| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Corresponding author: Jon Seger, Department of Biology, University of Utah, 257 S. 1400 East, Salt Lake City, UT 84112-0840., seger{at}bionix.biology.utah.edu (E-mail)
Communicating editor: S. YOKOYAMA
 | ABSTRACT |
|---|
 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
About 10% of mammalian odorant receptors are transcribed in testes, and odorant-receptor proteins have been detected on mature spermatozoa. Testis-expressed odorant receptors (TORs) are hypothesized to play roles in sperm chemotaxis, but they might also be ordinary nasal odorant receptors (NORs) that are expressed gratuitously in testes. Under the sperm-chemotaxis hypothesis, TORs should be subject to intense sexual selection and therefore should show higher rates of amino acid substitution than NORs, but under the gratuitous-expression hypothesis, TORs are misidentified NORs and therefore should evolve like other NORs. To test these predictions, we estimated synonymous and nonsynonymous divergences of orthologous NOR and TOR coding sequences from rat and mouse. Contrary to both hypotheses, TORs are on average more highly conserved than NORs, especially in certain domains of the OR protein. This pattern suggests that some TORs might perform internal nonolfactory functions in testes; for example, they might participate in the regulation of sperm development. However, the pattern is also consistent with a modified gratuitous-expression model in which NORs with specialized ligand specificities are both more highly conserved than typical NORs and more likely to be expressed in testes.
VERTEBRATE odorant receptors (ORs) were identified by 
There appear to be many fewer TORs (
50 per species in rodents;
|
Why are odorant receptors expressed in the testis? The scattered phylogenetic distribution of TORs within the OR gene family can be taken to support either of two artifactual explanations: first, that TORs are ordinary NORs transcribed gratuitously in the testis but not performing any function there; and second, that most putative testis "cDNA" clones are amplified from contaminating genomic DNA (R. AXEL, personal communication; but see
Alternatively, TOR proteins might mediate sperm chemotaxis, as suggested by
To test these predictions, we compared the evolution of orthologous NOR and TOR genes in rat and mouse. We found, to our surprise, that the amino acid sequences of 10 TORs are more highly conserved, on average, than those of 8 NORs. The greater conservation of TORs is concentrated in the extracellular end of the fourth transmembrane domain (TM4, thought to be involved in ligand binding) and the third intracellular loop (IC3, which interacts with G-proteins). This distinctive pattern of amino acid substitution contradicts straightforward predictions of the gratuitous-expression, nonexpression, and sperm-chemotaxis hypotheses. We consider two alternative models that might explain the relatively stringent conservation of at least some TORs. In the first model, some TORs are recruited to novel internal (developmental or physiological) functions that differ from those performed by canonical nasal odorant receptors. In the second model, some NORs evolve highly focused specificities for odorants of special ecological importance; this tight focus on single ligands causes these specialized NORs to evolve relatively slowly at the amino acid sequence level and to acquire increased levels of expression in the olfactory epithelium; as a side effect, they are at greater than average risk of being transcribed gratuitiously in nonolfactory tissues, especially the testis. We discuss the kinds of evidence needed to test these two models.
A phylogenetic analysis of 160 paralogous OR genes confirms that TOR and NOR lineages interdigitate extensively and suggests that recruitment between nasal and testicular expression patterns may occur in both directions. However, small differences in the assumed ease of recruitment in each direction lead to large differences in the estimated numbers of N 
T and T N recruitments. In the future, when patterns of nasal and testicular expression have been documented for many orthologs and closely related paralogs in species representing a range of divergence times, it should become possible to resolve histories of expression and of amino acid sequence change with enough precision to say whether sequence conservation precedes or follows expression in the testis. In either case, conserved ORs could be important to the evolution of the odorant-receptor gene family as a whole if they periodically give rise (by duplication or gene conversion) to new NOR lineages of greater average longevity than those derived from typical, less well-conserved NORs.
| MATERIALS AND METHODS |
|---|
| TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Sequence names:
We add lowercase species prefixes to names that do not otherwise indicate their source (m, mouse; r, rat; h, human; d, dog; p, pig), and we shorten some names. Thus F6 becomes rF6 and MTPCR09 becomes mT09, but CfOLF1 (from dog, Canis familiaris) remains CfOLF1. A convenient feature of this system is that corresponding postfixes can be used to identify orthologs (e.g., mT09r is the rat ortholog of mT09m).
