Tau Differences between Short-Day Responsive and Short-Day Nonresponsive White-Footed Mice (Peromyscus leucopus) Do Not Affect Reproductive Photoresponsiveness

Abstract

In laboratory-bred rodent populations, intraspecific variation in circadian system organization is a known cause of individual variation in reproductive photoresponsiveness. The authors sought to determine whether circadian system variation accounted for individual variation in reproductive photoresponsiveness in a single, highly genetically variable population of Peromyscus leucopus recently derived from the wild. Running-wheel activity patterns of male and female mice, aged 70 to 90 days, from artificially selected lines of reproductively photoresponsive (R) and nonresponsive (NR) lines were monitored under short-day photoperiod (8 h light, 16 h dark), long-day photoperiod (16 h light, 8 h dark), and constant darkness (DD). NR mice displayed a significantly longer mean free-running period (24.08 h) in DD compared with R mice (23.75 h), due in large part to a difference between NR and R females (24.25 h vs. 23.74 h, respectively). All other entrainment characteristics (alpha, phase angle of activity) under short days, long days, and DD were similar between R and NR mice. Variation in freerunning period and entrainment characteristics has been shown to affect photoresponsiveness in other rodent species by altering the manner in which the circadian system interprets short days. To determine whether variation in photoresponsiveness in P. leucopus is due to differences in free-running period instead of variation downstream from the central circadian clock in the pathway controlling photoresponsiveness, the authors exposed young R and NR mice to DD and measured the effect on reproductive organ development. If variation in free-running period affected how the circadian system of mice interpreted short days, then both R and NR mice exposed to DD should have exhibited a delay in gonadal development. Only R mice exhibited pubertal delay in DD. NR mice exhibited large paired testes, paired seminal vesicles, paired ovaries, and uterine weight typical of mice nonresponsive to short days, whereas R mice exhibited reproductive organ weight typical of mice responsive to short days. These data suggest that despite significant differences in free-running period between R and NR mice, individual variation in photoresponsiveness is not due to differences in how the circadian systems of R and NR mice interpret the LD cycle.

Heritable circadian period length in a wild bird population

“…With respect to seasonal timing, we found no relationship between t and lay date in captivity, which in our great tit population is a good proxy for laying date of these females in the wild (Visser et al 2009). Similarly, in studies of selection lines of rodents (Majoy & Heideman 2000), circadian differences were unrelated to individual differences in reproductive response to photoperiod. It has been argued that evolutionary adjustments of seasonal timing may occur downstream from the clock because circadian modifications are expected to have pleiotropic effects (Majoy & Heideman 2000;Bradshaw & Holzapfel 2007).…”

“…Similarly, in studies of selection lines of rodents (Majoy & Heideman 2000), circadian differences were unrelated to individual differences in reproductive response to photoperiod. It has been argued that evolutionary adjustments of seasonal timing may occur downstream from the clock because circadian modifications are expected to have pleiotropic effects (Majoy & Heideman 2000;Bradshaw & Holzapfel 2007). By contrast, recent findings from a closely related songbird, the blue tit, do point to a possible link between clock gene variants and seasonal timing (Liedvogel et al 2009).…”

“…In several avian species, including blue tits (Cyanistes caeruleus), EP success was highest for males that initiated morning song the earliest (Daan & Aschoff 1982;Dolan et al 2007;Poesel et al 2007;Murphy et al 2008 (Visser et al 1998;Visser 2008). Limited flexibility of breeding schedules could arise from mechanistic constraints, including circadian contributions, which we hereby examine (Silverin et al 1993;Majoy & Heideman 2000;Bradshaw & Holzapfel 2007;Liedvogel et al 2009;Visser et al in press). …”

Behavioral neuroendocrinology in nontraditional species of mammals: Things the ‘knockout’ mouse CAN'T tell us

“…Data from traditional laboratory models suggested that defects in the circadian system or melatonin production are major sources of variation in the pathway (reviewed by Majoy and Heideman, 2000). However, traditional models have been misleading when it comes to natural populations.…”

“…However, traditional models have been misleading when it comes to natural populations. In deer mice and white-footed mice, the evidence suggests that variation within natural populations is not due to an inability to secrete melatonin (Blank et al, 1988;Lynch et al, 1982;Ruf et al, 1997), to bind melatonin (Heideman et al, 1999b), to respond to melatonin (Blank, 1992;Heideman et al, 1999a), or an inability to produce normal circadian rhythms (Carlson et al, 1989;Majoy and Heideman, 2000).…”