Adaptive evolution of the circadian gene timeout in insects

Abstract

Most insects harbor two paralogous circadian genes, namely timeout and timeless. However, in the Hymenoptera only timeout is present. It remains unclear whether both genes, especially timeout in hymenopteran insects, have distinct evolutionary patterns. In this study, we examine the molecular evolution of both genes in 25 arthropod species, for which whole genome data are available, with addition of the daily expression of the timeout gene in a pollinating fig wasp, Ceratosolen solmsi (Hymenoptera: Chalcidoidea: Agaonidae). Timeless is under stronger purifying selection than timeout, and timeout has positively selected sites in insects, especially in the Hymenoptera. Within the Hymenoptera, the function of timeout may be conserved in bees and ants, but still evolving rapidly in some wasps such as the chalcids. In fig wasps, timeout is rhythmically expressed only in females when outside of the fig syconium but arrhythmically in male and female wasps inside the syconium. These plastic gene expressions reflect adaptive differences of males and females to their environment.

Figure 1. Gene structure analyses on timeout.

(a) Variation of gene lengths among arthropods. The orange columns represent hymenopteran species, and the blue columns represent non-hymenopteran species. (b) The genomic structure of timeout in Ceratosolen solmsi. The red bars represent exons, and the black reversed triangle denotes the nested gene, putatively encoding an insulin growing banding protein.

Figure 2. The phylogeny of timeout in 25 species of arthropod.

A red triangle before each species denotes the presence of timeless in a species, with a blue triangle denoting the existence of timeout. The branches a to i are the lineages tested for positive selection by a branch-site model using PAML software. The red colored branches denote the species in which a “nested” transcribed gene was detected in the intron of timeout.

Figure 3. The expression of timeout in the fig wasp Ceratosolen solmsi over a 24 h period in males (that do not leave their syconium), females still within their syconium, and females that had naturally dispersed from their syconium.