Genetic variation and covariation for resistance and tolerance to Cucumber mosaic virus in Mimulus guttatus (Phrymaceae): a test for costs and constraints

Abstract

Genetic variation for resistance and tolerance to pathogens may be maintained by costs represented as genetic tradeoffs between these traits and fitness. The evolution of resistance and tolerance also may be constrained by negative genetic correlations between these defense systems. Using a complete diallel, we measured genetic variation and covariation for and among performance, resistance, and tolerance traits in Mimulus guttatus challenged with a generalist pathogen, Cucumber mosaic virus (CMV). Viral coat protein was detected by enzyme-linked immunosorbent assay (ELISA) in all inoculated plants, indicating that all plants were susceptible to infection, although the ELISA absorbance varied quantitatively across plants. Plants inoculated with CMV had significantly reduced aboveground biomass and flower production relative to controls, although date of first flower was unaffected by infection. All three of these performance traits showed moderate to high narrow-sense heritability (h 2 ¼ 0.32-0.62) in both inoculated and control plants. We found phenotypic variation for both tolerance of and resistance to our strain of CMV, but both displayed very low narrow-sense heritability (h 2 o0.03). We found no evidence of a trade-off between resistance and tolerance. We also found no evidence for a cost of resistance or tolerance. In fact, a significant genetic correlation suggested that plants that were large when healthy had the greatest tolerance when infected. Significant, positive genetic correlations found between performance of uninfected and infected plants suggested that selection would likely favor the same M. guttatus genotypes whether CMV is present or not.

Genetic variation and constraints on the evolution of defense against spittlebug (Philaenus spumarius) herbivory in Mimulus guttatus

“…Our results suggest that any fitness costs of tolerance and resistance are not equivalent in M. guttatus, which would limit the opportunity for tradeoffs to arise. Similarly, Carr et al (2006) found no evidence for tradeoffs between tolerance and resistance to Cucumber mosaic virus in M. guttatus. The potential costs that we examined were presumably associated with constraints on resource allocation to other functions, specifically growth or reproduction.…”

“…These nonadditive sources of variation would have been impossible to observe without the diallel breeding design that we used; moreover, with a clonal or nested half-sib design, we would have overestimated the magnitude of heritability in our traits (Lynch and Walsh, 1998). Interestingly, another study involving a different population of M. guttatus also reported within-population nonadditive genetic variation for the same traits (Carr et al, 2006), which indicates that complex genetic effects for such traits may be more widespread than previously appreciated. In addition, there was some evidence for environment-dependent expression of nonadditive genetic variation; the presence of spittlebugs reduced the magnitude of nonadditive genetic variation in flower number (Table 2), which suggests that herbivory could dampen the influence of nonadditive genetic variation on natural selection for this trait.…”

“…Genetic variation for herbivory tolerance, however, has been reported for a number of other plant species (Strauss and Agrawal, 1999;Nú ñ ez-Farfán et al, 2007), and genetic variation for other traits in M. guttatus is often substantial (Carr and Fenster, 1994;Robertson et al, 1994;Fishman et al, 2002), including other forms of phenotypic plasticity (Murren et al, 2006). Carr et al (2006), on the other hand, found little evidence for genetic control of tolerance to Cucumber mosaic virus in a different population of M. guttatus. The low additive genetic variation for tolerance to spittlebug herbivory in the population we studied would limit its adaptive evolution, even under strong selection.…”

Standing genetic variation in host preference for mutualist microbial symbionts

“…[57,58]). However, a neutral or negative correlation does not conclusively eliminate the possibility of a cost.…”

“…greater association with mutualist partners) also have lower relative fitness in the absence of the exploiter (cf. [57,58]). However, we found the opposite (albeit nonsignificant) trend, where lines with a low proportion of non-fixing nodules showed higher relative biomass in the mutualist treatment (r ¼ 20.12676, p ¼ 0.1890; figure 2), indicating that no cost to filtering exploitative rhizobia partners was detected despite a large quantitative genetic design (n ¼ 110 plant lines).…”