Evolutionary significance of exine ultrastructure in the subfamily Barnadesioideae (Asteraceae) in the light of molecular phylogenetics

TELLERÍA, M.C.; PALAZZESI, L; BARREDA, V. D.

REVIEW OF PALAEOBOTANY AND PALYNOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2015 vol. 221 p. 32 - 46

0034-6667

Barnadesioideae (94 species) is the sister subfamily to the rest of the Asteraceae (23,000 species). Pollen grains inthis subfamily are structurally and sculpturally distinctive and diverse. Although pollen morphology has contributedto the taxonomy of the subfamily, there is a gap of knowledge concerning the evolution of the exine structure.This study aims at exploring the systematic and phylogenetic significance of optimizing selected pollencharacters of Barnadesioideae on the latest molecular phylogenetic tree. Transmission electron microscope(TEM) observations on pollen of selected species, some of them never explored so far, show that the exine probablyevolved from a thin pattern (ca. 1?3 μm), with a well-developed foot layer and solid and free columellae,present in sister family Calyceraceae, towards a thicker (N6?11 μm) and a more complex columellategranulatebilayered exine in Barnadesioideae (with very delicate columellae). The particular exine structure observedin the monotypic Schlechtendalia luzulaefolia, which combines compact and independent columellae(common in more derived Asteraceae) with a granular internal tectum as the inner ectexine layer (as inBarnadesioideae), reinforces its distant phylogenetic position within Barnadesioideae. More derived lineageswithin Asteraceae (e.g. Mutisioideae) retained some ancestral exine features although evolved an even thickerexine and a columellate trilayered exine (with robust columellae), rare in the angiosperm pollen grains.