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Spatial mosaic evolution of snail defensive traits.

Johnson SG, Hulsey CD, de León FJ - BMC Evol. Biol. (2007)

Bottom Line: Recent models suggest that escalating reciprocal selection among antagonistically interacting species is predicted to occur in areas of higher resource productivity.Using spatial autocorrelation to account for genetic and geographic divergence among populations, we found no autocorrelation among populations at small geographic and genetic distances for the two defensive traits.These local geographic differences may result from among-habitat variation in how resource productivity interacts to promote escalation in prey defenses.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biological Sciences, University of New Orleans, New Orleans, LA 70148, USA. sgjohnso@uno.edu

ABSTRACT

Background: Recent models suggest that escalating reciprocal selection among antagonistically interacting species is predicted to occur in areas of higher resource productivity. In a putatively coevolved interaction between a freshwater snail (Mexipyrgus churinceanus) and a molluscivorous cichlid (Herichthys minckleyi), we examined three components of this interaction: 1) spatial variation in two putative defensive traits, crushing resistance and shell pigmentation; 2) whether abiotic variables or frequency of molariform cichlids are associated with spatial patterns of crushing resistance and shell pigmentation and 3) whether variation in primary productivity accounted for small-scale variation in these defensive traits.

Results: Using spatial autocorrelation to account for genetic and geographic divergence among populations, we found no autocorrelation among populations at small geographic and genetic distances for the two defensive traits. There was also no correlation between abiotic variables (temperature and conductivity) and snail defensive traits. However, crushing resistance and frequency of pigmented shells were negatively correlated with molariform frequency. Crushing resistance and levels of pigmentation were significantly higher in habitats dominated by aquatic macrophytes, and both traits are phenotypically correlated.

Conclusion: Crushing resistance and pigmentation of M. churinceanus exhibit striking variation at small spatial scales often associated with differences in primary productivity, substrate coloration and the frequency of molariform cichlids. These local geographic differences may result from among-habitat variation in how resource productivity interacts to promote escalation in prey defenses.

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A) Average size-adjusted crushing resistance (95% Confidence Intervals) for M. churinceanus populations. B) Frequency of banded snails for M. churinceanus populations. Populations along the Y-axis are arranged on a transect from southwestern populations to the northern Rio Mesquite down to the southeastern lobe.
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Figure 3: A) Average size-adjusted crushing resistance (95% Confidence Intervals) for M. churinceanus populations. B) Frequency of banded snails for M. churinceanus populations. Populations along the Y-axis are arranged on a transect from southwestern populations to the northern Rio Mesquite down to the southeastern lobe.

Mentions: Mean and standard errors of crushing resistance and shell size are presented in Table 1. The five linear morphological variables had high positive loadings (> 0.96) on the first principal component, and over 94% of the total variance was explained by this first PC, which we interpret as a measure of overall shell size. Overall shell size (PC1 scores) showed considerable variation among populations (Table 1). Covariance PCA analysis revealed one major shape component representing a contrast between aperture size and spire length. This shape component explained about 5% of the total variance, and individuals with positive scores for this component have relatively large apertures and shortened spires. Multiple regression analysis of crushing resistance revealed that shell size and shape were both significantly positively related to crushing resistance. Shell size explained most of the variation in crushing resistance (standardized β = 0.662, df = 1, 503; R2 = 43.9%, p < 0.0001), whereas shell shape explained little variation in crushing resistance (standardized β = 0.077, df = 2, 503; R2 = 0.006%, p = 0.02). We next used analysis of covariance to examine the effect of population on crushing resistance using size as a covariate. The interaction term (shell size by population) was not significant (p = 0.24), and there was a highly significant effect of population on size-adjusted crushing resistance (F = 6.3, df = 18, 467, p < 0.001). There was considerable spatial variation in size-adjusted crushing resistance, ranging from 42.3 Newtons in a Rio Mesquites population to 92.3 Newtons in a Tierra Blanca population (Fig. 3a). The correlograms of size-adjusted crushing resistance and all distance measures were not significant (see Fig. 4 for correlogram using cytochrome b distance), indicating the absence of autocorrelation among populations at any scale.


Spatial mosaic evolution of snail defensive traits.

