1 Invertebrate Olfaction
Tom Finger
EVOLUTION OF TASTE
Thomas E. Finger
Dept. Cell & Developmental Biology
Rocky Mountain Taste & Smell Center
Univ. Colorado Medical School
Aurora CO 80045 USA
Contact:
Thomas Finger
Univ. Colorado Health Sciences Center
Room L18-11118, RC-1
12801 E. 17th Ave. MS 8108
PO Box 6511
Aurora CO 80045
<EMAIL_ADDRESS>KEYWORDS:
Amphid
Amphioxus
Cell type
Chemoreception
Drosophila
Fish
Insect
Labellum
Leech
Mammal
Nematode
Octopus
Solitary Chemoreceptor Cell
Taste Bud
Taste receptor
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SYNOPSIS: The sense of taste is well-defined only for vertebrates where there are readily
identified taste buds. These endorgans always are innervated by particular cranial nerves and
always serve in the regulation of feeding and food intake. Since invertebrates, including
amphioxus, have no such endorgans, comparisons across groups must be made on the basis of
analogy rather than homology. I adopt the term “taste” only for chemosensory endorgans
devoted to feeding-related behaviors without regard to endorgan structure, expression of
molecular receptors, chemical nature of the stimulus (e.g. volatility), or vehicle in which a
stimulus is presented (e.g. air vs water). In that context, nearly all metazoans have a sense of
taste, i.e. perioral or intraoral chemoreceptors that regulate feeding. This chapter explores the
diversity of such endorgans in both vertebrates and invertebrates. For the vertebrate lineage, this
modality is mediated by taste buds innervated only by the facial, glossopharyngeal or vagus
nerves. Taste buds in all vertebrates share many common features but two basic forms of
organization can be recognized. In non-mammalian vertebrates, taste buds consist of elongate
taste cells and Merkel-like basal cells. In contrast, taste buds in mammals contain only elongate
cells, but one of these (Type III taste cells) shares many characteristics with the Merkel-like
basal cells of non-mammalian vertebrates.
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01.1 INTRODUCTION
We humans recognize the sense of taste as those sensations arising from the oral cavity
and indicating information about the chemical quality of potential foodstuffs. The sense of taste
uniquely arises from the specialized sensory endorgans for this system: taste buds. In humans,
taste sensations are only those which we describe as salty, sweet, sour, bitter, and “umami” (the
taste of glutamate). All other oral sensations, e.g. the coolness of mint, the smoothness of fats or
the hotness of pepper, arise from the general cutaneous innervation of the epithelium and should
not be considered to be “taste”. So for humans and other vertebrates, “taste” is a system defined
by the sensory endorgans mediating the sensibility. For humans, and by extension, other
vertebrates, taste can be defined by the use of taste buds in the context of food selection.
When examining the evolution of the sense of taste, we are met with several difficulties.
First, how can the sense of taste be defined for organisms lacking taste buds and