Genetic variation in total number and locations of GnRH neurons identified using in situ hybridization in a wild-source population

Abstract

The evolution of brain function in the regulation of physiology may depend in part upon the numbers and locations of neurons. Wild populations of rodents contain natural genetic variation in the inhibition of reproduction by winter-like short photoperiod, and it has been hypothesized that this functional variation might be due in part to heritable variation in the numbers or location of gonadotropin releasing hormone (GnRH) neurons. A naturally variable wild-source population of white-footed mice was used to develop lines artificially selected for or against mature gonads in short, winter-like photoperiods. We compared a selection line that is reproductively inhibited in short photoperiod (Responsive) to a line that is weakly inhibited by short photoperiod (Nonresponsive) for differences in counts of neurons identified using in situ hybridization for GnRH mRNA. There was no effect of photoperiod, but there were 60% more GnRH neurons in total in the Nonresponsive selection line than the Responsive selection line. The lines differed specifically in numbers of GnRH neurons in more anterior regions, whereas numbers of GnRH neurons in posterior areas were not statistically different between lines. We compare these results to those of an earlier study that used immunohistochemical labeling for GnRH neurons. The results are consistent with the hypothesis that the selection lines and natural source population contain significant genetic variation in the number and location of GnRH neurons. The variation in GnRH neurons may contribute to functional variation in fertility that occurs in short photoperiods in the laboratory and in the wild source population in winter.

Timing as a sexually selected trait: the right mate at the right moment

“…Artificial directional selection experiments yielding lines of wild-derived Peromyscus mice with either strong or no responsiveness to short photoperiod (in inhibiting reproductive readiness [61]) support the hypothesis that fecundity or viability selection has shaped variation in the PNES. These selection-line Peromyscus mice showed clear differences in iodomelatonin binding in certain brain areas and in the number and location of GnRH neurons [62,63]. Furthermore, selection lines differed in the period of their free-running circadian rhythms (t), although that appeared to be unrelated to their photoperiodic responses [64].…”