| Summary: | This thesis addressed phenotypic and genetic variation in seasonal time keeping mechanisms of the tundra vole ( Microtus oeconomus ) and the common vole ( Microtus arvalis ). Voles ( Microtus ) are short-lived, non-hibernating and seasonally breeding rodents. The genus has rapidly evolved (< 2 million years) into one of the most speciose mammalian genera (Sitnikova et al. 2007; Triant and DeWoody 2006) and occupies a wide range of latitudes (14-78°N) with the tundra vole being the most wide spread species. Seasonality is strong at high latitudes with lower and more seasonally fluctuating ambient temperatures (Hut et al. 2013). Therefore, animals have evolved mechanisms to time their life cycles with the strongly cyclical environment. The annual day length cycle is the most reliable cue to predict upcoming changes and prepare accordingly. This information is integrated by the photoneuroendocrine system (PNES) that coordinates phenotypic changes such as seasonal molt and reproduction (D. Hazlerigg and Simonneaux 2015). In paper I, we showed that under laboratory conditions, short winter photoperiods alone reduced somatic growth (body mass) in tundra voles and gonadal growth (reproduction) in common voles. Since both vole species were caught at the same location (the Netherlands, 53°N), the different response can be ascribed to genetic variation between the species. This was possibly shaped by different selection pressures occurring during the more northern (tundra vole) and southern (common vole) paleogeographic history of the two species. Within and among vole species, the timing of breeding shows great year-to-year variation (Tast 1966; T. Ergon et al. 2001), which is apparently influenced by environmental conditions such as ambient temperature (Kriegsfeld, Trasy, and Nelson 2000). The breeding season starts in spring with the overwintering individuals producing the first spring-born cohort of pups. The short gestation and development times allow these spring-born cohorts to reproduce during the same breeding season as their parents and produce several subsequent cohorts until the end of the breeding season in autumn (Horton 1984a; Gliwicz 1996). In papers II and III, we investigated the critical photoperiod thresholds for initiation of accelerated reproductive maturation in voles on a spring developmental program and for the deceleration of development in voles on an autumn program. Further, we assessed the influence of ambient temperature (10°C or 21°C) on the response parameters. Seasonal gene expression, hormone levels, downstream body-mass and gonadal mass had different species-specific response thresholds to photoperiod and temperature. This indicates that the system has a hierarchical organization that allowed for independent modulation at various levels. The results of these experiments also emphasise the importance of the direction of day length change in setting maturation trajectories. In Paper IV we searched for signatures of selection across the genomes of tundra voles from a northern (70°N) and southern (53°N) population. A signature of selection is a reduction in population diversity at a certain genomic position because of positive selection on a favoured allele. We found selection on a paralogue of the Aldh1a1 gene located between the Aldh1a1 and Aldh1a7 genes. We found two additional Aldh1a1 -like paralogues on the same locus. Other voles investigated also had two or three paralogues, which are not present in mouse and rat genomes. Aldh1a1 has a central role in photoperiodic retinoic acid signaling in the rodent hypothalamus, which may be involved in seasonal body mass regulation (Helfer, Barrett, and Morgan 2019; Shearer, Stoney, Nanescu, et al. 2012). Aldh1a7 is also considered as a paralogue of Aldh1a1 (90% amino acid sequence homology in the mouse) but it is not involved in retinoic acid signaling (Hsu et al. 1999). The paralogues found in the vole had the highest sequence homology with Aldh1a7 . Future research has to clarify the function of this gene and whether this selection pressure is associated with latitude. Taken together we found various levels of flexibility within the vole PNES where ambient temperature and photoperiodic history can modulate the seasonal response which is possibly affected by evolution at different latitudes. Reproductive opportunism and an ability to override photoperiodic information may be favoured in voles living at higher latitudes which may lead to genetic differences between and within species.
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