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Free Content Distribution of orbicules in Annonaceae mirrors evolutionary trend in angiosperms

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Background and aims – Orbicules or Ubisch bodies have been recorded in many angiosperm families and although the first observations date back to 1865, their function in the anther remains enigmatic. In flowering plants a general evolutionary trend is observed from common occurrence of orbicules in early diverging lineages towards a more patchy distribution in derived clades of eudicots. Annonaceae was our family of choice for an in depth study of orbicule distribution in early diverging angiosperms since it met the following three criteria: (1) high tapetum diversity, (2) orbicule presence and absence recorded, and (3) recent phylogeny at genus level available.

Key results – Our SEM data of eighteen species show that orbicules are more common in Annonaceae than previously perceived. The resulting orbicule distribution pattern on the family topology indicates a consistent absence of orbicules in the 'long branch clade' while orbicules are present in Anaxagorea, the ambavioids, and the 'short branch clade'. Presence of orbicules is the ancestral condition in the family. Morphologically, Annonaceae orbicules are small (< 1 μm), spherical and smooth.

Conclusions – The orbicule distribution pattern in Annonaceae reflects the general evolutionary trend in flowering plants. We hypothesize that orbicule presence can be considered as a powerful proxy for non-amoeboid tapetum characterization in Annonaceae.

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Keywords: ANNONACEAE; ORBICULES; PALYNOLOGY; SPOROPOLLENIN; STARCH GRANULES; TAPETUM; UBISCH BODIES

Document Type: Research Article

Publication date: July 1, 2010

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