Pollen ultrastructure and systematic affinities ofAnaxagorea (Annonaceae)

Abstract

LM, SEM and TEM observations document similar palynological features for paleo- and neotropical species ofAnaxagorea. Pollen grains are solitary, heteropolar, ellipsoidal to globose, and broadly sulcate. The exine consists of a smooth and microperforated tectum, a granular to “protocolumellar” infratectal layer, and sometimes an initial, not foliated basal layer. The intine is stratified and tends to extrude with its considerably swelling alveolarfibrillar outer layer through the aperture, already inside the unopened pollen sacs. This appears as a new and irreversible harmomegathic mechanism, limited toAnaxagorea, Meiocarpidium and possibly otherAnnonaceae. These palynological data are discussed and coordinated with those from gross-morphological, anatomical, karyological, phytochemical and other studies. All this offers convincing proof for the sometimes questioned systematic and phylogenetic coherence of the widely disjunct S. & C. American and SE. Asian members ofAnaxagorea. Furthermore, a broad comparison with otherAnnonaceae genera demonstrates somewhat closer links ofAnaxagorea with several small and relictual African genera, i.e.Meiocarpidium andPolyceratocarpus, but alsoLettowianthus andPiptostigma;Cleistopholis andAmbavia are more isolated. These genera which exhibit many “primitive” features are provisionally arranged under “tribe I” of the so-called “Fusaea subfamily”. There are loose affinities with “tribe II”, an assembly of more advanced and widespread, often proliferating generic groups, including the S. AsianCananga, the pantropicalXylopia (withPseudannona from Mauritius), a possible link toAnaxagorea and “tribe I”, and the S. AsianGoniothalamus (with the AustralasianRichella s. str. and the W. AfricanNeostenanthera andBoutiquea); the neotropicalFusaea andDuckeanthus apparently are more remote. Because of relationships to several otherAnnonaceae groups and difficulties with its circumscription and definition, the maintainance of the “Fusaea subfamily” as a whole appears questionable.