Neuroendocrine Mechanisms of Vole Population Self-regulation

Arvicolinae rodents (voles and lemmings) go through distinct population cycles that influence ecosystem dynamics throughout the northern hemisphere. The existence of an intrinsic regulatory mechanism driving these population cycles has been speculated about since the 1960s, however, the mechanism has never been demonstrated. In this thesis work, I tested the hypothesis that high population density, a condition of the cycle peak, induces epigenetic changes in genes central in regulating the reproductive axis and the stress axis of voles. I manipulated population density in meadow voles (Microtus pennsylvanicus) using predator-proof field enclosures. In the high density enclosures, fewer females were reproductive, as has been found in peak density populations in the wild. Non-reproductive females did not appear to be social subordinates, as they were equally able to secure a territory as reproductive females and there were no differences in stress (fecal corticosterone) levels between classes. However, breeding females at high density had higher corticosterone levels than breeding females at low density, potentially reflecting a cost of competition amongst females. I collected brain tissue from juvenile animals born at high and low density and quantified mRNA expression in the candidate genes gonadotropin releasing hormone (GnRH; GNRH1), estrogen receptor alpha (ERα; ESR1), glucocorticoid receptor (GR; NR3C1) and mineralocorticoid receptor (MR; NR3C2). I found that voles of both sexes born in high densities had lower GnRH expression, and lower levels of androgens in the feces, indicating down-regulation of the reproductive axis. There were no density treatment differences in ERα in either sex. Male voles born in high densities also had lower GR and MR expression, aligning with predictions of an increased stress response and weaker negative feedback in a high competition environment. Females, contrary to predictions, had higher GR expression levels at high density, and no change in MR, suggesting faster down-regulation of the stress axis. I sequenced a region of the M. pennsylvanicus genome that is highly conserved with the mouse GnRH promoter, and examined it for methylation differences between population densities. In the CG site within a sequence highly similar to the mouse EGR-1 transcription factor binding site, meadow voles had no differences in methylation levels associated with density treatment. Other CG sites in the meadow vole GnRH promoter should be examined in future work. These findings provide evidence for an intrinsic physiological mechanism in vole population dynamics by density-induced suppression of the reproductive axis. However, the suppression of GnRH is not driven by methylation in a key region of the GnRH promoter. Arvicolinae rodents (voles and lemmings) go through distinct population cycles that influence ecosystem dynamics throughout the northern hemisphere. The existence of an intrinsic regulatory mechanism driving these population cycles has been speculated about since the 1960s, however, the mechanism has never been demonstrated. In this thesis work, I tested the hypothesis that high population density, a condition of the cycle peak, induces epigenetic changes in genes central in regulating the reproductive axis and the stress axis of voles. I manipulated population density in meadow voles (Microtus pennsylvanicus) using predator-proof field enclosures. In the high density enclosures, fewer females were reproductive, as has been found in peak density populations in the wild. Non-reproductive females did not appear to be social subordinates, as they were equally able to secure a territory as reproductive females and there were no differences in stress (fecal corticosterone) levels between classes. However, breeding females at high density had higher corticosterone levels than breeding females at low density, potentially reflecting a cost of competition amongst females. I collected brain tissue from juvenile animals born at high and low density and quantified mRNA expression in the candidate genes gonadotropin releasing hormone (GnRH; GNRH1), estrogen receptor alpha (ERα; ESR1), glucocorticoid receptor (GR; NR3C1) and mineralocorticoid receptor (MR; NR3C2). I found that voles of both sexes born in high densities had lower GnRH expression, and lower levels of androgens in the feces, indicating down-regulation of the reproductive axis. There were no density treatment differences in ERα in either sex. Male voles born in high densities also had lower GR and MR expression, aligning with predictions of an increased stress response and weaker negative feedback in a high competition environment. Females, contrary to predictions, had higher GR expression levels at high density, and no change in MR, suggesting faster down-regulation of the stress axis. I sequenced a region of the M. pennsylvanicus genome that is highly conserved with the mouse GnRH promoter, and examined it for methylation differences between population densities. In the CG site within a sequence highly similar to the mouse EGR-1 transcription factor binding site, meadow voles had no differences in methylation levels associated with density treatment. Other CG sites in the meadow vole GnRH promoter should be examined in future work. These findings provide evidence for an intrinsic physiological mechanism in vole population dynamics by density-induced suppression of the reproductive axis. However, the suppression of GnRH is not driven by methylation in a key region of the GnRH promoter.

Keywords: Endocrinology
Epigenetics
Methylation
Population ecology
Reproductive biology
Stress axis
Advisor: Boonstra, Rudy