Genome size can affect the phenotype of plants by a simple physical effect of the DNA material at the cellular level. Pollen contains the bare necessities to initiate and sustain pollen tube growth and carries the haploid genome. This work investigates the extent to which the nuclear DNA content affects pollen size in an evolutionary context within Streptocarpus (Gesneriaceae), by correlating genome size with pollen size of 38 samples representing 36 taxa in a phylogenetic framework. Streptocarpus was found to possess an average genome size among diploid species of 0.82 pg (1C). Significant genome downsizing of up to 44.4% was observed among the polyploid species which are exclusively found in Madagascar. The pollen size ranged between 11.27 μm and 25.55 μm at the diploid level, but 1C values were not found to drive pollen size. On the other hand, 1C values in most polyploids showed a strong positive correlation with pollen size, near linear in species of sect. Parasaintpaulia. In a phylogenetic context, polyploidy has evolved at least twice in the genus, and contrary to pollen size, genome size was strongly lineage-specific rather than adaptive in Streptocarpus. Repeated parallel increases and decreases in genome size (1C, and 1Cx) during the evolution of the genus were inferred. Overall, in Streptocarpus at least, pollen size is a limited predictor of genome size and only partly reflecting ploidy level, but may be of taxonomic value. The study demonstrates that the relationship between pollen size and genome size is not straightforward, and their evolutionary trajectories unlinked.
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