PCR and sequencing:
Published NOR or TOR cDNA sequences from rat or mouse were used to design primers that specifically amplify both the original sequence (in the source species) and a presumptive ortholog (in the other species) from genomic DNA (Sprague-Dawley rat or BALB/c mouse; Clontech, Palo Alto, CA). Several partial TOR sequences were extended to nearly full length by inverse PCR. Primer sequences and reaction conditions can be obtained from the first author (A.B.). PCR products were sequenced directly on ABI (Columbia, MD) 373 and 377 automated fluorescent sequencing machines.
The original sequences are rF6 (M64378), rF12 (M64381), rI8 (M64387), rI9 (M64388), mK7 (L14566), mK20 (U28770), mM31 (U28777), mT09 (X89681), mT15 (X89683), mT33 (X89685), rT07 (X89697), rT09 (X89698), and rT18 (X89702). We redetermined all except rI9 as positive controls, and these differ by no more than three nucleotide substitutions from the originals. Several genes from each tissue are represented by nearly full-length coding sequences (TM1 through TM7), and several are represented by shorter sequences. Five genes are represented by published sequences: the apparently orthologous relationships of mOR3/rT44 (M84005, X89706), rT05/mT07 (X89695, X89680), rT19/mT18 (X89703, X89-684), and rT38/mT53 (X89705, X89691) were noted independently by us and by
Sequence divergence:
Synonymous (KS) and nonsynonymous (KA) substitutions were estimated by the method of
Phylogeny:
We aligned DNA sequences for mammalian odorant receptors derived from tissue-specific cDNA libraries, or for which a site of expression was determined directly. We reduced sets of alleles, closely related paralogs, and obvious orthologs to a single representative each. Many of the sequences are fragments representing one-third to one-half of a complete coding region; our alignment comprises a 112-codon region of shared homology that begins immediately after the MAYDRYVAIC motif at the boundary between TM3 and IC2, which is frequently used as a binding site for degenerate primers. One hundred and sixty sequences (94 NORs and 66 TORs) are included. Accession numbers and the alignment can be obtained from the second author (J.S.). Phylogenetic relationships were estimated by DNA maximum-likelihood analysis as implemented in DNAML 3.6 (
| RESULTS |
|---|
| TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Divergence between orthologs:
Synonymous substitution rates vary widely among hundreds of genes that have been sequenced in rat and mouse (
|
First, some TORs are highly conserved. Three of the 10 in Table 1 have KA/KS ratios <0.05, and 1 has identical amino acid sequences in rat and mouse, with a KA/KS <0.01 despite a typical level of synonymous divergence. The most highly conserved NOR shows a KA/KS of 0.07 and 5 amino acid differences; a typical full-length NOR (KA/KS 
0.20) would show 21 amino acid differences. For comparison, a sample of 14 other (non-OR) GPCR rat-mouse ortholog pairs from the GPCR database shows a mean (combined) KA/KS of 0.13 (data not shown), and a sample of 470 rat-mouse ortholog pairs of all kinds shows a mean KA/KS of 0.19, with 23% of the individual values <0.05 and 6% <0.01 (
Second, the average relative difference between NORs and TORs increases when the samples are culled to remove ortholog pairs with synonymous divergences (KS) >0.2 (slightly above the mean KS for all rat-mouse comparisons). The resulting samples, restricted to include only pairs with KS 
0.2, should be relatively unlikely to include putative ortholog pairs that are, in fact, closely related paralogs. If the overall nonsynonymous difference between NORs and TORs had been caused by a greater number of misidentified paralogs in the NOR sample, then the exclusion of pairs with KS > 0.2 should have reduced the difference between the samples rather than increased it.
