Organization and distribution of glomeruli in the bowhead whale olfactory bulb

Abstract

Although modern baleen whales (Mysticeti) retain a functional olfactory systemthatincludes olfactory bulbs, cranial nerve I and olfactory receptor genes, their olfactorycapabilities have been reduced to a great degree. This reduction likely occurred asa selective response to their fully aquatic lifestyle. The glomeruli that occur in theolfactory bulb can be divided into two non-overlapping domains, a dorsal domainand a ventral domain. Recent molecular studies revealed that all modern whaleshave lost olfactory receptor genes and marker genes that are specific to the dorsaldomain. Here we show that olfactory bulbs of bowhead whales (Balaena mysticetus)lack glomeruli on the dorsal side, consistent with the molecular data. In addition, weestimate that there are more than 4,000 glomeruli elsewhere in the bowhead whaleolfactory bulb, which is surprising given that bowhead whales possess only 80 intactolfactory receptor genes. Olfactory sensory neurons that express the same olfactoryreceptors in rodents generally project to two specific glomeruli in an olfactorybulb, implying an approximate 1:2 ratio of the number of olfactory receptors tothe number of glomeruli. Here we show that this ratio does not apply to bowheadwhales, reiterating the conceptual limits of using rodents as model organisms forunderstanding the initial coding of odor information among mammals.