Plants adapt to accommodate a long proboscis in South Africa.

Our Grant-Stebbins week continues. Today it’s a question of looking at the geographical context of floral adaptations. Why might you find a plant here and not there? Is it the pollinators that cause it?

Matching floral and pollinator traits through guild convergence and pollinator ecotype formation by Ethan Newman, John Manning and Bruce Anderson examines seventeen members of a pollination guild. These are plants that might look similar and appeal to similar pollinators, but are not closely related. These plants are all pollinated by Prosoeca longipennis a fly with a long proboscis, though the longipennis actually refers to long wings.

South African flowers.
A sub-set of floral guild members pollinated by Prosoeca longipennis, namely (A) Pelargonium pinnatum (Geraniaceae), (B) Gladiolus oppositiflorus (Iridaceae). (C) Pelargonium dipetalum (Geraniaceae), (D) Nerine humilis (Amaryllidaceae), (E) Pelargonium carneum (Geraniaceae), (F) Geissorhiza fourcadei (Iridaceae), (G) Wahlenbergia guthrie (Campanulaceae), (H) Tritoniopsis antholyza (Iridaceae) and (I) Gladiolus engysiphon (Iridaceae).

All photos, Newman et al. except B, Petra Wester.

How long a P. longipennis proboscis is depends on which flies you observe. It’s found across a wide range of South Africa. The flowers that attract P. longipennis all have similar attributes. They have long corollas, tubes of petals that made the head of the flower. They have similar colours and no scent. Newman et al. also note another feature that makes them specific to P. longipennis – they flower in the autumn.

Observations in the field were able to confirm the existence of a guild and add more members to them. It was then a matter of examining different sites and seeing how they correlated with the local flies. What they found was there was a strong correlation between the tube length of a flower and the average length of the fly’s proboscis at each site. If a site had significantly longer P. longipennis, then the flowers would also have significantly longer tubes.

However, not all of the plants relied on P. longipennis. Some could be found at sites where the flies didn’t visit. Here the same plants would have morphologically different floral displays, showing there was a correlation between the flowers and the local pollinators.

What Newman et al. show with their work is that a variety of plants with different evolutionary histories are all hitting on the same solution. The common factor is they’re all trying to attract P. longipennis to pollinate them. At the same time they also show that while corolla size matters, and colour might matter, the size of the reward doesn’t. Being able to put together a description of the guild means that they can make a prediction. They say that from what they have observed, P. longipennis should visit Watsonia plants, in particular W. galpinii

This kind of prediction reminds me of the Angraecum sesquipedale prediction that Darwin made. em>Angraecum sesquipedale is an orchid that has an amazingly long tube. Darwin predicted that a moth with an equally amazingly long tongue would be found that pollinated it. He didn’t live to see his prediction proved right. Hopefully Newman, Manning and Anderson will have their prediction confirmed a lot sooner. If they’re right they will not just know what fly pollinates the flower. If they know the location they’ll have a good idea of how long its proboscis is too.

You can pick up this paper from the Annals of Botany.