Third, each of three TOR ortholog pairs is significantly more strongly conserved than a closely related NOR pair (Fig 1 and Fig 2). These comparisons are of particular interest because, in principle, some odorant-receptor subfamilies might be inherently less tolerant of amino acid substitutions than others; if our 10 orthologous TOR pairs happened to be sampled largely from such subfamilies, then their slower evolution might have nothing to do with expression in the testis (L. B. BUCK, personal communication). On this hypothesis, there should be a large phylogenetic component to the variation in KA/KS ratios. In particular, closely related NOR/TOR pairs should show similar ratios. This specific prediction is not satisfied by the three such cases in our data set (Fig 2), and the more general prediction is not supported by the overall distribution of KA/KS ratios on the phylogeny (Fig 1). In principle, the high average KA/KS ratios of NORs might be an artifact if some are pseudogenes, but this seems unlikely because all show ratios significantly <1.0, and all show several strictly conserved amino acids shared by other odorant receptors (data not shown).
|
Fourth, amino acid substitutions are distributed heterogeneously among functional domains of the OR protein, both in NORs and TORs (Table 2), but the pattern differs significantly between the two tissues (Table 3). For example, the fourth transmembrane domain (TM4) includes amino acid positions that vary extensively among paralogous members of the OR family; this variation is concentrated in the extracellular end of the domain and along the face of the 
-helix that is inferred to orient inward toward TM5 and to participate in ligand binding ( 0.2), only 6 amino acid substitutions occur in the TM4 domains of eight TORs, but 12 occur in five NORs; all 6 of the TOR substitutions are located near the cytoplasmic end of TM4, while the 12 NOR substitutions are distributed roughly uniformly along the length of TM4 (data not shown). TM1 and especially IC3 also appear to be more strongly conserved in TORs than in NORs (Table 2).
|
|
In summary, some of the TORs in our sample appear to have experienced stronger or more constant purifying selection than typical NORs, especially in certain functional domains of the protein. This distinctive history of selection appears to be inconsistent with the sperm-chemotaxis hypothesis and with at least the simplest version of the gratuitous-expression hypothesis.
Phylogenetic distribution of testis expression:
NOR and TOR lineages interdigitate extensively in the odorant receptor phylogeny (Fig 1 Fig 2 Fig 3), suggesting that recruitments between tissues have occurred on many occasions and in both directions. This pattern is somewhat surprising. If TORs were functionally distinct from NORs then they might be expected to be evolutionarily distinct as well; they might be expected to derive from one or a few recruitments and thus to form one or a few clades adjacent to or nested within the larger family of NORs. However, if TORs are simply misidentified NORs, then they should be scattered randomly through the family.
|
In fact, the observed distribution appears to be nearly random. When the history of expression is estimated by parsimony analysis ( TOR derivations is expected under the random-assignment null model because there are many more NORs than TORs on the tree; nasal expression therefore tends to reconstruct as ancestral under the default assumption that recruitments in either direction are equally likely a priori.
However, there is no reason to expect recruitments in both directions to occur with equal ease. If the penalty for NOR TOR changes is increased from 1 evolutionary step to 1.2 steps, then the most parsimonious expression history includes just 18 NOR TOR recruitments and the number of TOR NOR recruitments increases to 34, despite the numerical preponderance of NORs. An equivalent (20%) penalty bias in the other direction gives 48 unambiguous NOR TOR recruitments and 2 unambiguous TOR NOR recruitments (Fig 3). In the absence of detailed information on patterns of expression for orthologs and for entire subfamilies of paralogs in several species, we doubt that robust estimates of these relative recruitment probabilities can be made.
