Intraspecific variability in the parasitoid wasp Trichogramma chilonis: can we predict the outcome of hybridization?

Abstract

In the framework of biological control, the selection of effective natural enemies determines the final pest control. Thus, the genetic improvement of biocontrol agents could enhance the efficiency of biocontrol programs. Although promising, this approach has rarely been applied in this field. At the intraspecific level, hybridization between divergent populations of biocontrol agents is expected to promote hybrid vigor (heterosis), but it is not clear to what extent. An even more difficult task is the ability to predict the fitness of hybrids from the biological characteristics of their parents. We investigated these general questions by crossing seven populations of the parasitoid wasp Trichogramma chilonis (Hymenoptera: Trichogrammatidae). Our results show different levels of mating compatibilities among populations, including asymmetric or almost complete reproductive isolation. Hybrids' performance (fitness of the F(1) generation) ranges from inbreeding depression to heterosis. It was possible, to some extent, to predict hybrid fitness from pairwise genetic and phenotypic distances among parents, in accordance with the 'dominance' hypothesis. This may provide general guidelines for the genetic improvement of biological control agents.

Keywords: biological control; genetic improvement; heterosis; intraspecific hybridization; reproductive compatibilities.

Figure 1

Relationship between mating incompatibility index (I_PSI) and pairwise genetic distance obtained using the cytochrome c oxidase subunit 1 genetic marker (COI). Values of I_PSI close to 1 indicate incompatible crosses.

Figure 2

Genetic differentiation between the seven parental populations of Trichogramma chilonis. Estimates were obtained after fitting the most parsimonious generalized linear model to the observed data (see Material and methods). Error bars represent 95% confidence intervals.

Figure 3

Composite figure illustrating the variable outcomes of hybridization on female fecundity. (A) Bubble plot representing the mean fecundity of F₁ generation. Estimates were obtained from the most parsimonious generalized linear mixed-effects model (GLMM) (see text for details). Bubbles along the diagonal represent the homotypic crosses (parents from the same population) that were detailed in Fig. 2. (B) Hybrid breakdown induced by the crossing between a female of Taiwan and a male of Pakistan. Statistical tests are given in Table 4. The reciprocal cross was not available. (C) Inbreeding depression in the Japanese population. (D) Evidences for heterosis. Statistical tests are given in Table 4.

Figure 4

Relationship between F₁ fitness and genetic distance between parents obtained using the cytochrome c oxidase subunit 1 genetic marker (COI). The line represents the quadratic effect of COI.

Figure 5

Relationship between F₁ fitness [residual of previous generalized linear mixed-effects model (GLMM); see text for details] and phenotypic difference between parents.