Invasive populations of Brassica nigra express fewer glucosinolate
compounds and are more deterrent to generalist herbivores than native
The shifting defence hypothesis attributes the invasion success of many
plant species to the species’ post- introduction evolution of traits
that confer high fitness including reduced allocation to physiologically
and ecologically costly chemical defence compounds against herbivores.
However, to date, most of the studies that compared invasive and
conspecific native plants for differences in defence compounds focused
only on one or a few compounds. Thus, it remains unclear whether
invasive plants may evolve a lower diversity of defence compounds in
response to a simplified herbivore community assemblages in the
introduced range. Therefore, the present study used B. nigra plants from
11 invasive- and 14 native range populations to test whether invasive B.
nigra plants: i) express a lower diversity of glucosinolate compounds
and higher concentration of a single dominant glucosinolate compound
owing to a reduced diversity of herbivores in the introduced range; ii)
are less preferred by generalist herbivores and attract more specialist
herbivore species than B. nigra from the native range when grown in a
common garden in the native range; iii) are less preferred by generalist
herbivores than native-range B. nigra in no- choice feeding bioassays.
In support of the shifting defence hypothesis, invasive B. nigra plants
expressed a lower diversity of glucosinolates and a higher concentration
of sinigrin (a major glucosinolate compound in B. nigra) than
native-range B. nigra plants. Moreover, the invasive B. nigra plants
were more deterrent to two generalist herbivore species in no-choice
feeding bioassays than native B. nigra plants. Brassica nigra plants
that expressed higher concentrations and diversities of glucosinolate
compounds were visited by a more diverse specialist herbivore community
than B. nigra plants with low concentrations and diversities of
glucosinolates. The biogeographical differences in glucosinolate
profiles observed here may be the result of differential herbivore
selection pressures in the respective invaded and native ranges.