Pollen Evolution and Development in Ericaceae, with Particular Reference to Pseudomonads and Variable Pollen Sterility in Styphelioideae

Abstract

Many epacrids (Ericales: Ericaceae: Styphelioideae) possess the highly unusual character of variable sterility of some members of the pollen tetrad, producing tetrads, triads, dyads, and monads. In Leucopogon and Styphelia (Styphelioideae: Styphelieae), remarkably, three of the four cells produced by meiosis regularly degenerate. In this article, pollen structure was examined using TEM in nine species of Styphelioideae from five different tribes, together with pollen development in Leucopogon parviflorus. Both living and rehydrated herbarium material was used. Data on pollen units in Ericaceae, on the basis of these original observations and the results of an extensive literature survey, were optimized on an rbcL phylogeny, and a hypothesis for pollen evolution in Ericaceae is proposed. Monad pollen is plesiomorphic in Ericaceae, and pollen dispersed in permanent tetrahedral tetrads has evolved from this condition, possibly because of mutations in genes controlling callose deposition and the degradation of the microsporocyte primary cell wall. Tetrads with variable pollen sterility have arisen several times, mostly in Styphelioideae, possibly due to the action of lethal genes, and are ancestral to pseudomonads. Tetrads with nuclear migration followed by asymmetric cytokinesis occur in the later‐branching genera Leucopogon and Styphelia, where three of the four meiotic products regularly die, an example of programmed cell death, forming pseudomonads. There are striking similarities between programmed cell death in microsporogenesis in Styphelioideae and in the sedge family Cyperaceae (Poales) and in the formation of the seed plant megaspore.