and intergeneric variation by principal components analysis. Compared to other Anthemideae taxa, the structural elements of Artemisia grains have reduced dimensions and variability. Otherwise, structural radiation of Anthemideae pollen has produced a phenetic continuum.