roses are red

Blogging on Peer-Reviewed Research

But not, says a press release, for the usual reasons.

Over my muesli this morning I read an item in the Herald (sorry, the link’s to the identical item in a UK paper cos the Herald website doesn’t carry it) saying that a group of scientists had ‘disproved the theory’ that competition for pollinators led to the evolution of brightly coloured flowers in many flowering plants. And it specifically mentioned red roses, saying in the sub-headline: Scientists have discovered why roses are red, claiming their striking colour has evolved to deter predators.

This was of course a red rag to a bull, so I went off to find the original paper on which this press release was based (Hanley et al. 2009).

You’ve probably picked the first point that annoyed me a little. Yes, I know it’s journalistic license – but the researchers haven’t disproved a theory (closer, perhaps, to an hypothesis) about the evolution of colour in flowers. What they have demonstrated is that the assumed drivers might not be working in some types of red flower.

And the type of flower’s important too – the work was done in Hakea species, not roses. The roses we have today have been moved a long way from the original wild roses, as plant breeders have selected for particular features of roses that they find attractive: perfume, yes, but colour probably comes at the top of the list. (Personally, I go for perfume first, although I do like a nice dark red hybrid-tea rose.) Given that some of the earliest rose cultivars were white or very pale, I suspect that the hand of man has more to do with red roses than evolution of predator deterrents or pollinator attractants.

Hakea, by the way, is an Australian genus of rather lovely flowering plants, some but not all of which have red flowers:


Anyway, what did the researchers do? They collected information on the structure of leaves & flowers, the colour of the flower heads, & the chemical defences present (or absent) in the flowers of plants from 51 species of Hakea. At the same time they decided whether the flowers were pollinated by birds or by insects, on the basis of the distance between the stigma (where pollen is deposited) and the floral nectary (which holds a sweet reward for the pollinator). The chemical defences bit may sound unusual – but some animals do regularly eat flowers as part of their normal diet: one study (cited by Hanley et al.) found that white-tailed deer ate >80% of iris flowers in the study area. Plants can put a lot of resources into growing flowers, so you can see how there would be quite strong selection pressure in favour of mutations conferring protection from grazing. Not to mention the fact that having your flowers eaten would rather put a damper on your sex life!

Hakea flowers turn out to be quite varied. Some have small flowers, enclosed in a spiny embrace that keeps away all but insect pollinators. Others have large, red inflorescences that are easy for birds to get at – & possibly to eat. So, the team looked at the amounts of cyanide compounds found in the different types of Hakea flowers, and related this to the probable nature of their pollinators. (Because insect pollinators in Western Australia are seldom >15mm long, plants with a stigma-nectary distance greater than this are probably bird-pollinated.) They used the data to test several hypotheses: 1) species with [highly accessible] flowers are bird-pollinated’ 2) highly accessible flowers are cyanogenic; 3) highly accessible flowers are red; 4) red flowers are cyanogenic; & 5) species with red flowers are cyanogenic (Hanley et al. 2009).

 What did they find out? Flowers in bird-pollinated species were more likely than those of insect-pollinated plants to be red, and highly accessible. ‘Highly accessible’ flowers tended to contain high levels of cyanide, although this relationship was not highly significant. And bird-pollinated flowers also tended to have high cyanide levels. This led to the conclusion that highly accessible, bird-pollinated flowers contain enhanced chemical defences to deter potential florivores. Birds see red colours well, & it’s possible that the red pigments in Hakea signal both the presence of a sugar meal and also the distasteful nature of the petals themselves.

So yes, this is an interesting paper that suggests that the evolution of flower colour & form is not as straightforward as scientists have assumed – pollinator choices probably are not the only selection pressure operating in floral evolution. The paper ends with the statement that adaptations against both foliar and floral herbivory should be considered in the search for a clearer understanding of floral trait evolution. Our understanding is probed, and extended, rather than overturned.

And roses weren’t mentioned at all…

M. Hanley, B.B. Lamont & W.S.Armbruster (2009) Pollination and plant defence traits co-vary in Western Australian HakeasNew Phytologist 182: 251-260  

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