Sharp hook-hairs of the tree Macaranga trachyphylla in Borneo. PHOTO: RITABRATA CHOWDHURY
Sharp hook-hairs on the Macaranga trachyphylla tree kill most caterpillars, except Arhopala amphimuta, which may point to a co-evolutionary adaptation.
Vefence mechanisms in plants can give rise to selection pressures that drive new adaptations in both attacking insects and the plant itself. Although such evolutionary arms races are a well-known phenomenon, virtually all documented examples involve chemical plant defences. Researchers from Brunei and Cambridge now suggest that physical defences can also drive co-evolution between plant and insect (Biology Letters, 4 June). The study shows that the hook-shaped hairs on the stems and leaf stalks of the Bornean tree Macaranga trachyphylla pose no danger to caterpillars of the butterfly species Arhopala amphimuta – which occurs naturally on this tree species – whilst they are frequently lethal to caterpillars of related butterfly species that normally inhabit other Macaranga species. Hook-hairs occur within the genus Macaranga exclusively on M. trachyphylla, presumably as a response to caterpillar herbivory. They pierce the bodies and legs of three of the four Arhopala species that the research team collected and placed on different parts of the tree, but A. amphimuta emerges from the test unscathed.
Shelter
Almost all Macaranga tree species protect themselves against herbivory by entering into an ‘agreement’ with ants. Some – including M. trachyphylla – even offer permanent shelter to their specific ant partners, which efficiently keep most other insects at bay. The Arhopala caterpillars, however, manage to appease these zealous guardians by supplying the ants with nectar-like bodily secretions, and then proceed to feed quite happily on the tree’s leaves. ‘The butterfly species involved here belong to the family of blues’, says evolutionary ecologist Arjen Biere of the Netherlands Institute of Ecology (NIOO-KNAW). ‘This is a group of butterfly species well known for their positive interactions with ants and for their arsenal of specialised organs that facilitate this partnership. The species in this study are no exception.’
Response
Exactly which adaptations enable the offspring of A. amphimuta to also escape the hook-hairs – presumably the tree’s second line of defence – remains unclear from the recent publication. The butterfly species is moreover also found on other Macaranga species that lack sharp hairs. It is therefore uncertain whether the as yet unknown caterpillar trait is genuinely an evolutionary response to the hairs of M. trachyphylla – as the authors suggest.
Premature
‘It seems somewhat premature to me to present this research as an example of a co-evolutionary arms race based on a physical plant defence. It is a possibility, but without additional studies the caterpillar’s adaptation could equally be the result of an entirely different selection pressure, which happens by chance to be advantageous on a tree with sharp hook-shaped hairs’, says Biere. ‘In some species of blues, the association with ants has resulted in the evolution of a thickened caterpillar skin to protect them against aggression from their ant partners. I therefore think it would be interesting to investigate whether that is the case here, and whether it might explain why the hairs of M. trachyphylla do not pierce these caterpillars.’