Genomic imprinting mediates sexual experience-dependent olfactory learning in male mice William T. Swaney*†‡, James P. Curley*, Frances A. Champagne*†, and Eric B. Keverne* *Sub-Department of Animal Behaviour, University of Cambridge, High Street, Madingley, Cambridge CB3 8AA, United Kingdom; and †Department of Psychology, Columbia University, New York, NY 10027 Edited by Donald W. Pfaff, The Rockefeller University, New York, NY, and approved February 21, 2007 (received for review October 25, 2006) Mammalian imprinted genes are generally thought to have mother results in a complementary phenotype affecting pup evolved as a result of conflict between parents; however, recent growth, pup suckling/maternal milk letdown, and pup thermoreg- knockout studies suggest that coadaptation between mother and ulation/maternal nest-building (11), showing that these interdepen- offspring may have been a significant factor. We present evidence dent behaviors are coregulated by the same imprinted locus. Peg3 that the same imprinted gene that regulates mammalian maternal is strongly expressed in both the developing embryo and the care and offspring development also regulates male sexual behav- hypothalamus (9, 12), which controls maternal care in the mouse; ior and olfaction. We have shown that the behavior of male mice in light of these findings, it has been proposed that coadaptation carrying a knockout of the imprinted gene Peg3 does not change between mother and offspring may also have been a factor in the with sexual experience and that the mice are consequently unable evolution of imprinting in mammals. to improve their copulatory abilities or olfactory interest in female For a paternally expressed gene to be transmitted down the odor cues after mating experience. Forebrain activation, as indexed patriline and become fixed in the population, it would be antici- pated to also have adaptive effects on male behavior, enhancing by female odor-induced c-Fos protein induction, fails to increase male survival or reproductive success, and so we examined three with sexual experience, providing a neural basis for the behavioral different aspects of male behavior: sexual behavior, olfaction, and deficits that the male mice display. The behavioral and neural sexual experience-dependent changes in behavior in Peg3 knockout effects of the Peg3 knockout show that this imprinted gene has (KO) animals. Olfaction is the most important sensory faculty in evolved to regulate multiple and varied aspects of reproduction, mice, and female chemosignals detected by the main and accessory from male sexual behavior to female maternal care, and the olfactory systems play a vital role in triggering sexual behavior in development of offspring. Moreover, sexual experience-driven male mice, as studies involving ablation of the olfactory systems (13) behavioral changes may represent an adaptive response that or KO of olfactory genes (14, 15) have shown. As well as regulating enables males to increase their reproductive potential over their male sexual behavior, the hypothalamus receives input from the lifespan, and the effects we have found suggest that the evolu- accessory olfactory system (16), and so the behavioral and neuronal tion of genomic imprinting has been influenced by coadaptation responses of Peg3-KO males to female odors were also investigated between males and females as well as between females and to determine how the mutation affects reproduction-directed ol- offspring. faction and male sexual behavior. Sexual experience has been shown to have important effects on accessory olfactory system ͉ coadaptation ͉ mammalian brain ͉ reward ͉ male behavior in different rodent species, modulating interest in sexual behavior female semiochemicals (17, 18), mating behavior (19, 20), and increasing male fertility (21). Such experience-dependent changes in behavior are likely to be adaptive, enabling males to enhance he expression of certain mammalian autosomal genes in a their sexual behavior throughout life, and so we tested both virgin Tparent-of-origin fashion was first demonstrated in the 1980s and and sexually experienced males to determine whether sexual expe- termed ‘‘genomic imprinting’’ (1); since then, close to 100 mater- rience plays a role in sexual behavior in male mice and whether it nally and paternally expressed imprinted genes have been identified has similar effects in Peg3-KO mutants and wild-type males. (2). Over the last 20 years, the essential roles that imprinted genes We assessed the neural basis of these behaviors by using c-Fos play in development, particularly that of the brain, have become expression as a marker of brain activation in response to female clear (3, 4), suggesting that imprinted genes might have enabled the urinary chemosignals in wild-type (wt) sexually experienced (wt- evolutionary expansion of the mammalian brain (5). Most im- SE) males, wt virgin (wt-V) males, Peg3-KO sexually experienced printed genes are strongly expressed in the