The stickiness of Onagraceae pollen

Oenothera pollen on the anthers.

I am particularly fascinated by how the properties of host pollen affect bees. I discuss this topic a bit in my paper “Convergent evolution of pollen transport mode in two distantly related bee genera (Hymenoptera: Andrenidae and Melittidae).” In that paper, I suggest that the properties of pollen could influence how it is carried. In particular, it seems that very sticky pollen types are often carried in a dry state, even in bee groups where most species moisten the pollen in order to carry it.

The plant family Onagraceae is particularly well-known for having sticky pollen. It has been well-documented that Onagraceae pollen has “viscin threads” which make the pollen sticky and difficult to handle (Linsley and MacSwain 1958, Roberts and Vallespir 1978, Hesse 1981). In fact, many Onagraceae-specialist bees have evolved specialized pollen-transporting hairs in order to effectively manipulate the pollen (Linsley 1958, Thorp 1979).

However, it’s hard to get a sense of what exactly is meant by “viscin threads,” especially since if you look up “viscin” in the dictionary, it refers to the mucous-like substance in Mistletoe fruits.

I believe the term “cobwebby” is the most accurate synonym for “viscin threads.” I feel like it captures the sticky, but not too sticky, nature of the threads. In my experience, these viscin threads adhere in a very similar manner to cobwebs, since their adherence feels more like it is due to static cling of the filamentous threads rather than to any inherent stickiness in the threads themselves.

One last thing I want to mention about Onagraceae pollen is that I commonly see two distinct size-classes in the pollen, big grains and small grains. I have never seen this mentioned before in the literature and I don’t know why this happens, but it seems to be fairly consistent in different individual plants and even different species. I had noticed different-sized Onagraceae pollen in the scopa of bees previously, and had initially chalked it up to them visiting multiple species of Onagraceae. However, now it’s not clear whether that is the case.

Zoomed-in photo demonstrating the different sizes of pollen.

References

Hesse M (1981) Pollenkitt and viscin threads: their role in cementing pollen grains. Grana 20: 37–41.

Linsley E (1958) The ecology of solitary bees. Hilgardia 27: 543–585.

Linsley EG, MacSwain JW (1958) The significance of floral constancy among bees of the genus Diadasia (Hymenoptera, Anthophoridae). Evolution 12: 219–223.

Roberts RB, Vallespir SR (1978) Specialization of hairs bearing pollen and oil on the legs of bees (Apoidea: Hymenoptera). Annals of the Entomological Society of America 71: 619–627.

Thorp RW (1979) Structural, behavioral, and physiological adaptations of bees (Apoidea) for collecting pollen. Annals of the Missouri Botanical Garden 66: 788–812.