Domestic Dogs (canis Familiaris,), Wolf (canis Lupus) Olfactory System

The watchdog (canis familiaris), the world's oldest domesticate, is a very close relative of the grey wolf (Canis lupus) and the two species diverged anywhere from 145 00 to 15 000 years ago. The wolf, ancestor of watchdog, whose behavior and physiological features are adapt to field lives, has their own domains that are not allowed to be invaded. In order to probe into the developmental course of watchdogs' olfactory system and their structure difference between watchdogs of different age and different sex, the methods of histological and immunohistochemical were used in this paper to detect the olfactory bulb's(OB) histological structure and the expression of c-Fos in its each layer in watchdogs of different age and different sex. We also use molecular methods to study olfactory receptor genes of watchdogs and wolves comparatively for the purpose of exploring the influence of domestication on watchdogs' olfactory receptor genes. At the same time, we compared the composition of olfactory receptor genes of watchdogs and wolves with other animals' in the interest of discussing the reasons for the powerful sense of smell held by canine. The results of our experiments showed as follows:1. The watchdogs' OB, which lies behind the ethmoid and at the frontal part of the brain, was oval. Its main structures include ONL, GL, EPL, ML, IPL and GRL. And it accepts odorant signals transmitted by olfactory receptor neurons whose axons are bundled in groups to penetrate the ethmoidal cribriform plate of bone, reaching the OB of the brain where they converge to terminate with post-synaptic cells to form synaptic structures called glomeruli.2. The appearance of primary structures in the OB of juvenile watchdogs demonstrates that they already possess the potential ability for distinguishing odorants. The results of statistical analysis told us that there was no significant difference on the width of OB's each layer and the cells' density of ONL, EPL and GRL between juvenile watchdogs of different sex (P>0.05). However, the difference on the density of GL's cells reached the remarkably significant level (P>0.01). The apparent higher density of cells in GL of female juvenile demonstrated their faster development of this layer. On the other hand, the rank of cells' density is GRL>GL>ONL>EPL.3. There was also no significant difference on the width of OB's each layer and the cells' density of ONL, EPL and GRL between adult watchdogs of different sex (P>0.05). Nevertheless, the difference of mitral cells' number and their morphology between adult males and adult females was apparent (P<0.05). Just like its name, the morphology of females' mitral cells is mitral, and it also has evident prominency and heavy color in nucleolus, whereas the color of the oval mitral cells of males is light. Mitral cells are the main efferent cells in the OB, and they are also the largest cells in the OB. The difference mentioned above might be one of the reasons for the difference of smell sensitivity between males and females as the result of the importance of mitral cells in olfactory system. In addition, the cells' density rank is the same as that in juveniles' OB.4. The comparison of OB's structure between juveniles and adults showed that the OB's weight, volume and width increased evidently with the growth of age. However, the density of cells of its each layer decreased significantly as the watchdogs grow old. This change suggested that the OB was developing during the course of watchdogs' growth.5. We found c-Fos immunoreactivity neurons (c-Fos-ir) in every layer of juveniles' OB according to the results got from the immunohistochemical experiments. The difference of the expression ratio of c-Fos in GL between female juvenile and males was at the significant level (P<0.05), but there was no significant difference on the expression ratio of c-Fos in OB' other layers between juveniles of different sex (P>0.05). c-Fos-ir neurons mainly distributed in GRL, ML and GL, and the rank of its expression ratio is ML>GRL>EPL>GL>ONL.6. We also found many c-Fos immunoreactivity neurons in every layer of adults' OB, and there was no significant difference on the expression ratio of c-Fos in OB' each layer between adult watchdogs of different sex (P>0.05). c-Fos-ir neurons mainly distributed.in GRL, ML and GL, and the rank of its expression ratio is ML>EPL>ONL>GL>GRL.7. The c-Fos-ir neurons in OB of adults were much clearer, their color was much heavier and their number was much larger than that in juveniles' OB. All of these implied that cells in juveniles' OB were during the course of development and cytogenesis, and the apoptosis event was fewer accordingly; the OB's cells of juveniles might have a weak ability for memorizing odorant information. On the other hand, cells in OB of adult watchdogs were much more active and they might have stronger memory abilities than that of juveniles.8. The results of molecular experiments illuminated that the percentage of pesudogenes in watchdogs' olfactory receptor gene repertoire was larger than that in wolves, but the difference did not reach the significant level (X2=1.388, P=0.239>0.05). This indicated that domestication did not reduce the number of functional olfactory receptor genes in watchdogs evidently as the result of the purpose of its domestication. We found that the proportion of pseudogenes in olfactory receptor gene repertoire of watchdogs and wolves were much smaller than that in other species, this demonstrated that canine have a stronger ability for discerning odorants than that of other animals.