The genetic architecture underlying diapause termination in a planktonic crustacean

Abstract

Diapause is a feature of the life cycle of many invertebrates by which unfavourable environmental conditions can be outlived. The seasonal timing of diapause allows organisms to adapt to seasonal changes in habitat suitability and thus is key to their fitness. In the planktonic crustacean Daphnia, various cues can induce the production of diapause stages that are resistant to heat, drought or freezing and contain one to two embryos in developmental arrest. Daphnia is a keystone species of many freshwater ecosystems, where it acts as the main link between phytoplankton and higher trophic levels. The correct seasonal timing of diapause termination is essential to maintain trophic interactions and is achieved via a genetically based interpretation of environmental cues like photoperiod and temperature. Field monitoring and modelling studies raised concerns on whether populations can advance their seasonal release from diapause to advances in spring phenology under global change, or if a failure to adapt will cause trophic mismatches negatively affecting ecosystem functioning. Our capacity to understand and predict the evolution of diapause timing requires information about the genetic architecture underlying this trait. In this study, we identified eight quantitative trait loci (QTLs) and four epistatic interactions that together explained 66.5% of the variation in diapause termination in Daphnia magna using QTL mapping. Our results suggest that the most significant QTL is modulating diapause termination dependent on photoperiod and is involved in three of the four detected epistatic interactions. Candidate genes at this QTL could be identified through the integration with genome data and included the presynaptic active zone protein bruchpilot. Our findings contribute to understanding the genomic control of seasonal diapause timing in an ecological relevant species.

The expression of circadian clock genes in Daphnia magna diapause

“…spider mites 30 ) the photoperiodic clock necessary for termination of diapause is probably not identical to the circadian clock. However, the involvement of cry 2 in diapause termination of Daphnia is likely: In a QTL study, Czypionka et al 31 have identified three isoforms of an ELKS/Rab6 interacting/Cast 396 family member protein (ERC; homologous to the gene bruchpilot ( brp ) in insects) to be potentially involved in diapause termination. Interestingly, brp interacts with the circadian clock although it is not part of the core circadian system 32 .…”

A region of the sex chromosome associated with population differences in diapause induction contains highly divergent alleles at clock genes

“…The evolution of plasticity depends on the strength of selection and the predictability of environmental fluctuations (Leimar et al 2006). To better understand the evolution of adaptive plasticity it is necessary to characterize its genetic background, and this has only been performed in very few studies (Czypionka et al 2018). One open point of discussion on adaptive plasticity considers that either specific loci are responsible for determining a plastic response, or alternatively that loci are co-opted by a number of traits that are adapted to a certain environment (Via et al 1995;Sgrò et al 2016).…”

Daphnia as a versatile model system in ecology and evolution

“…The position of two deleterious, but naturally segregating mutations were mapped [ 8 ]. The same standing QTL panel was later used to map the position of resistance genes to different parasites [ 71 , 116 ] and genes related to sex induction and breaking of diapause [ 5 , 117 ]. The mapping of phenotypes to genotypes has become much easier by the use of next generation sequencing for large number of genotypes, conducting genome wide association studies (GWAS).…”