placenta (6) and are (p3-SE) males, and Peg3-KO virgin (p3-V) males. involved in regulating fetal development (7). These developmental roles, in particular those of Igf2, Igf2r, Igf2AS, and H19, have been Results cited as evidence to support the conflict theory for the evolution of Peg3 KO Males’ Interest in Female Urine Is Unaffected by Sexual genomic imprinting (8). This theory posits that imprinted genes Experience. Adult male C57BL/6J (B6) mice were tested on their evolved with placentation in mammals as a result of parental ability to discriminate between male and female urine and their conflict over offspring investment. The fetus/placenta is a geneti- cally half-paternal structure that provides the father with an op- Author contributions: W.T.S., J.P.C., and E.B.K. designed research; W.T.S. performed re- portunity to manipulate the mother’s investment in his offspring to search; W.T.S. analyzed data; and W.T.S., J.P.C., F.A.C., and E.B.K. wrote the paper. the possible detriment of her potential future offspring. Thus, The authors declare no conflict of interest. paternally expressed genes are predicted to enhance offspring growth, whereas maternally expressed genes are predicted to resist This article is a PNAS Direct Submission. this as a result of an evolutionary ‘‘arms race’’ over levels of Abbreviations: AOB, accessory olfactory bulb; B6, C57BL/6J; BNSTp, posterior part of the bed nucleus of the stria terminalis; KO, knockout; MeAmgA, anterior medial amygdala; investment in offspring. However, work by our group on the MeAmgPD, posterior–dorsal medial amygdala; MPOA, medial preoptic area; NAc, nucleus paternally expressed gene Peg3 has shown that as well as regulating accumbens; p3-SE, sexually experienced Peg3-KO; p3-V, virgin Peg3-KO; VMH, ventrome- pup development, this gene also regulates mothers’ maternal be- dial hypothalamus; wt, wild type; wt-SE, sexually experienced wt; wt-V, virgin wt. havior (9), a finding that is not predicted by the conflict theory (10). ‡To whom correspondence should be addressed. E-mail: [email protected]. Targeted deletion of Peg3 in either pups or independently in the © 2007 by The National Academy of Sciences of the USA 6084–6089 ͉ PNAS ͉ April 3, 2007 ͉ vol. 104 ͉ no. 14 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0609471104 Downloaded by guest on September 24, 2021 Fig. 2. Sexual behavior of wt-SE (n ϭ 16), p3-SE (n ϭ 10), wt-V (n ϭ 14), and p3-V (n ϭ 10) males in 1-h sexual behavior tests. (a) Mean latencies to mount females. wt-SE males mounted females faster than all other groups. (b and c) Frequency of mounting (b) and intromission (c) during tests. wt-SE males mounted and intromitted more frequently than all other groups. Compari- sons are with wt-SE males (** indicate P Ͻ 0.01; *** indicate P Ͻ 0.001; error bars indicate SEM). Fig. 1. Mean time spent sniffing water, male urine, and female urine by greater preference for female urine than wt-V males (P ϭ 0.004), ϭ ϭ ϭ ϭ wt-SE (n 7), p3-SE (n 7), wt-V (n 6), and p3-V (n 7) males in olfactory p3-SE males (P ϭ 0.040), and p3-V males (P ϭ 0.009), but no other tests. (a and b) Habituation–dishabituation tests. All groups sniffed the first urine stimulus more than the final water stimulus, and wt-SE, wt-V, and p3-V group differences were found. males increased their olfactory investigations when the urine types were switched. (c) Urine preference tests. Significance is represented by an asterisk Sexual Behavior Is Enhanced by Sexual Experience in wt Males but Not for comparison of interest in male and female urine and by a dagger for in Peg3-KO Males. To assess whether the Peg3-KO had effects on comparison with the preference of wt-SE males. All groups spent longer male mating behavior similar to those seen on male olfactory sniffing female urine than male urine, but wt-SE males had a significantly behavior, we tested the sexual behavior of wt-SE, wt-V, p3-SE, and greater preference for female urine than all other groups. (* and † indicate P Ͻ p3-V males. Males were paired with receptive virgin B6 females; 0.05; **, ##, and †† indicate P Ͻ 0.01; *** and ### indicate P Ͻ 0.001; and error their sexual behavior over the following hour was recorded and bars indicate SEM.) analyzed. Significantly more wt-SE males than p3-SE males mounted (P ϭ 0.018), intromitted (P Ͻ 0.001), and ejaculated (P ϭ preference for female urine over male urine in olfactory tests in 0.001) during testing; however, no other group differences in the which they could physically contact the urine and so use both the proportion of males performing sexual behavior were found. La- accessory and main olfactory systems. Habituation–dishabituation tency data were analyzed for males that performed each behavior tests were used to measure the ability of subject males to discrim- measured, but because of the very low incidence of intromission and ejaculation among p3-SE males, it was only possible to analyze inate between urine and water and between male and female urine.