In the Light of Evolution Volume VI: Brain and Behavior (2013) / Chapter Skim
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2 Adaptive Evolution of Voltage-Gated Sodium Channels: The First 800 Million Years--Harold H. Zakon
2 Adaptive Evolution of Voltage-Gated Sodium Channels: The First 800 Million Years--Harold H. Zakon
Pages 21-36
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From page 21... ...
, whereas vertebrate Nav channels are of the Nav1 family. Approximately 500 Mya in early chordates Nav channels evolved a motif that allowed them to cluster at axon initial segments; 50 million years later with the evolution of myelin, Nav channels "capitalized" on this property and clustered at nodes of Ranvier.
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From page 22... ...
An additional rationale is that although many proteins, such as immunoglobins, sperm and egg receptors, olfactory receptors, opsins, and surface proteins of pathogens, are routinely studied in the field of molecular evolution, only recently have ion channels begun to receive greater attention (Lopreato et al., 2001; Geffeney et al., 2005; Zakon et al., 2006, 2011; Arnegard et al., 2010; Liu et al., 2011) ; of these studies, the majority are on Nav channels.
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From page 23... ...
There is uncertainty about the origin of the 4x6TM family, which more likely evolved in eukaryotes than prokaryotes, as indicated in this figure. A more precise and detailed relationship among Cav and Nav channels in basal metazoans and their sister group, the choanoflagellates, is given in Fig.
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From page 24... ...
. Their relationship to eukaryotic Nav channels is unclear, and they will not be discussed in this review.
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From page 25... ...
. Analysis of putative Cav and Nav channel genes from fungi, choanoflagellates, and metazoans confirm this speculation and show that choanoflagellates have a channel that groups with recognized Nav channels with strong support (Fig.
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From page 26... ...
. This motif is present in the Nav channels of anthozoan cnidaria (anemones, coral)
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From page 27... ...
in the lamprey. In lampreys, and presumably nonvertebrate chordates, the high-density clustering of Nav channels adjacent to the soma ensures sufficient current injection into the high-resistance axon in the face of current shunting by the low-resistance soma (Kole et al., 2008)
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From page 28... ...
. Furthermore, an analysis of Cav, transient receptor potential, and various K+ channel subfamilies shows that there was no widespread duplication and retention of other ion channel genes in the tetrapod 6TM family since the teleost–tetrapod divergence (Zakon et al., 2011)
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From page 29... ...
There is no indication of any loss of Nav channel genes despite losses of surrounding genes in both teleosts and tetrapods. Furthermore, although not shown here, no other ion channel gene family duplicated after the teleost and tetrapod divergence.
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From page 30... ...
scn4ab, which encodes Nav1.4ab, are expressed in the muscles of teleost fish. In the two lineages of weakly electric fishes, the mormyroidea and gymnotiformes, the gene for Nav1.4a (scn4aa)
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From page 31... ...
ADAPTIVE EVOLUTION OF NAV CHANNELS: WEAKLY ELECTRIC FISH In most organisms ion channels cause behavior indirectly by triggering muscle movements. Weakly electric fish, however, emit electric signals directly into the water, and these are shaped by the biophysical properties of Nav and Kv channels in their electric organs.
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From page 32... ...
Has the duplication of a muscle-expressing Nav channel gene facilitated the evolution of multiple novel muscle-derived structures in teleosts? ADAPTIVE EVOLUTION OF NAV CHANNELS: TETRODOTOXIN RESISTANCE The best-studied cases of adaptive evolution of Nav channel genes involve the evolution of resistance to the various neurotoxins that act on Nav channels.
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From page 33... ...
Sequencing of Nav channel genes from Fugu and other pufferfishes shows that many of the same TTX-resistant amino acid substitutions have occurred multiple times in various Nav channels and lineages of pufferfishes (Yotsu-Yamashita et al., 2000; Venkatesh et al., 2005; Jost et al., 2008)
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From page 34... ...
Given that invertebrates have only a single Nav channel gene, this seems likely, and it will be interesting to see whether other invertebrate egg-predators are resistant to TTX. ADAPTIVE EVOLUTION OF NAV CHANNELS: PROTON INSENSITIVITY Naked mole rats (Heterocephalus glaber)
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From page 35... ...
It is likely that the Nav channels of choanoflagellates and early metazoans were permeable to both Na+ and Ca2+ and evolved enhanced selectivity to Na+ in parallel in early bilaterians and jellyfish. Although it is convenient to think that invertebrates possess only a single Nav1 channel gene, it is worth scouring the wealth of new genomes to determine whether there are any lineage-specific duplications, and if so, what this might mean.
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From page 36... ...
For example, fast-firing inhibitory neurons in mammals express different Nav channels than more slowly firing pyramidal neurons. Do we see a similar functional partitioning of Nav channel types in teleosts?
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