Seasonal Diphenism in Daphnia from Temperate Environments: Organismal Traits and Molecular Regulation

Abstract

In temperate waters, post-diapause and subitaneous offspring in Daphnia substantially vary in organismal traits. Post-diapause offspring have elevated metabolic activity. Under rich food, they grow faster, mature at a larger body size and exhibit a greater allocation to progeny resulting in a higher relative fitness. In contrast, subitaneous females are more resistant to starvation and have a higher fitness across limited food concentrations.

These offspring types result from different developmental programs of the same genetic background, representing the phenomenon of phenotypic plasticity. A high-throughput proteomic analysis revealed 176 proteins that were differentially expressed among offspring phenotypes. There were more upregulated proteins with oxyreductase and binding activity in post-diapause offspring, whereas more upregulated proteins with transporter and transferase activity were seen in subitaneous offspring. Over 1.5-fold more of the proteins that were upregulated in post-diapause phenotype are involved in metabolism and biosynthesis. The greatest difference, a fivefold upregulation in post-diapause compared to subitaneous offspring, was recorded for the target of rapamycin-like (TOR) protein. Expression of ribosomal proteins in this offspring phenotype was also increased. These upregulations suggest that the TOR signalling pathway is involved and can be responsible for the regulation of the developmental program underlying post-diapause and subitaneous offspring phenotypes. Gene regulatory patterns observed in post-diapause and subitaneous offspring were in general agreement with observed organismal traits of these Daphnia phenotypes.

In temperate waters, Daphnia have evolved two alternative seasonal phenotypes matching environmental conditions in which they occur and perform in accordance with predictions of seasonal polyphenism (diphenism in this case). Due to higher metabolic activity, which must lead to increased resource acquisition, post-diapause offspring are superior to subitaneous offspring under high food conditions, which are expected during early season, but inferior under limiting food environments, which frequently occur later in the season. In seasonal climate, the adjustment of resource acquisition with respect to resource availability may be a general evolutionary trend for multivoltine organisms resulting in a seasonal polyphenism.