Pollen Development in Cosmelieae (Ericaceae: Styphelioideae): Are Monads in Andersonia macranthera Unique?

Abstract

Styphelioideae (Ericaceae) is noted for producing tetrad pollen with various patterns of microspore abortion, and the existence of other pollen units such as polyads and true monads has been understudied. This study compared the development of putatively monadinous pollen in Andersonia macranthera (Cosmelieae) with that of permanent tetrads in other members of the tribe. Whole fresh pollen, anther squashes, and sectioned materials were used. Microsporogenesis was uniformly simultaneous. In species with permanent tetrads, the microspores remained attached along their contiguous, inner walls and developed as a single, synexinous unit. In contrast, in A. macranthera the microspores separated, and each developed into a pollen unit consisting of a single microgametophyte. Andersonia macranthera is confirmed as being the only member of the tribe with this pattern of pollen formation. Tetrads with varying numbers of sterile microgametophytes were observed in most taxa, lending credence to the suggestion that this feature is underreported among Styphelioideae. These observations strongly suggest that existing classifications of pollen type in epacrids require reconsideration.