The development of an asymmetric accessory olfactory bulb in Octodon degus and the influence of social behavior in its establishment

Abstract

The vomeronasal system (VNS) has been extensively associated with the regulation of sexual and social behaviors in mammals. This system comprises two types of vomeronasal neurons sending parallel projections from the vomeronasal organ (VNO) to the anterior and posterior portions of the accessory olfactory bulb (aAOB and pAOB, respectively). Members of caviomorph rodents develop either a larger aAOB or pAOB bias, which has been associated with ecological traits related to the pheromonal communication displayed by these species. However, it is still unknown how and when such AOB asymmetries emerge and whether experience may influence their establishment. To explore these factors, we studied the development of the VNS in Octodon degus, a caviomorph that exhibits an asymmetric AOB, in which the aAOB is larger and has more glomeruli than the pAOB. We found that both VNO and AOB are present at birth, but exhibiting substantial immature traits. Both VNS structures develop postnatally, showing mature patterns by the end of the first postnatal month. By the second postnatal week, aAOB becomes larger in size than pAOB, however, in the glomerular layer such bias appears later, by the end of the first postnatal month. The AOB asymmetry increases in extent until adulthood, as in this period the pAOB shows a minor growing rate than the aAOB. Additionally, we found that animals raised in restricted social contexts displayed a reduced AOB asymmetry in comparison to wild degus. Overall, we suggest that social experiences contribute to the development of the AOB asymmetry in degus. Key pointsO_LIIn degus, the vomeronasal organ and the accessory olfactory bulb are present by birth, both displaying immature anatomical traits. More mature, adult-like vomeronasal structures are suggested to be present by the end of the first postnatal month. C_LIO_LIA bias toward a larger volume of the anterior portion of the accessory olfactory bulb compared to its posterior portion, as reported in adult degus, emerges within the first postnatal month, but continues to increase in extent until adulthood. C_LIO_LIThe AOB asymmetry is reduced in captivity-reared compared to wild-captured degus and it is absent entirely in individuals raised in isolation conditions. C_LI