Evolution of resistance and tolerance to herbivores: testing the trade-off hypothesis

Introduction

Plants rely on resistance and tolerance to avoid the negative fitness effects of damage by herbivores or pathogens (Simms & Triplett, 1994; Fineblum & Rausher, 1995; Strauss & Agrawal, 1999; Núñez-Farfán, Fornoni & Valverde, 2007). Resistant plants prevent or reduce the amount of damage through chemical secondary metabolites and physical defenses (Ehrlich & Raven, 1964; Levin, 1973; Berenbaum, Zangerl & Nitao, 1986; Wink, 2003). Tolerant plants can lessen the negative impact of herbivore damage on fitness, once it has occurred (Rausher, 1992b; Stowe et al., 2000). Unlike resistance, tolerance does not prevent herbivory but maintains fitness by eliciting compensatory physiological plant responses after damage by herbivores.

Natural selection for increased resistance to herbivores has been detected in different plant species as well as for plant traits associated to resistance (Berenbaum, Zangerl & Nitao, 1986; Simms & Rausher, 1989; Mauricio & Rausher, 1997; Shonle & Bergelson, 2000). However, investment in plant defense is thought to involve fitness costs such that optimal defense does not necessarily imply maximal investment (Fagerström, Larsson & Tenow, 1987; Simms & Rausher, 1987). Thus, potential trade-offs between different defensive traits might arise (see Mauricio, 1998). Furthermore, if fitness costs of herbivory can be reduced by tolerance, selection on resistance traits would be relaxed, nil, or even selected against if leaf damage could be partially or completely compensated by tolerance (Abrahamson & Weis, 1997; Fineblum & Rausher, 1995; Mauricio, Rausher & Burdick, 1997). Hence, the simultaneous investment in tolerance and resistance may imply a greater total cost than the possession of only one pure strategy (van der Meijeden, Wijn & Verkaar, 1988; Herms & Mattson, 1992; Fineblum & Rausher, 1995; Mauricio, Rausher & Burdick, 1997). However, if the fitness benefit of investment in tolerance and resistance is higher than its cost, the evolution of a mixed defense strategy is a possible outcome (Fornoni et al., 2004a; Carmona & Fornoni, 2013).

Nevertheless, evidence of a trade-off between plant defensive strategies is scarce, and may depend on the sort of traits involved in the defensive response. A review of literature indicates little support for a negative genetic correlation between tolerance and resistance across different plant species, and suggests that a fruitful approach is to assess the relationship between tolerance and specific plant resistance traits rather that the correlation between tolerance with total resistance (Leimu & Koricheva, 2006). Thus, this study aimed to determine if total resistance and a component of it (leaf trichomes) are genetically correlated with plant tolerance. We carried out an experiment to expose maternal half-sib families of the annual plant Datura stramonium to their natural herbivores in order to (1) assess genetic variation in plant tolerance, total resistance, and leaf trichome density, and (2) measure selection on trichome density and resistance to herbivores. Finally, (3) we estimated the genetic correlation between tolerance and total resistance and leaf trichomes.

Materials and Methods

Results

The amount of damage received by plants exposed to herbivores (control) was significantly higher than that received by plants in the insecticide group (Fig. 1). Although the levels of damage were low in both groups, the ANOVA indicated that plants that received the insecticide application were significantly less damaged (∼15%) than those who did not (F_1,135 = 5.83, P = 0.017). However, in spite of the fact that plants performing better in the absence of herbivores (insecticide group), the differences in the average values of vigor, reproductive and resistance traits between the control and the insecticide group were not significant (Table 1).

Genetic variance

ANOVA detected a significant family effect of the number of flowers produced per plant and the two resistance traits estimated: total resistance (F_27,125 = 1.57, P = 0.04) and leaf trichome density (F_27,125 = 3.62, P < 0.001) (Table 2; Fig. 2). Heritability of leaf trichome number was high (h² = 0.641), compared to total resistance (h² = 0.259). In contrast, we failed to detect genetic variation of tolerance to damage, since the family × relative damage by herbivores was not significant (F_27,125 = 0.08, n.s.).

Table 2:

Analysis of variance of reproductive (flowers) and resistance traits (total resistance and leaf trichomes density).

The family is the random effect and block is a fixed factor. A significant effect of family (P < 0.05) indicates that there is genetic variance. The analysis was performed with plants exposed to herbivores.

Character	Source of variation	d.f.	F	P
Flower number	Family	27	1.94	0.01
	Block	2	6.32	0.002
	Error	90
	Total	119
Total resistance	Family	27	1.64	0.04
	Block	2	0.81	0.44
	Error	96
	Total	125
Leaf trichome density	Family	27	3.06	<0.0001
	Block	2	4.62	0.012
	Error	96
	Total	125

DOI: 10.7717/peerj.789/table-2

Notes:

Values in bold indicate P < 0.05.

Natural selection on resistance and tolerance

Positive directional selection was detected on leaf trichome density (resistance component) (Fig. 3). However, directional selection was not significant for total resistance. No non-linear selection (curvilinear) for any of the resistance attributes (leaf trichomes and total resistance) was detected (Table 4). Also, neither directional nor non-linear selection acting on tolerance was found.

Table 4: Linear (β), and non-linear (γ) selection gradients on resistance traits to herbivores for Datura stramonium plants growing in Ticuman, Morelos.

