Phenotypic plasticity in nematodes: Evolutionary and ecological significance

Abstract

Model systems, including C. elegans, have been successfully studied to understand the genetic control of development. A genotype's phenotype determines its evolutionary fitness in natural environments, which are typically harsh, heterogeneous and dynamic. Phenotypic plasticity, the process by which one genome can produce different phenotypes in response to the environment, allows genotypes to better match their phenotype to their environment. Phenotypic plasticity is rife among nematodes, seen both as differences among life-cycles stages, perhaps best exemplified by parasitic nematodes, as well as developmental choices, such as shown by the C. elegans dauer/non-dauer developmental choice. Understanding the genetic basis of phenotypically plastic traits will probably explain the function of many genes whose function still remains unclear. Understanding the adaptive benefits of phenotypically plastic traits requires that we understand how plasticity differs among genotypes, and the effects of this in diverse, different environments.

Keywords: Caenorhabditis; Parastrongyloides; Pristionchus; Strongyloides; adaptation; reaction norms.

Figure 1. The life-cycle choices of (A) C. elegans and other free-living nematodes, (B) S. ratti where gray box 1 is a sex-determination event, and gray box 2 is a female larva-only developmental choice between direct and indirect development (larval stages, except the infective L3 stage, have been omitted for clarity) and (C) P. trichosuri as for S. ratti, except that the P. trichosuri life-cycle differs (1) by having a dioecious parasitic generation and (2) the progeny of the free-living adult generation can form multiple, secondary free-living adult generations.

Figure 2. Reaction norms A reaction norm displays the value of a trait in a minimum of two environments. Thus, in (A), as the environment (e.g., temperature, or quantity of food) changes from E₁ to E₄, then the value of the trait changes from T1 to T₄. The difference in the trait value, which is the slope of the line, is therefore the norm of reaction, more easily thought of as the sensitivity of the phenotype to the environment. This can be summarized numerically as T₄ – T1. Different genotypes may have different reaction norms. In (B) a second genotype (blue line) has different trait values (T₃ and T₆), though the sensitivity is the same, (T₄ – T1) = (T₆ – T₃). A third genotype (red line) has different trait values (T2 and T₃), and a different sensitivity, in this case less sensitive, (T₃ – T2) < (T₄ – T1). For traits measured in two environments (A and B), only a linear reaction norm can be interpolated. Measurement in more environments, may reveal nonlinear reaction norms (C). For binary traits measured as proportions (e.g., for C. elegans the proportion of developing larvae that form dauer larvae, and for analogous traits in Strongyloides and Parastrongyloides) the trait values have 0 and 1 minima and maxima, respectively. As a consequence, the trait value in E₁ limits the maximum trait sensitivity, because the trait has a maximum of 1 in E₂ (for the case where the slope is positive, in A or B). Such proportion data also have a non-normal error structure (errors cannot lie outside of the 0 - 1 range) which requires a different error structure.