New research: Secondary Chemistry Expression in Senecio

Every species has a natural suite of enemies (predators, parasites, diseases) that has co-evolved with it in the context of its indigenous range.   Some species, upon being transported outside their indigenous range exhibit rapid spread and high densities and become regarded as invasive aliens.   The enemy release hypothesis (ERH) suggests that one of the reasons for the success of invasive alien species is their escape from their natural enemies upon their arrival in a new environment.   Indeed this has been demonstrated in a number of studies, but is by no means a universal explanation for those species apparently performing better in their adventive range compared to their indigenous range.

One of the ways in which plant species can defend themselves against herbivorous enemies is the production of secondary metabolic compounds which diminish the palatability of plant tissue (e.g., tannins, alkaloids, phyto-oestrogens).   This raises the potential for examining evidence for the ERH by measuring the expression of these compounds under various conditions – for example indigenous and adventives species within and without their native range.   We proposed to embark on such a study for New Zealand weeds using the genus Senecio as the subject.   A number of species of the genus have been shown to produce pyrrolizidine alkaloids known to be anti-herbivory compounds – perhaps the most studied being S. jacobaea (now Jacobaea vulgaris, but we’re not too fussy in this context).   However, this is not a trivial task, as the range of secondary compounds and their various chemotypes is enormous – the investigations into ragwort have a basis in decades of investigation of the chemistry.   Extending such investigations to other invasive species presents some challenges but new developments in chemical analytical technologies and new approaches to the analysis of complex chemical profiles offer powerful tools for untangling the web.

In the first instance, we propose to chemically screen different populations of a range of Senecio spp. to address the hypothesis “If the invasive nature of a naturalised population of a plant species is primarily due to its defensive chemistry, the range of chemotypes in the invasive zone should show uniformity compared to the diversity in its natural range where adapted specialist herbivores provide a countervailing selective pressure.   Conversely if the features favouring invasion are not primarily phytochemical, in principle the range of chemotypes in the invasive and natural range should not differ”.

To do this we have begun to compare the chemical profiles of different populations of a range of invasive Senecio spp. in New Zealand.   So far we have looked at S. angulatus, S. elegans, S. glastifolius, S. mikanioides (a.k.a. Delairea odorata, but remember we’re not fussy!) and S. skirrhodon.   Preliminary analysis of the chemotype profiles do indicate a degree of uniformity within spenbsp; We would be pleased to hear from any Wellington BotSoc members who can help us locate populations of Senecio spp. that we could sample for our studies.

Mike Dodd and Geoff Lane, AgResearch Grasslands, Palmerston North.