It is surprising (on any hypothesis) to find the known TORs distributed as randomly on the OR phylogeny as they are, because most of them were cloned in one laboratory during what amounts to one study (
| DISCUSSION |
|---|
| TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Testis-expressed odorant receptors appear to represent a population of genes that differs in significant respects from the population of canonical nasal odorant receptors. Typical TORs appear to evolve more slowly than typical NORs, especially in certain domains of the OR protein, and some are very well conserved between rat and mouse. These differences contradict predictions derived from both the sperm-chemotaxis and the gratuitous-expression hypotheses. The most obvious alternative explanation is that at least some TORs perform nonolfactory internal functions and that selection for precise or efficient performance of such functions conserves the amino acid sequences of these TORs. Compared to many kinds of genes, typical NORs evolve fairly rapidly. This could reflect changing olfactory environments, or weak selective constraints associated with the broadly overlapping ligand specificities of most NORs (see below), or both.
The hypothesis that nasal odorant receptors might occasionally be recruited to participate in various developmental processes (e.g., sperm maturation) seems plausible given that NORs mediate axonal pathfinding by olfactory sensory neurons (1% of mammalian genes and undoubtedly possess diverse functional properties, so perhaps it should be expected that some members of this huge family would acquire novel functions in nonolfactory tissues (
But might the testis expression of conserved TORs be a consequence of their conservation, rather than their conservation being a consequence of nonolfactory functions? We considered several models that reverse the chain of causation in this way. Most rest on questionable assumptions, but one seems plausible. It begins with the observation that some odorants are likely to be ecologically critical, in the sense that an individual's fitness will depend strongly on detecting these odorants at low concentrations and distinguishing them accurately from other, chemically similar odorants. In response to selection for "focused" olfactory functions, some NORs would become specialized for detection of certain odorants, to the exclusion of others. Such specialist NORs would tend to evolve relatively slowly after they became optimized for detection of single odorants. Such NORs might also acquire relatively high levels of expression in the olfactory epithelium so as to lower the animal's threshold for detection of the critical odorants, either by increasing receptor concentrations within individual sensory neurons or by increasing the numbers of neurons that select these NORs for expression. As an incidental consequence of the properties that lead to high expression in the olfactory epithelium, some such odorant-specialist NORs might come to be expressed gratuitously in testes (see below). Odorant receptors cloned from testis cDNA libraries therefore would tend to show greater than average levels of amino acid sequence conservation.
This model rests on testable assumptions. For example, it assumes that odorant receptors vary in their degrees of ligand specificity. Typically only one odorant receptor is expressed in a given olfactory sensory neuron, and neurons expressing a given receptor are distributed more or less randomly within one of several zones in the olfactory epithelium (
The assumption that different NORs might be expressed at different levels in the olfactory epithelium also seems plausible, and some evidence supports it. Widely varying numbers of olfactory neurons are labeled by in situ hybridizations with probes made from different odorant-receptor genes (e.g.,
The focused-olfaction model predicts that odorants recognized by highly conserved receptors will be ones of special ecological importance; this implies that such odorants signal either great danger or great opportunity. The identities of such odorants, once determined, should be consistent with this interpretation. For example, animals exposed to such odorants might show signs of unusual agitation or interest in the source of the odor. These predicted associations could also be pursued in the opposite direction, from an ecologically derived understanding of significant odorants, toward an analysis of the expression, tuning, and evolutionary conservation of their receptors. Related species with similar ecologies should tend to share more of these "most significant" odorants than equally related species with dissimilar ecologies; thus, under the focused-olfaction model (but not under the internal-function model), ecologically similar species should share greater numbers of highly conserved TORs than ecologically dissimilar species, all else being equal.
