DNA metabarcoding traces most of Melbourne's spring grass pollen to perennial ryegrass

Two techniques ran in sequence because each answers a different question. Metabarcoding pulls short marker sequences out of environmental DNA and matches them against a library of known sequences, which is how you learn which species left traces in a sample ^[11]. It is poor at measuring how much of each ^[15]. So the team took the four grasses that came up most often and measured those with quantitative PCR ^[16]. Ryegrass DNA came out by far the most abundant ^[17].

Across the season, ryegrass DNA and the measured airborne grass pollen count rose and fell together. The authors take that as evidence that ryegrass is the main grass pollen in Melbourne's air for most of spring ^[18].

The co-movement identifies the dominant taxon, and it means the count Melbourne already publishes each day from October to December tracks ryegrass closely for most of the season ^[3][18]. Species labels change what that daily number can be interpreted to mean. qPCR reports DNA copies, while allergen exposure is a dose of protein, and the measurements here are of DNA ^[16].

Two consecutive years of weekly sampling ^[10] sit inside a hazard that recurs in Melbourne on average once every seven years ^[7], so the sampled springs were most likely ordinary ones ^[24]. Since 2016 the city has issued emergency thunderstorm asthma warnings on the basis of predicted grass pollen counts and the weather ^[19]. A weekly air sample gives at most one measurement per seven daily forecasts ^[26].

The clinical use is more direct. Most people with thunderstorm asthma are allergic to ryegrass pollen, and many also react to other grasses including Timothy, Bermuda and Bahia ^[20]. In two years of Melbourne air, Timothy DNA did not appear at all and the subtropical grasses barely registered ^[14][13]. A Victorian patient who tests positive to Timothy is therefore sensitised to a grass the air sampling did not find, which is a different exposure picture from a ryegrass-driven one, and the authors note evidence that the more strongly allergic a person is, the greater their asthma risk ^[21].

Air sampling on its own cannot supply that link, and the authors write that clinicians could treat hay fever sufferers far more reliably with a better understanding of how spring asthma symptoms relate to grass pollen exposure ^[23]. That is the question behind CARISTA, which is recruiting up to 600 hay fever sufferers across Victoria for allergy and lung function testing ^[22].