EVOLUTION OF POLLEN AND TAPETAL CHARACTERS IN OCHNACEAE (MALPIGHIALES)

摘要

Ochnaceae are members of the recently recircumscribed and extremely diverse rosid order Malpighiales. They consist of three lineages, Ochnoideae, Quiinoideae, and Medusagyne oppositifolia (jellyfish tree), a critically endangered endemic from the Seychelles. However, relationships among these lineages are unclear, as are relationships between Ochnaceae and other Malpighiales. In this article, pollen and tapetum characters are evaluated in relation to recent hypotheses of relationships within Ochnaceae and between Ochnaceae and their putative relatives, with a focus on the remarkable pollen apertures with cone-like exine protrusions in Medusagyne. The comparative morphology and structure of pollen and tapetum are examined using TEM in selected Chrysobalanaceae (Chrysobalanus), Euphroniaceae (Euphronia), and Ochnaceae (Cespedesia, Medusagyne, Ochna, and Touroulia). Pollen and tapetum characters in Ochnaceae and selected outgroups are optimized onto a recent molecular phylogeny. The optimizations indicate that pollen morphological and structural characters are potentially informative in elucidating hypotheses of relationship within Ochnaceae. Very small pollen (E = 11-30 μm) is a synapomorphy for Ochnaceae, including Medusagyne, and is possibly related to "buzz" pollination in many taxa. Striate pollen could indicate a relationship between Medusagyne and Ochnoideae. Apertures with compound pores and large atria, apomorphic for Medusagyne, suggest a closer relationship with Ochnaceae with costate apertures and may have a protective function. The optimizations also indicate that clusioids and a clade of five families (Balanopaceae-Trigoniaceae-Dichapetalaceae-Euphroniaceae-Chrysobalanaceae) are more closely related to each other than to Ochnaceae. Ochnaceae pollen has probably become specialized in various ways, in response to environmental conditions and pollinators.