Johnson SG, Hulsey CD, de León FJ - BMC Evol. Biol. (2007)

A) Average size-adjusted crushing resistance (95% Confidence Intervals) for M. churinceanus populations. B) Frequency of banded snails for M. churinceanus populations. Populations along the Y-axis are arranged on a transect from southwestern populations to the northern Rio Mesquite down to the southeastern lobe.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC1851949&req=5

Figure 3: A) Average size-adjusted crushing resistance (95% Confidence Intervals) for M. churinceanus populations. B) Frequency of banded snails for M. churinceanus populations. Populations along the Y-axis are arranged on a transect from southwestern populations to the northern Rio Mesquite down to the southeastern lobe.
Mentions: Mean and standard errors of crushing resistance and shell size are presented in Table 1. The five linear morphological variables had high positive loadings (> 0.96) on the first principal component, and over 94% of the total variance was explained by this first PC, which we interpret as a measure of overall shell size. Overall shell size (PC1 scores) showed considerable variation among populations (Table 1). Covariance PCA analysis revealed one major shape component representing a contrast between aperture size and spire length. This shape component explained about 5% of the total variance, and individuals with positive scores for this component have relatively large apertures and shortened spires. Multiple regression analysis of crushing resistance revealed that shell size and shape were both significantly positively related to crushing resistance. Shell size explained most of the variation in crushing resistance (standardized β = 0.662, df = 1, 503; R2 = 43.9%, p < 0.0001), whereas shell shape explained little variation in crushing resistance (standardized β = 0.077, df = 2, 503; R2 = 0.006%, p = 0.02). We next used analysis of covariance to examine the effect of population on crushing resistance using size as a covariate. The interaction term (shell size by population) was not significant (p = 0.24), and there was a highly significant effect of population on size-adjusted crushing resistance (F = 6.3, df = 18, 467, p < 0.001). There was considerable spatial variation in size-adjusted crushing resistance, ranging from 42.3 Newtons in a Rio Mesquites population to 92.3 Newtons in a Tierra Blanca population (Fig. 3a). The correlograms of size-adjusted crushing resistance and all distance measures were not significant (see Fig. 4 for correlogram using cytochrome b distance), indicating the absence of autocorrelation among populations at any scale.

Bottom Line: Recent models suggest that escalating reciprocal selection among antagonistically interacting species is predicted to occur in areas of higher resource productivity.Using spatial autocorrelation to account for genetic and geographic divergence among populations, we found no autocorrelation among populations at small geographic and genetic distances for the two defensive traits.These local geographic differences may result from among-habitat variation in how resource productivity interacts to promote escalation in prey defenses.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biological Sciences, University of New Orleans, New Orleans, LA 70148, USA. sgjohnso@uno.edu

ABSTRACT

Background: Recent models suggest that escalating reciprocal selection among antagonistically interacting species is predicted to occur in areas of higher resource productivity. In a putatively coevolved interaction between a freshwater snail (Mexipyrgus churinceanus) and a molluscivorous cichlid (Herichthys minckleyi), we examined three components of this interaction: 1) spatial variation in two putative defensive traits, crushing resistance and shell pigmentation; 2) whether abiotic variables or frequency of molariform cichlids are associated with spatial patterns of crushing resistance and shell pigmentation and 3) whether variation in primary productivity accounted for small-scale variation in these defensive traits.

Results: Using spatial autocorrelation to account for genetic and geographic divergence among populations, we found no autocorrelation among populations at small geographic and genetic distances for the two defensive traits. There was also no correlation between abiotic variables (temperature and conductivity) and snail defensive traits. However, crushing resistance and frequency of pigmented shells were negatively correlated with molariform frequency. Crushing resistance and levels of pigmentation were significantly higher in habitats dominated by aquatic macrophytes, and both traits are phenotypically correlated.

Conclusion: Crushing resistance and pigmentation of M. churinceanus exhibit striking variation at small spatial scales often associated with differences in primary productivity, substrate coloration and the frequency of molariform cichlids. These local geographic differences may result from among-habitat variation in how resource productivity interacts to promote escalation in prey defenses.

Show MeSH
Related in: MedlinePlus