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Evolution of a new function by degenerative mutation in cephalochordate steroid receptors.

Bridgham JT, Brown JE, Rodríguez-Marí A, Catchen JM, Thornton JW - PLoS Genet. (2008)

Bottom Line: BfSR is specifically activated by estrogens and recognizes estrogen response elements (EREs) in DNA; BfER does not activate transcription in response to steroid hormones but binds EREs, where it competitively represses BfSR.These results corroborate previous findings that the ancestral steroid receptor was estrogen-sensitive and indicate that, after duplication, BfSR retained the ancestral function, while BfER evolved the capacity to negatively regulate BfSR.Our findings suggest that after duplication of genes whose functions depend on specific molecular interactions, high-probability degenerative mutations can yield novel functions, which are then exposed to positive or negative selection; in either case, the probability of neofunctionalization relative to gene loss is increased compared to existing models.

View Article: PubMed Central - PubMed

Affiliation: Center for Ecology and Evolutionary Biology, University of Oregon, Eugene, Oregon, United States of America.

ABSTRACT
Gene duplication is the predominant mechanism for the evolution of new genes. Major existing models of this process assume that duplicate genes are redundant; degenerative mutations in one copy can therefore accumulate close to neutrally, usually leading to loss from the genome. When gene products dimerize or interact with other molecules for their functions, however, degenerative mutations in one copy may produce repressor alleles that inhibit the function of the other and are therefore exposed to selection. Here, we describe the evolution of a duplicate repressor by simple degenerative mutations in the steroid hormone receptors (SRs), a biologically crucial vertebrate gene family. We isolated and characterized the SRs of the cephalochordate Branchiostoma floridae, which diverged from other chordates just after duplication of the ancestral SR. The B. floridae genome contains two SRs: BfER, an ortholog of the vertebrate estrogen receptors, and BfSR, an ortholog of the vertebrate receptors for androgens, progestins, and corticosteroids. BfSR is specifically activated by estrogens and recognizes estrogen response elements (EREs) in DNA; BfER does not activate transcription in response to steroid hormones but binds EREs, where it competitively represses BfSR. The two genes are partially coexpressed, particularly in ovary and testis, suggesting an ancient role in germ cell development. These results corroborate previous findings that the ancestral steroid receptor was estrogen-sensitive and indicate that, after duplication, BfSR retained the ancestral function, while BfER evolved the capacity to negatively regulate BfSR. Either of two historical mutations that occurred during BfER evolution is sufficient to generate a competitive repressor. Our findings suggest that after duplication of genes whose functions depend on specific molecular interactions, high-probability degenerative mutations can yield novel functions, which are then exposed to positive or negative selection; in either case, the probability of neofunctionalization relative to gene loss is increased compared to existing models.

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BfER represses BfSR-mediated transcription on estrogen response elements.A) Full-length BfSR activates transcription of an ERE-driven reporter gene in the presence of 1 µM estradiol (black bars), but activation is inhibited by transfection of increasing quantities of BfER. The quantity of each plasmid transfected is indicated in ng/well. White bars, vehicle-only (no hormone) control. B) Neither BfER nor BfSR activates reporter transcription from an SRE binding site. Cells were transfected with an SRE-driven luciferase reporter and full-length BfER, BfSR, empty vector (pcDNA3), or positive control (HsGR; human GR). Black bars, hormone treatment at 1 uM; white bars, vehicle only.
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pgen-1000191-g004: BfER represses BfSR-mediated transcription on estrogen response elements.A) Full-length BfSR activates transcription of an ERE-driven reporter gene in the presence of 1 µM estradiol (black bars), but activation is inhibited by transfection of increasing quantities of BfER. The quantity of each plasmid transfected is indicated in ng/well. White bars, vehicle-only (no hormone) control. B) Neither BfER nor BfSR activates reporter transcription from an SRE binding site. Cells were transfected with an SRE-driven luciferase reporter and full-length BfER, BfSR, empty vector (pcDNA3), or positive control (HsGR; human GR). Black bars, hormone treatment at 1 uM; white bars, vehicle only.

Mentions: To determine the molecular functions of the B. floridae steroid receptors, we assayed both full-length proteins and specific functional domains in a cell culture system. Surprisingly, we found that BfSR retains the ER-like functions of the ancestral receptor, but BfER does not. BfSR's LBD activates transcription in the presence of nanomolar concentrations of estradiol and estrone, but is insensitive to ketosteroids, including a broad panel of androgens, progestins, and corticoids (Figures 3A, 3B). In the presence of estrogens, the full-length BfSR activates transcription of an ERE-driven reporter gene (Figures 4A, S5), but it does not activate genes driven by the SRE recognized by vertebrate kSRs (Figure 4B). These data indicate that the derived ligand-binding and DNA-recognition properties of the vertebrate kSRs evolved by neofunctionalization after the cephalochordate/vertebrate divergence.