Male and female ecologies often differ, especially with respect to reproductive strategies. The focused-olfaction model therefore suggests that some specialist odorant receptors might be expressed mainly or exclusively in the members of one sex. Indeed, a potential mechanistic explanation for the gratuitous expression of some conserved ORs in testes might be that they are specialized to detect odorants of particular importance to males and that their expression tends to increase in response to androgens (appropriately in the olfactory epithelium, gratuitously in the testes). Sexually dimorphic expression of conserved odorant receptors would support the focused-olfaction model, but dimorphic expression is not a strong or necessary prediction of the model in its general form. Many components of the RNA polymerase II transcription complex appear to be expressed at high levels during the early haploid phases of meiosis, giving rise to "a permissive environment for transcription initiation" (
Four TORs in our data set (rT19/mT18, rT09, rT05/mT07, and rT38/mT53) were tested for expression in the olfactory epithelia of mice and/or rats by
Distinctive predictions can be derived also from the internal-function model. For example, mice carrying targeted disruptions of conserved TORs (or carrying constructs that overexpress such genes) might show reproductive deficits either in traditional phenotypic screens or in tests of reproductive success in competitive seminatural social settings, which can reveal subtle functional differences that would otherwise be difficult to detect (
If typical NORs respond to several different odorants, and if typical odorants stimulate sensory neurons expressing several different NORs, then most NORs are somewhat functionally redundant. How can such a large set of individually nonessential genes be maintained? As subfamilies of NORs diversify, selection on their individual member genes should often become rather weak. Disabling mutations of weakly selected ORs might therefore be carried to fixation at relatively high rates, converting formerly functional NORs to pseudogenes. NOR "deaths" would need to be replenished from some source. To the degree that successful new NOR lineages tend to derive from relatively well-conserved sequences, and to the degree that the birth-and-death process turns over quickly, most functional NORs might come to descend (if not immediately, then at modest removes) from a relatively small number of highly conserved (perhaps often testis-expressed) ancestors. In this model, conserved OR lineages (whatever the reason for their conservation) would form the "trunks" of most subfamily gene trees and would thereby contribute strongly to the shape and diversity of the entire odorant-receptor family.
| ACKNOWLEDGMENTS |
|---|
We thank Fred Adler, Richard Axel, Linda Buck, Joshua Cherry, Patrice Corneli, Joe Dickinson, Maria Ganfornina, Barbara Graves, Glenn Herrick, Stephen Heinemann, Laurie Issel-Tarver, Kevin Johnson, David Mason, Dustin Penn, Wayne Potts, John Roth, Victoria Rowntree, Diego Sanchez, Pierre Vanderhaeghen, Lisa Vawter, Loren Walensky, David Witherspoon, and two anonymous reviewers for helpful discussions and for comments on the manuscript. We thank Joe Felsenstein for the prerelease version of DNAML 3.6, and we thank Doron Lancet and Yitzhak Pilpel for a preprint of
Manuscript received January 10, 2000; Accepted for publication June 15, 2000.
| LITERATURE CITED |
|---|
| TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
ABE, K., Y. KUSAKABE, K. TANEMURA, Y. EMORI, and S. ARAI, 1993 Multiple genes for G protein-coupled receptors and their expression in lingual epithelia. FEBS Lett. 316:253-256[Medline].
ANDERSSON, M., 1994 Sexual Selection. Princeton University Press, Princeton, NJ.
ARNQVIST, G., 1998 Comparative evidence for the evolution of genitalia by sexual selection. Nature 393:784-786.
ASAI, H., H. KASAI, Y. MATSUDA, N. YAMAZAKI, and F. NAGAWA et al., 1996 Genomic structure and transcription of a murine odorant receptor gene: differential initiation of transcription in the olfactory and testicular cells. Biochem. Biophys. Res. Commun. 221:240-247[Medline].
BUCK, L., 2000 The molecular architecture of odor and pheromone sensing in mammals. Cell 100:611-618[Medline].
BUCK, L. and R. AXEL, 1991 A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65:175-187[Medline].
CHESS, A., I. SIMON, H. CEDAR, and R. AXEL, 1994 Allelic inactivation regulates olfactory receptor gene expression. Cell 78:823-834[Medline].
