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Adaptive melanin response of the soil fungus Aspergillus niger to UV radiation stress at "Evolution Canyon", Mount Carmel, Israel.

Singaravelan N, Grishkan I, Beharav A, Wakamatsu K, Ito S, Nevo E - PLoS ONE (2008)

Bottom Line: We test the working hypothesis that the AS strains exhibit higher melanin content than strains from the ES resulting in higher UV resistance.The results indicated that mean conidial melanin concentration of AS strains were threefold higher than ES strains and the former resisted UVA irradiation better than the latter.We conclude that melanin in A. niger is an adaptive trait against UVR generated by natural selection.

View Article: PubMed Central - PubMed

Affiliation: Institute of Evolution, University of Haifa, Mount Carmel, Israel.

ABSTRACT

Background: Adaptation is an evolutionary process in which traits in a population are tailored by natural selection to better meet the challenges presented by the local environment. The major discussion relating to natural selection concerns the portraying of the cause and effect relationship between a presumably adaptive trait and selection agents generating it. Therefore, it is necessary to identify trait(s) that evolve in direct response to selection, enhancing the organism's fitness. "Evolution Canyon" (EC) in Israel mirrors a microcosmic evolutionary system across life and is ideal to study natural selection and local adaptation under sharply, microclimatically divergent environments. The south-facing, tropical, sunny and xeric "African" slope (AS) receives 200%-800% higher solar radiation than the north-facing, temperate, shady and mesic "European" slope (ES), 200 meters apart. Thus, solar ultraviolet radiation (UVR) is a major selection agent in EC influencing the organism-environment interaction. Melanin is a trait postulated to have evolved for UV-screening in microorganisms. Here we investigate the cause and effect relationship between differential UVR on the opposing slopes of EC and the conidial melanin concentration of the filamentous soil fungus Aspergillus niger. We test the working hypothesis that the AS strains exhibit higher melanin content than strains from the ES resulting in higher UV resistance.

Methodology/principal findings: We measured conidial melanin concentration of 80 strains from the EC using a spectrophotometer. The results indicated that mean conidial melanin concentration of AS strains were threefold higher than ES strains and the former resisted UVA irradiation better than the latter. Comparisons of melanin in the conidia of A. niger strains from sunny and shady microniches on the predominantly sunny AS and predominantly shady ES indicated that shady conditions on the AS have no influence on the selection on melanin; in contrast, the sunny strains from the ES displayed higher melanin concentrations.

Conclusions/significance: We conclude that melanin in A. niger is an adaptive trait against UVR generated by natural selection.

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Related in: MedlinePlus

Relative culturability of A. niger strains after 4 h exposure to UVA irradiation plotted against conidial melanin concentration.Regression equations: linear for AS strains (open circles) y = 34.383x+68.414, R2 = 0.9166, P<0.001), and logarithmic curve for ES strains (solid circles) y = 16.334Ln(x)+100.67 R2 = 0.9187: p<0.001).
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pone-0002993-g002: Relative culturability of A. niger strains after 4 h exposure to UVA irradiation plotted against conidial melanin concentration.Regression equations: linear for AS strains (open circles) y = 34.383x+68.414, R2 = 0.9166, P<0.001), and logarithmic curve for ES strains (solid circles) y = 16.334Ln(x)+100.67 R2 = 0.9187: p<0.001).

Mentions: To investigate whether the adaptation of higher melanin concentration in the AS strains indeed leads to increased fitness, we studied the UV-resistance potential of selected strains (covering stations and habitats) from both slopes. The relative culturability after UVA irradiation of the AS strains (79.9%±1.36) was significantly higher than that of the ES strains (57.75%±3.71) (non-parametric Mann-Whitney rank sum test: T = 474, n1 = 18, n2 = 19, P<0.001) (Fig. 2). However, plots and regressions indicated that these differences were dependent on the positive relationship between relative conidial culturability and the concentration of melanin. The melanin content (A420/106) values, within a range between 0.2 and 0.4, of strains from both slopes exhibit a fairly equal range of relative conidial culturability (75%–85%). The positive relationship between relative conidial culturability (after exposure to UVA) and concentration of melanin was indicated also by linear regression for both sunny and shady microniches. Regression equations for AS strains (Fig. 3A) were: y = 71.42+26.97x, R2 = 0.884, p<0.0001 in the sunny microniche; y = 66.31+38.75x, R2 = 0.973, p = 0 .0003 in the shady microniche. The slopes of the two regression lines were significantly different (t = 2.71, df = 14, P = 0.006). For ES strains (Fig. 3B): y = 45.46+119.57x, R2 = 0.801, p = 0.007 in the sunny microniche; y = 31.75+343.02x, R2 = 0.9089, p<0.0001 in the shady microniche. Slopes of the two regression lines were significantly different (t = 4.54, df = 15, P = 0.0004).


