Cantharidin is conserved across phylogeographic lineages and present in both morphs of Iberian Berberomeloe blister beetles (Coleoptera, Meloidae)

Abstract

Intra-specific coloration polymorphism coupled with an ancient process of lineage differentiation in Berberomeloe majalis (Linnaeus, 1758) offers the opportunity to analyse the temporal scenario in which the correlation between toxicity and coloration might have evolved. Based on phylogenetic and phylogeographic analyses, we identified the timing for the split between red-striped and entirely black morphotypes of B. majalis. To evaluate whether coloration patterns and toxicity are related in this species, we quantified the concentration of cantharidin across morphotypes and phylogeographic lineages. Phylogenetic analyses based on cox1 mitochondrial DNA sequences recovered three major clades where both morphotypes were intermingled, indicating a multiple homoplastic origin for the entirely black coloration. Our analyses showed that cantharidin content did not differ between morphs of B. majalis; however, it significantly increased in haemolymph in females kept isolated from males, which reveals the females' ability either to concentrate cantharidin towards haemolymph or to synthesize cantharidin themselves. Lack of monophyly and absence of genetic isolation in both morphotypes favour the hypothesis of a recent homoplastic phenomenon to explain the loss of the striped pattern. Our phylogenetic and phylogeographic analyses show that changes in coloration are recent, suggesting that the ancient pressures that fixed and maintained red-striped colorations are no longer acting today on B. majalis. The absence of change in cantharidin content (i.e. entirely black and red-striped specimens are equally poisonous) suggests that the evolution of colour polymorphisms in B. majalis is probably decoupled from toxicity.

Cantharidin content in two Mediterranean species of blister beetles, Lydus trimaculatus and Mylabris variabilis (Coleoptera: Meloidae)

“…As we carried out multiple hemolymph extractions over the entire lifetime of L. trimaculatus and M. variabilis individuals, our estimates were higher than those previously obtained from 2–4 (0.03 ± 0.05 mg/g) or 8–10 (0.13 ± 0.14 mg/g) day old Berberomeloe majalis (Bravo et al . ).…”

“…Despite the defensive and reproductive role of autohemorrhaging, CA concentration in the exuded hemolymph has been rarely quantified in blister beetles (see Bravo et al 2017). The analysis of hemolymphbased solutions (n replicates = 5) revealed a mean CA content of 0.79 (SD = 0.01) and 0.26 (SD = 0.01) mg for each gram of the total amount of exuded hemolymph collected from 139 L. trimaculatus and 100 M. variabilis, respectively.…”

“…; Bravo et al . ) and transfer large quantities of this terpene to females during copulation (Selander ; Bologna ; Carrel et al . ; Dettner ; Nikbakhtzadeh ; Nikbakhtzadeh et al .…”

“…Hence, CA is supposed to have a primary defensive role, but also to be involved in reproduction. In fact, males of meloid beetles usually produce more CA than females (Capinera et al 1985;Blodgett et al 1991;Carrel et al 1993;Nikbakhtzadeh & Tirgari 2002;Mebs et al 2009;Bravo et al 2017) and transfer large quantities of this terpene to females during copulation (Selander 1964;Bologna 1991;Carrel et al 1993;Dettner 1997;Nikbakhtzadeh 2004;Nikbakhtzadeh et al 2012). Females are supposed to use the transferred CA for covering eggs to protect them from potential predators (Sierra et al 1976;Carrel et al 1993;Eisner et al 2002).…”

“…because these are supposed to be responsible for poisoning livestock feeding on alfalfa hay contaminated with CA (Capinera et al 1985;Blodgett et al 1991Blodgett et al , 1992Carrel et al 1993;Blodgett & Denke 1999). In contrast, apart from some pioneering approaches (reported in Beauregard 1890), few investigations have been undertaken on other species, with sparse data on CA content only available for few other members of the subfamily Meloinae, primarily collected in Iran, South Africa (Nikbakhtzadeh & Tirgari 2002;Mebs et al 2009;Nikbakhtzadeh et al 2012) and, more recently, in the Iberian Peninsula (Bravo et al 2017).…”