CLARK, A. G., D. J. BEGUN, and T. PROUT, 1999 Female x male interactions in Drosophila sperm competition. Science 283:217-220
DREYER, W. J., 1998 The area code hypothesis revisited: olfactory receptors and other related transmembrane receptors may function as the last digits in a cell surface code for assembling embryos. Proc. Natl. Acad. Sci. USA 95:9072-9077
DRUTEL, G., J. M. ARRANG, J. DIAZ, C. WISNEWSKY, and K. SCHWARTZ et al., 1995 Cloning of OL1, a putative olfactory receptor and its expression in the developing rat heart. Recept. Channels 3:33-40[Medline].
DUCHAMP-VIRET, P., M. A. CHAPUT, and A. DUCHAMP, 1999 Odor response properties of rat olfactory receptor neurons. Science 284:2171-2174
EBERHARD, W. G., 1996 Female Control: Sexual Selection by Cryptic Female Choice. Princeton University Press, Princeton, NJ.
EBRAHIMI, A. W. and A. CHESS, 2000 Olfactory neurons are interdependent in maintaining axonal projections. Curr. Biol. 10:219-222[Medline].
FELSENSTEIN, J., 1981 Evolutionary trees from DNA sequences: a maximum likelihood approach. J. Mol. Evol. 17:368-376[Medline].
FELSENSTEIN, J., 1989 PHYLIP—phylogeny inference package (version 3.2). Cladistics 5:164-166.
FREITAG, J., G. LUDWIG, I. ANDREINI, P. RÖSSLER, and H. BREER, 1998 Olfactory receptors in aquatic and terrestrial vertebrates. J. Comp. Physiol. A 183:635-650[Medline].
HOLLAND, B. and W. R. RICE, 1999 Experimental removal of sexual selection reverses intersexual antagonistic coevolution and removes a reproductive load. Proc. Natl. Acad. Sci. USA 96:5083-5088
KRAUTWURST, D., K. W. YAU, and R. R. REED, 1998 Identification of ligands for olfactory receptors by functional expression of a receptor library. Cell 95:917-926[Medline].
LEVY, N. S., H. A. BAKALYAR, and R. R. REED, 1991 Signal transduction in olfactory neurons. J. Ster. Biochem. Mol. Biol. 39:633-637[Medline].
LI, W.-H., 1993 Unbiased estimation of the rates of synonymous and nonsynonymous substitution. J. Mol. Evol. 36:96-99[Medline].
LI, W.-H., 1997 Molecular Evolution. Sinauer, Sunderland, MA.
MADDISON, W. P., and D. R. MADDISON, 1992 MacClade: Analysis of Phylogeny and Character Evolution, Version 3. Sinauer, Sunderland, MA.
MAKALOWSKI, W. and M. S. BOGUSKI, 1998 Evolutionary parameters of the transcribed mammalian genome: an analysis of 2,820 orthologous rodent and human sequences. Proc. Natl. Acad. Sci USA 95:9407-9412
MALNIC, B., J. HIRONO, T. SATO, and L. B. BUCK, 1999 Combinatorial receptor codes for odors. Cell 96:713-723[Medline].
MATARAZZO, V., A. TIRARD, M. RENUCCI, A. BELAICH, and J. L. CLEMENT, 1998 Isolation of putative olfactory receptor sequences from pig nasal epithelium. Neurosci. Lett. 249:87-90[Medline].
MCVEAN, G. T. and L. D. HURST, 1997 Evidence for a selectively favourable reduction in the mutation rate of the X chromosome. Nature 386:388-392[Medline].
METZ, E. C., R. ROBLES-SIKISAKA, and V. D. VACQUIER, 1998 Nonsynonymous substitution in abalone sperm fertilization genes exceeds substitution in introns and mitochondrial DNA. Proc. Natl. Acad. Sci. USA 95:10676-10681
MOMBAERTS, P., 1999a Molecular biology of odorant receptors in vertebrates. Annu. Rev. Neurosci. 22:487-509[Medline].
MOMBAERTS, P., 1999b Seven-transmembrane proteins as odorant and chemosensory receptors. Science 286:707-711
MOMBAERTS, P., F. WANG, C. DULAC, S. K. CHAO, and A. NEMES et al., 1996 Visualizing an olfactory sensory map. Cell 87:675-686[Medline].
