Evolutionary diversification of bean
beetles: climate-dependent traits and development associated with pest status
Kyushu University, Japan
A large proportion of beetle species in
the genus Callosobruchus are economically
important pests of stored, dry post-harvest beans of the tribe Phaseoleae.
However, the evolution of this key feeding habit is poorly understood. Here,
using the phylogeny of the Asian and African Callosobruchus based on three mitochondrial genes, we first test the extent to which
the ability to use dry beans is affected by ancestral traits, or phylogenetic
signals, and then test for associations with the following variables: (1) host
taxa in the wild, (2) a suite of climatic variables (i.e. annual mean
temperature and number of arid months) and (3) the presence/absence of
endosymbiotic bacteria, Wolbachia. Our
phylogenetic analysis shows that both dry-bean use and host-plant use exhibited
high phylogenetic signals, and transitions in the two were significantly
correlated. The ability to use dry beans was suggested to promote habitat
shifts from tropical to temperate regions. Adaptation to arid climate was also
associated with the ability to reproduce on dry beans. Thus, our analysis
suggests that physiological adaptations to an arid climate and to Phaseolinae
hosts both render beetles predisposed to become pests of cultivated beans.
Adaptation to dry food can indirectly enhance cold hardiness. Adaptation to low
ambient humidity may have further served as pre-adaptation to the use of seeds
with low water content. Long arid seasons and the associated availability of
legume seeds may tend to favor repeated reproduction. This study predicts that
new stored-product pests will likely evolve under arid climate.