Character	β 1	t	P	ANOVA of the multiple linear regression model	γ	t	P	ANOVA of the multiple quadratic regression model
Total resistance	0.179 (0.145)	1.23	0.22	F = 57.27 P < 0.0001 R2 = 0.31	−0.002 (0.061)	−0.04	0.96	F = 14.02 P < 0.0001 R2 = 0.31
Leaf trichome density	0.89 (0.117)	7.57	<0.0001		0.024 (0.065)	0.37	0.7

DOI: 10.7717/peerj.789/table-4

Notes:

Values in bold indicate P < 0.05.

Discussion

Selection to increase leaf trichome was detected in the population of D. stramonium. This character was negatively related to damage by herbivores. However, we did not detect selection on total resistance suggesting that individual variation in resistance includes other components besides leaf trichome density (Agrawal, 2011). Although we found evidence of genetic variation in resistance and leaf trichome density, we failed to detect genetic variation for plant tolerance to damage. Hence, we found not support for the trade-off hypothesis between plant resistance and tolerance, or between tolerance and a specific resistance trait (leaf trichome density).

Furthermore, we did not detect fitness costs of leaf trichome density in the absence of herbivores. In fact, the effect of this trait fitness was positive even in the absence of herbivores, suggesting that such resistance trait could be correlated with other traits that were subject to selection (Björkman & Anderson, 1990; Roy, Stanton & Epplely, 1999), or that leaf trichomes may have another function besides defense. Empirical evidence has shown that, in addition to being a mechanical barrier to herbivores (Baur, Blinder & Benz, 1991; Agrawal et al., 2009), nonglandular trichomes (not-producing chemical compounds) can reduce the amount of heat on the leaf surface (Ehleringer, Björkman & Mooney, 1976; Vogelmann, 1993), thus reducing water loss through evapotranspiration. Nevertheless, our results are consistent with a defensive role, even when defense would not be the primary function of leaf trichomes.

In the present study, we did not detect genetic variance for tolerance contrasting with results reported for the same population by Fornoni, Valverde & Núñez-Farfán (2003). A possible explanation is that the low levels of leaf damage recorded in this study (86% of individuals that received leaf damage, experienced less than 10% of loss) prevented the expression of differences in tolerance i.e., lack of genetic variation. Previous evidence indicates that D. stramonium in the same population receives, on average, more damage by herbivores (21.65% ± 0.7; mean ± SE) than the average level recorded in this study. In the Ticuman population, the detection of genetic variance occurred when the damage surpassed 10% of total leaf area (see Fornoni, Valverde & Núñez-Farfán, 2003). Thus, if tolerance is a genotype’s reaction norm of fitness in a damage gradient, differences in the reaction norms (i.e., G × E interaction, implying tolerance) could be detected when damage attains higher values. But at low levels of leaf damage, only part of the reaction norm is apparent in the narrow range of the damage gradient and no differences among genotypes is present (Fig. 4). This fact might possibly preclude the detection of a significant correlation between defensive strategies (or a specific resistance trait). Previous studies have shown how the correlation between resistance and tolerance may vary depending on the biotic environment (e.g., levels of leaf damage due to herbivory). For instance, Fornoni, Valverde & Núñez-Farfán (2003) assessed the correlation between defensive strategies by conducting a reciprocal transplants experiment between two natural populations of D. stramonium. They found that the detection of a trade-off between total resistance and tolerance occurred only for native plants growing in the population with the highest levels of leaf damage (i.e., Ticuman). In contrast, in the population where the average level of leaf damage was lower (i.e., Santo Domingo) no trade-off was detected, suggesting that the amount of leaf damage may restrict the detection of a significant correlation between plant defenses.

The expression of a negative correlation could also be restricted by patterns of resource acquisition and allocation to defense. Theoretically, if variation in resource allocation is greater than variation in acquisition (i.e., increasing defense costs), a negative correlation between defenses is a possible outcome (Houle, 1991; Mole, 1994). Allocation patterns depend mainly on the frequency and intensity of herbivory during the plant’s life (Basey, Jenkins & Busher, 1988; Boucher, 1985; Langenheim & Stubblebine, 1983). The low levels of leaf damage detected in this study may indicate (besides of resistance traits acting on plant consumers) a low abundance of herbivores. This factor could reduce the variance of resource allocation to defense. Consequently, defense costs could be diminished, and plants could simultaneously allocate resources to different classes of defensive traits, i.e., a trade-off between defensive strategies would not be favored.

On the other hand, the trade-off hypothesis between defensive strategies is based on the assumption of redundancy. However, the defensive role of plant defense-related traits (strategies) would depend on the identity and diversity of herbivores attacking the plants. Previous studies on D. stramonium have shown how different herbivores could modify the selection pattern on resistance traits (Shonle & Bergelson, 2000; Lankau, 2007), or defense strategies (Carmona & Fornoni, 2013), and how geographic variation of the herbivore community is related to variation in the selective patterns exerted by plant consumers on chemical and physical resistance traits.

Conclusions

Altogether, our results suggest that resistance could evolve independently from plant tolerance in the Ticuman population. Leaf trichome density is a heritable resistance component and thus it has no restrictions on evolving, and the pattern of selection should favor those individuals with high levels of leaf trichomes. Even when no evidence of a trade-off between plant resistance and tolerance was found, it should not be excluded because the expression of tolerance and its correlation with resistance seems to be a function of the magnitude of damage by herbivores in this population.

Supplemental Information