NEF, S. and P. NEF, 1997 Olfaction: transient expression of a putative odorant receptor in the avian notochord. Proc. Natl. Acad. Sci. USA 94:4766-4771
NEF, P., I. HERMANS-BORGMEYER, H. ARTIÈRES-PIN, L. BEASLEY, and V. E. DIONNE et al., 1992 Olfaction in birds: differential embryonic expression of nine putative odorant receptor genes in the avian olfactory system. Proc. Natl. Acad. Sci. USA 89:8948-8952
NIKAIDO, M., A. P. ROONEY, and N. OKADA, 1999 Phylogenetic relationships among cetartiodactyls based on insertions of short and long interspersed elements: hippopotamuses are the closest extant relatives of whales. Proc. Natl. Acad. Sci. USA 96:10261-10266
OELSCHLÄGER, H. A., 1989 Early development of the olfactory and terminalis systems in baleen whales. Brain Behav. Evol. 34:171-183[Medline].
PAMILO, P. and N. O. BIANCHI, 1993 Evolution of the Zfx and Zfy genes: rates and interdependence between the genes. Mol. Biol. Evol. 10:271-281[Abstract].
PARMENTIER, M., F. LIBERT, S. SCHURMANS, S. SCHIFFMANN, and A. LEFORT et al., 1992 Expression of members of the putative olfactory receptor gene family in mammalian germ cells. Nature 355:453-455[Medline].
PARTRIDGE, L. and L. D. HURST, 1998 Sex and conflict. Science 281:2003-2008
PILPEL, Y. and D. LANCET, 1999 The variable and conserved interfaces of modeled olfactory receptor proteins. Protein Sci. 8:969-977[Abstract].
POTTS, W. K., C. MANNING, and E. K. WAKELAND, 1994 The role of infectious disease, inbreeding and mating preferences in maintaining MHC genetic diversity: an experimental test. Philos. Trans. R. Soc. Lond. Ser. B 346:369-378[Medline].
RAMING, K., S. KONZELMANN, and H. BREER, 1998 Identification of a novel G-protein coupled receptor expressed in distinct brain regions and a defined olfactory zone. Recept. Channels 6:141-151[Medline].
RESSLER, K. J., S. L. SULLIVAN, and L. B. BUCK, 1993 A zonal organization of odorant receptor gene expression in the olfactory epithelium. Cell 73:597-609[Medline].
RESSLER, K. J., S. L. SULLIVAN, and L. B. BUCK, 1994 Information coding in the olfactory system: evidence for a stereotyped and highly organized epitope map in the olfactory bulb. Cell 79:1245-1255[Medline].
RICE, W. R., 1996 Sexually antagonistic male adaptation triggered by experimental arrest of female evolution. Nature 381:232-234[Medline].
RICE, W. R., 1998 Male fitness increases when females are eliminated from gene pool: implications for the Y chromosome. Proc. Natl. Acad. Sci. USA 95:6217-6221
RUBIN, B. D. and L. C. KATZ, 1999 Optical imaging of odorant representation in the mammalian olfactory bulb. Neuron 23:499-511[Medline].
SCHMIDT, E. E., 1996 Transcriptional promiscuity in testes. Curr. Biol. 6:768-769[Medline].
SOKAL, R. R., and J. F. ROHLF, 1995 Biometry: The Principles and Practice of Statistics in Biological Research, Third Edition. Freeman, New York.
SULLIVAN, S. L., M. C. ADAMSON, K. J. RESSLER, C. A. KOZAK, and L. B. BUCK, 1996 The chromosomal distribution of mouse odorant receptor genes. Proc. Natl. Acad. Sci. USA 93:884-888
TOUHARA, K., S. SENGOKU, K. INAKI, A. TSUBOI, and J. HIRONO et al., 1999 Functional identification and reconstitution of an odorant receptor in single olfactory neurons. Proc. Natl. Acad. Sci. USA 96:4040-4045
TSAUR, S.-C. and C. I. WU, 1997 Positive selection and the molecular evolution of a gene of male reproduction, Acp 26Aa of Drosophila.. Mol. Biol. Evol. 14:544-549[Abstract].
