Abstract
Although modern baleen whales (Mysticeti) retain a functional olfactory system that includes olfactory bulbs, cranial nerve I and olfactory receptor genes, their olfactory capabilities have been reduced to a great degree. This reduction likely occurred as a selective response to their fully aquatic lifestyle. The glomeruli that occur in the olfactory bulb can be divided into two non-overlapping domains, a dorsal domain and a ventral domain. Recent molecular studies revealed that all modern whales have lost olfactory receptor genes and marker genes that are specific to the dorsal domain. Here we show that olfactory bulbs of bowhead whales (Balaena mysticetus) lack glomeruli on the dorsal side, consistent with the molecular data. In addition, we estimate that there are more than 4,000 glomeruli elsewhere in the bowhead whale olfactory bulb, which is surprising given that bowhead whales possess only 80 intact olfactory receptor genes. Olfactory sensory neurons that express the same olfactory receptors in rodents generally project to two specific glomeruli in an olfactory bulb, implying an approximate 1:2 ratio of the number of olfactory receptors to the number of glomeruli. Here we show that this ratio does not apply to bowhead whales, reiterating the conceptual limits of using rodents as model organisms for understanding the initial coding of odor information among mammals.
Highlights
Terrestrial mammals generally have a well-developed sense of smell that can discriminate millions of odors using hundreds or thousands of olfactory receptors (ORs) (Nei, Niimura & Nozawa, 2008)
Odorants are detected by ORs expressed in the cell membrance of the olfactory sensory neurons (OSNs), which project to the glomeruli of the olfactory bulbs (OBs)
We provide the distribution of glomeruli in bowhead whales (Balaena mysticetus) and present data that test whether the mysticete OR:glomeruli ratio compares with the 1:2 ratio observed in mice and rats
Summary
Terrestrial mammals generally have a well-developed sense of smell that can discriminate millions of odors using hundreds or thousands of olfactory receptors (ORs) (Nei, Niimura & Nozawa, 2008). Genomic analyses have revealed that all modern mysticetes lack functional OMACS genes (Kishida et al, 2015) Based on these findings, it appears that, mysticetes have fully equipped olfactory systems, their OB lacks the D domain (Kishida et al, 2015). It appears that, mysticetes have fully equipped olfactory systems, their OB lacks the D domain (Kishida et al, 2015) These molecular data suggest that mysticetes lack glomeruli on the dorsal side of their OB. We provide the distribution of glomeruli in bowhead whales (Balaena mysticetus) and present data that test whether the mysticete OR:glomeruli ratio compares with the 1:2 ratio observed in mice and rats
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