Developmental evolution of flowering plant pollen tube cell walls: callose synthase (CalS) gene expression patterns

We investigated expression of CalS in pollen and pollen tubes of selected non-flowering seed plants (gymnosperms) and angiosperms within lineages that diverged below the monocot/eudicot node. First, we determined the nearly full length coding sequence of a CalS5 orthologue from Cabomba caroliniana (CcCalS5) (Nymphaeales). Semi-quantitative RT-PCR demonstrated low CcCalS5 expression within several vegetative tissues, but strong expression in mature pollen. CalS transcripts were detected in pollen tubes of several species within Nymphaeales and Austrobaileyales, and comparative analyses with a phylogenetically diverse group of sequenced genomes indicated homology to CalS5. We also report in silico evidence of a putative CalS5 orthologue from Amborella. Among gymnosperms, CalS5 transcripts were recovered from germinating pollen of Gnetum and Ginkgo, but a novel CalS paralog was instead amplified from germinating pollen of Pinus taeda.The finding that CalS5 is the predominant callose synthase in pollen tubes of both early-diverging and model system angiosperms is an indicator of the homology of their novel callosic pollen tube walls and callose plugs. The data suggest that CalS5 had transient expression and pollen-specific functions in early seed plants and was then recruited to novel expression patterns and functions within pollen tube walls in an ancestor of extant angiosperms.The pollen tube is a unique feature of male gametophytes of seed plants. In cycads and Ginkgo, pollen tubes are long-lived and function solely as haustorial, highly branched structures that grow invasively into female tissues [1-3]. In conifers and Gnetales pollen tubes function in a new way to deliver non-motile sperm to the egg (siphonogamy), while generally retaining a haustorial growth pattern [2,3]. Flowering plant (angiosperm) pollen tubes have lost most features of haustorial growth - their pollen tubes are typically short-lived and seem to function exclusively to deliver sperm to the egg