The role of phenotypic variation on a tritrophic interaction

Brockhouse, Suzanne (2005) The role of phenotypic variation on a tritrophic interaction. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Plant defence highlights the need to approach plant-insect interactions from a tritrophic perspective. Glucosinolates are a group of defensive chemicals produced extensively by members of the Brassicaceous family. The constitutive production of glucosinolates directly deters generalist herbivores. However, special herbivores are attracted to these chemicals and utilise them to locate suitable host plants. Upon herbivore attach, glucosinolates are hydrolysed by the enzyme myrosianase, which causes the release of a specific blend of volatile products. These herbivore-induced volatiles are thought to form an indirect method of plant defence via the action of natural enemies.

A wild Brassica system was identified to investigate the differences in glucosinolate genotype on a specialist herbivore and its parasitoid. HPLC analysis revealed that Brassica oleracea subsp. Oleracea (wild cabbage) at Kimmeridge and Durdle Door produce quantitative and qualitative differences in aliphatic glucosinolates. Air entertainment experiments revealed that 3-butenylisothiocyanate was released. Laboratory and field investigations concurred to reveal that Brevicoryne brassicae and Diaeretiella rapae representing the second and third trophic level respectively were unable to differentiate between the glucosinolate differences produced by Brassica oleracea subsp. oleracea from Kimmeridge and Durdle Door. However, "host-line" variation occurred in Brevicoryne brassicae, whilst behavioural differences between laboratory-reared and field-collected Diaeretiella rapae were observed. The results of all of the investigations are discussed in relation to the evolution of plant chemical defence and factors responsible for the glucosinolate polymorphism observed in these wild Brassica populations.