Adaptive evolution of vertebrate-type cryptochrome in the ancestors of Hymenoptera

Abstract

One of the most mysterious aspects of insect clock mechanisms is that some insects, including Hymenoptera and Tribolium, only express a vertebrate-type cryptochrome (cry2). It is unknown whether or not cry2 underwent adaptive evolution in these insects. In the present study, we cloned and sequenced the full-length cry2 from a fig pollinator species, Ceratosolen solmsi (Hymenoptera: Chalcidoidea: Agaonidae), and examined the molecular evolution and daily expression of this gene. Our results suggest that cry2 underwent positive selection in the branch leading to hymenopteran insects. The function of CRY2 might have been fixed since undergoing natural selection in the ancestor of Hymenoptera. Male pollinators showed stronger rhythmicity in the host figs, which reflect an adaptation to their life cycles.

Figure 1.

(a) Phylogenetic reconstruction of cry2 in holometabolic insects. The ML tree is constructed from full-length coding sequences of cry2. Bootstrap values are shown at the nodes. The branches tested for positive selection are labelled as a–g. Asterisk (*) indicates that positively selected sites are detected. The species that lack cry1 are highlighted in blue. (b) Phylogeny of the holometabolic insects, character mapping of possession of cryptochrome genes and proposed ancestral state. Triangles indicate cry1 (red) and cry2 (blue).

Figure 2.

(a) Daily expression of cry2 in female and (b) male C. solmsi. Fold expression of cry2 (mRNA abundance of opsin genes relative to reference genes) at each time point are represented by bars. LF (blue), fig-female light; DF (red), fig-female dark; F (green), emerged females light; LM (blue), fig-male light; DM (red), fig-male dark. Asterisk (*) indicates significant rhythmic expression. Morphological dimorphism between female and male of C. solmsi is present.