Adaptive melanin response of the soil fungus Aspergillus niger to UV radiation stress at "Evolution Canyon", Mount Carmel, Israel.

Singaravelan N, Grishkan I, Beharav A, Wakamatsu K, Ito S, Nevo E - PLoS ONE (2008)

Relative culturability of A. niger strains after 4 h exposure to UVA irradiation plotted against conidial melanin concentration.Regression equations: linear for AS strains (open circles) y = 34.383x+68.414, R2 = 0.9166, P<0.001), and logarithmic curve for ES strains (solid circles) y = 16.334Ln(x)+100.67 R2 = 0.9187: p<0.001).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2500164&req=5

pone-0002993-g002: Relative culturability of A. niger strains after 4 h exposure to UVA irradiation plotted against conidial melanin concentration.Regression equations: linear for AS strains (open circles) y = 34.383x+68.414, R2 = 0.9166, P<0.001), and logarithmic curve for ES strains (solid circles) y = 16.334Ln(x)+100.67 R2 = 0.9187: p<0.001).
Mentions: To investigate whether the adaptation of higher melanin concentration in the AS strains indeed leads to increased fitness, we studied the UV-resistance potential of selected strains (covering stations and habitats) from both slopes. The relative culturability after UVA irradiation of the AS strains (79.9%±1.36) was significantly higher than that of the ES strains (57.75%±3.71) (non-parametric Mann-Whitney rank sum test: T = 474, n1 = 18, n2 = 19, P<0.001) (Fig. 2). However, plots and regressions indicated that these differences were dependent on the positive relationship between relative conidial culturability and the concentration of melanin. The melanin content (A420/106) values, within a range between 0.2 and 0.4, of strains from both slopes exhibit a fairly equal range of relative conidial culturability (75%–85%). The positive relationship between relative conidial culturability (after exposure to UVA) and concentration of melanin was indicated also by linear regression for both sunny and shady microniches. Regression equations for AS strains (Fig. 3A) were: y = 71.42+26.97x, R2 = 0.884, p<0.0001 in the sunny microniche; y = 66.31+38.75x, R2 = 0.973, p = 0 .0003 in the shady microniche. The slopes of the two regression lines were significantly different (t = 2.71, df = 14, P = 0.006). For ES strains (Fig. 3B): y = 45.46+119.57x, R2 = 0.801, p = 0.007 in the sunny microniche; y = 31.75+343.02x, R2 = 0.9089, p<0.0001 in the shady microniche. Slopes of the two regression lines were significantly different (t = 4.54, df = 15, P = 0.0004).

Bottom Line: We test the working hypothesis that the AS strains exhibit higher melanin content than strains from the ES resulting in higher UV resistance.The results indicated that mean conidial melanin concentration of AS strains were threefold higher than ES strains and the former resisted UVA irradiation better than the latter.We conclude that melanin in A. niger is an adaptive trait against UVR generated by natural selection.

View Article: PubMed Central - PubMed

Affiliation: Institute of Evolution, University of Haifa, Mount Carmel, Israel.

ABSTRACT

Background: Adaptation is an evolutionary process in which traits in a population are tailored by natural selection to better meet the challenges presented by the local environment. The major discussion relating to natural selection concerns the portraying of the cause and effect relationship between a presumably adaptive trait and selection agents generating it. Therefore, it is necessary to identify trait(s) that evolve in direct response to selection, enhancing the organism's fitness. "Evolution Canyon" (EC) in Israel mirrors a microcosmic evolutionary system across life and is ideal to study natural selection and local adaptation under sharply, microclimatically divergent environments. The south-facing, tropical, sunny and xeric "African" slope (AS) receives 200%-800% higher solar radiation than the north-facing, temperate, shady and mesic "European" slope (ES), 200 meters apart. Thus, solar ultraviolet radiation (UVR) is a major selection agent in EC influencing the organism-environment interaction. Melanin is a trait postulated to have evolved for UV-screening in microorganisms. Here we investigate the cause and effect relationship between differential UVR on the opposing slopes of EC and the conidial melanin concentration of the filamentous soil fungus Aspergillus niger. We test the working hypothesis that the AS strains exhibit higher melanin content than strains from the ES resulting in higher UV resistance.

Methodology/principal findings: We measured conidial melanin concentration of 80 strains from the EC using a spectrophotometer. The results indicated that mean conidial melanin concentration of AS strains were threefold higher than ES strains and the former resisted UVA irradiation better than the latter. Comparisons of melanin in the conidia of A. niger strains from sunny and shady microniches on the predominantly sunny AS and predominantly shady ES indicated that shady conditions on the AS have no influence on the selection on melanin; in contrast, the sunny strains from the ES displayed higher melanin concentrations.

Conclusions/significance: We conclude that melanin in A. niger is an adaptive trait against UVR generated by natural selection.

Show MeSH
Related in: MedlinePlus