Genetic, ecological, behavioral and geographic differentiation of populations in a thistle weevil: implications for speciation and biocontrol
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2008-02Author
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<jats:title>Abstract</jats:title><jats:p>Because weevils are used as biocontrol agents against thistles, it is important to document and understand host shifts and the evolution of host‐specificity in these insects. Furthermore, such host shifts are of fundamental interest to mechanisms of speciation. The mediterranean weevil <jats:italic>Larinus cynarae</jats:italic> normally parasitizes either one of two thistle genera, <jats:italic>Onopordum</jats:italic> and <jats:italic>Cynara</jats:italic>, being locally monophagous. In Sardinia, however, both host genera are used. We used three types of data to help understand this complex host use: (i) weevil attack rates on the two host genera among 53 different populations in Sardinia and nearby Corsica, (ii) host preference in a lab setting, and (iii) genetic (allozyme) differentiation among weevil populations exploiting the same or different hosts. Using a subset of populations from northern Sardinia, we attempted to relate interpopulation differences in host preference to gene flow among populations by comparing pairwise differences in oviposition preference (<jats:italic>Qst</jats:italic>) and in allozyme frequencies (<jats:italic>Fst</jats:italic>). Overall, <jats:italic>Qst</jats:italic> and <jats:italic>Fst</jats:italic> were positively correlated. <jats:italic>Fst</jats:italic> was positively correlated with geographic distance among pairs of populations using the same host, but not among different‐host population pairs. As mating occurs on the hosts, this result suggests reinforcement. Genetic evidence indicates <jats:italic>Cynara</jats:italic> as the ancestral host of the weevils from both islands and our current studies suggest repeated attempts to colonize <jats:italic>Onopordum</jats:italic>, with a successful shift in Corsica and a partial shift in Sardinia. This scenario would explain why in Sardinia the level of attack was higher on <jats:italic>Cynara</jats:italic> than on <jats:italic>Onopordum</jats:italic> and why, when given a choice in the laboratory, Sardinian weevils preferred <jats:italic>Cynara</jats:italic> even when sampled from <jats:italic>Onopordum</jats:italic>. The lability of host shifts in <jats:italic>L. cynarae</jats:italic> supports caution in using these or related weevils as biocontrol agents of exotic thistles.</jats:p>
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