VACQUIER, V. D., 1998 Evolution of gamete recognition proteins. Science 281:1995-1998
VANDERHAEGHEN, P., S. SCHURMANS, G. VASSART, and M. PARMENTIER, 1993 Olfactory receptors are displayed on dog mature sperm cells. J. Cell Biol. 123:1441-1452
VANDERHAEGHEN, P., S. SCHURMANS, G. VASSART, and M. PARMENTIER, 1997 Specific repertoire of olfactory receptor genes in the male germ cells of several mammalian species. Genomics 39:239-246[Medline].
VASSAR, R., J. NGAI, and R. AXEL, 1993 Spatial segregation of odorant receptor expression in the mammalian olfactory epithelium. Cell 74:309-318[Medline].
WALENSKY, L. D., J. ROSKAMS, R. J. LEFKOWITZ, S. H. SNYDER, and G. V. RONNETT, 1995 Odorant receptors and desensitization proteins colocalize in mammalian sperm. Mol. Med. 1:130-141[Medline].
WALENSKY, L. D., M. RUAT, R. E. BAKIN, S. BLACKSHAW, and G. V. RONNETT et al., 1998 Two novel odorant receptor families expressed in spermatids undergo 5'-splicing. J. Biol. Chem. 273:9378-9387
WANG, F., A. NEMES, M. MENDELSOHN, and R. AXEL, 1998 Odorant receptors govern the formation of a precise topographic map. Cell 93:47-60[Medline].
WOLFE, K. H. and P. M. SHARP, 1993 Mammalian gene evolution: nucleotide sequence divergence between mouse and rat. J. Mol. Evol. 37:441-456[Medline].
WOLFE, K. H., P. M. SHARP, and W.-H. LI, 1989 Mutation rates differ among regions of the mammalian genome. Nature 337:283-285[Medline].
YOKOYAMA, S., and W. T. STARMER, 1996 Evolution of G-protein coupled receptor superfamily, pp. 93–119 in Human Genome Evolution, edited by M. S. JACKSON, G. DOVER and T. STRACHAN. BIOS Scientific, Oxford.
This article has been cited by other articles:
 |
|
 |
|
  S. S. Steiger, A. E. Fidler, and B. Kempenaers Detection of Olfactory Receptor Transcripts in Bird Testes J. Hered., June 20, 2008; (2008) esn051v1. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. D. Emes, S. A. Beatson, C. P. Ponting, and L. Goodstadt Evolution and Comparative Genomics of Odorant- and Pheromone-Associated Genes in Rodents Genome Res., April 1, 2004; 14(4): 591 - 602. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. Amadou, R. M. Younger, S. Sims, L. H. Matthews, J. Rogers, A. Kumanovics, A. Ziegler, S. Beck, and K. Fischer Lindahl Co-duplication of olfactory receptor and MHC class I genes in the mouse major histocompatibility complex Hum. Mol. Genet., November 15, 2003; 12(22): 3025 - 3040. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Volz, A. Ehlers, R. Younger, S. Forbes, J. Trowsdale, D. Schnorr, S. Beck, and A. Ziegler Complex Transcription and Splicing of Odorant Receptor Genes J. Biol. Chem., May 23, 2003; 278(22): 19691 - 19701. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. M. Young and B. J. Trask The sense of smell: genomics of vertebrate odorant receptors Hum. Mol. Genet., May 15, 2002; 11(10): 1153 - 1160. [Abstract] [Full Text] [PDF]
|
|
| ||||||||
= 0.12). The TOR pair (rT18) shows typical synonymous divergence (KS = 0.18) and extremely strong amino acid conservation (no amino acid differences in 263 codons; 
values