ABSTRACT
Grass pollen is one of the leading causes of pollinosis, affecting 10-30% of the world's population. The allergenicity of pollen from different Poaceae species is not the same and is estimated from moderate to high. Aerobiological monitoring is a standard method that allows one to track and predict the dynamics of allergen concentration in the air. Poaceae is a stenopalynous family, and thus grass pollen can usually be identified only at the family level with optical microscopy. Molecular methods, in particular the DNA barcoding technique, can be used to conduct a more accurate analysis of aerobiological samples containing the DNA of various plant species. This study aimed to test the possibility of using the ITS1 and ITS2 nuclear loci for determining the presence of grass pollen from air samples via metabarcoding and to compare the analysis results with the results of phenological observations. Based on the high-throughput sequencing data, we analyzed the changes in the composition of aerobiological samples taken in the Moscow and Ryazan regions for three years during the period of active flowering of grasses. Ten genera of the Poaceae family were detected in airborne pollen samples. The representation for most of them for ITS1 and ITS2 barcodes was similar. At the same time, in some samples, the presence of specific genera was characterized by only one sequence: either ITS1 or ITS2. Based on the analysis of the abundance of both barcode reads in the samples, the following order could describe the change with time in the dominant species in the air: Poa, Alopecurus, and Arrhenatherum in early mid-June, Lolium, Bromus, Dactylis, and Briza in mid-late June, Phleum, Elymus in late June to early July, and Calamagrostis in early mid-July. In most samples, the number of taxa found via metabarcoding analysis was higher compared to that in the phenological observations. The semi-quantitative analysis of high-throughput sequencing data well reflects the abundance of only major grass species at the flowering stage.
ABSTRACT
PREMISE: The sporoderm of seed-plant pollen grains typically has apertures in which the outer sporopollenin-bearing layer is relatively sparse. The apertures allow regulation of the internal volume of the pollen grain during desiccation and rehydration (harmomegathy) and also serve as sites of pollen germination. A small fraction of angiosperms undergo pollination in water or at the water surface, where desiccation is unlikely. Their pollen grains commonly lack apertures, though with some notable exceptions. We tested a hypothesis that in some angiosperm aquatics that inhabit water of unstable salinity, the pollen apertures accommodate osmotic effects that occur during pollination in such conditions. METHODS: Pollen grains of the tepaloid clade of the monocot order Alismatales, which contains ecologically diverse aquatic and marshy plants, were examined using light microscopy and scanning electron microscopy. We used Ruppia as a model to test pollen grain response in water of various salinities. Pollen aperture evolution was also analyzed using molecular tree topologies. RESULTS: Phylogenetic optimizations demonstrated an evolutionary loss and two subsequent regains of the aperturate condition in the tepaloid clade of Alismatales. Both of the taxa that have reverted to aperturate pollen (Ruppia, Ruppiaceae; Althenia, Potamogetonaceae) are adapted to changeable water salinity. Direct experiments with Ruppia showed that the pollen apertures have a role in a harmomegathic response to differences in water salinity. CONCLUSIONS: Our results showed that the inferred regain of pollen apertures represents an adaptation to changeable water salinity. We invoke a loss-and-regain scenario, prompting questions that are testable using developmental genetics and plant physiology.