Evolution of a new function by degenerative mutation in cephalochordate steroid receptors.

Bridgham JT, Brown JE, Rodríguez-Marí A, Catchen JM, Thornton JW - PLoS Genet. (2008)

BfER represses BfSR-mediated transcription on estrogen response elements.A) Full-length BfSR activates transcription of an ERE-driven reporter gene in the presence of 1 µM estradiol (black bars), but activation is inhibited by transfection of increasing quantities of BfER. The quantity of each plasmid transfected is indicated in ng/well. White bars, vehicle-only (no hormone) control. B) Neither BfER nor BfSR activates reporter transcription from an SRE binding site. Cells were transfected with an SRE-driven luciferase reporter and full-length BfER, BfSR, empty vector (pcDNA3), or positive control (HsGR; human GR). Black bars, hormone treatment at 1 uM; white bars, vehicle only.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2527136&req=5

pgen-1000191-g004: BfER represses BfSR-mediated transcription on estrogen response elements.A) Full-length BfSR activates transcription of an ERE-driven reporter gene in the presence of 1 µM estradiol (black bars), but activation is inhibited by transfection of increasing quantities of BfER. The quantity of each plasmid transfected is indicated in ng/well. White bars, vehicle-only (no hormone) control. B) Neither BfER nor BfSR activates reporter transcription from an SRE binding site. Cells were transfected with an SRE-driven luciferase reporter and full-length BfER, BfSR, empty vector (pcDNA3), or positive control (HsGR; human GR). Black bars, hormone treatment at 1 uM; white bars, vehicle only.
Mentions: To determine the molecular functions of the B. floridae steroid receptors, we assayed both full-length proteins and specific functional domains in a cell culture system. Surprisingly, we found that BfSR retains the ER-like functions of the ancestral receptor, but BfER does not. BfSR's LBD activates transcription in the presence of nanomolar concentrations of estradiol and estrone, but is insensitive to ketosteroids, including a broad panel of androgens, progestins, and corticoids (Figures 3A, 3B). In the presence of estrogens, the full-length BfSR activates transcription of an ERE-driven reporter gene (Figures 4A, S5), but it does not activate genes driven by the SRE recognized by vertebrate kSRs (Figure 4B). These data indicate that the derived ligand-binding and DNA-recognition properties of the vertebrate kSRs evolved by neofunctionalization after the cephalochordate/vertebrate divergence.

Bottom Line: BfSR is specifically activated by estrogens and recognizes estrogen response elements (EREs) in DNA; BfER does not activate transcription in response to steroid hormones but binds EREs, where it competitively represses BfSR.These results corroborate previous findings that the ancestral steroid receptor was estrogen-sensitive and indicate that, after duplication, BfSR retained the ancestral function, while BfER evolved the capacity to negatively regulate BfSR.Our findings suggest that after duplication of genes whose functions depend on specific molecular interactions, high-probability degenerative mutations can yield novel functions, which are then exposed to positive or negative selection; in either case, the probability of neofunctionalization relative to gene loss is increased compared to existing models.

View Article: PubMed Central - PubMed

Affiliation: Center for Ecology and Evolutionary Biology, University of Oregon, Eugene, Oregon, United States of America.

ABSTRACT
Gene duplication is the predominant mechanism for the evolution of new genes. Major existing models of this process assume that duplicate genes are redundant; degenerative mutations in one copy can therefore accumulate close to neutrally, usually leading to loss from the genome. When gene products dimerize or interact with other molecules for their functions, however, degenerative mutations in one copy may produce repressor alleles that inhibit the function of the other and are therefore exposed to selection. Here, we describe the evolution of a duplicate repressor by simple degenerative mutations in the steroid hormone receptors (SRs), a biologically crucial vertebrate gene family. We isolated and characterized the SRs of the cephalochordate Branchiostoma floridae, which diverged from other chordates just after duplication of the ancestral SR. The B. floridae genome contains two SRs: BfER, an ortholog of the vertebrate estrogen receptors, and BfSR, an ortholog of the vertebrate receptors for androgens, progestins, and corticosteroids. BfSR is specifically activated by estrogens and recognizes estrogen response elements (EREs) in DNA; BfER does not activate transcription in response to steroid hormones but binds EREs, where it competitively represses BfSR. The two genes are partially coexpressed, particularly in ovary and testis, suggesting an ancient role in germ cell development. These results corroborate previous findings that the ancestral steroid receptor was estrogen-sensitive and indicate that, after duplication, BfSR retained the ancestral function, while BfER evolved the capacity to negatively regulate BfSR. Either of two historical mutations that occurred during BfER evolution is sufficient to generate a competitive repressor. Our findings suggest that after duplication of genes whose functions depend on specific molecular interactions, high-probability degenerative mutations can yield novel functions, which are then exposed to positive or negative selection; in either case, the probability of neofunctionalization relative to gene loss is increased compared to existing models.

Show MeSH