Full Text DQ S

Biology 258 Phenotypic Plasticity Week 4: Pigliucci Ch 3 Discussion Leader: Jon Allen

References: Bradshaw, A.D. 1965. Evolutionary significance of phenotypic plasticity in plants. Advances in Genetics 13:115-155. Dodson, S. 1989. Predator-induced reaction norms. Bioscience. 447-452. Johannsen, W. 1911. The genotype concept of heredity. American Naturalist 45: 129-159. Lewontin, R.C. 1974. The analysis of variance and the analysis of causes. American Journal of Human Genetics. 26:400-411. Via, S. and R. Lande. 1985. Genotype-environment interaction and the evolution of phenotypic plasticity. Evolution. 39:505-522.

Discussion Topics

Pigliucci Johannsen and Woltereck No questions, but Woltereck’s misinterpretation of his results must be one of the great ironies in biology. Pg 50 (Jon)

Schmalhausen -We should consider internal (developmental constraints) and external (natural selection) factors to be equivalent sources of evolutionary changes, yet given the limitation on controlling internal factors are we limited in our ability to understand the plasticity we observe? Pg. 51 (Marty) -How can we conclude that “developmental mechanisms are more conserved across species than plasticity” since it’s so difficult to quantify the two in comparable ways? Pg. 55 (Willow) “plasticity is a higher order phenomenon (i.e. plasticity can be the result of developmental mechanisms) and so you can’t compare the two. Pigliucci is an $%#@&!” (Margaret)

Bradshaw -Could Bradshaw’s description of how stabilizing selection can mask genotypic variation be used to explain some of the conflict between Johannsen and Woltereck’s debate about the relationship of genotype to phenotype? Pg. 58 (Amber) -Can we study situations where plasticity facilitates directional selection in the field or in the lab? Pg. 57-58 (Will) -How does Bradshaw (or Pigliucci) define ‘genotype’ and what definition is necessary to make Bradshaw’s conclusions valid? (Willow)

Lewontin -If ANOVA and reaction norms are so ‘tightly linked’ then how can Lewontin suggest that the ANOVA is flawed while the reaction norm should be a central tool for plasticity studies? Pg. 61 (Marty) -Is Lewontin’s criticism of ANOVA still valid today? Or do most researchers understand the limitations? (Greg) -Can Lewontin’s arguments be applied to any study that looks for patterns (not processes) and if so, is this just a restatement of ‘correlation is not causation’? (Will) Via and Lande When does 1985 become ‘historical’? For cars, it takes 25 years to be considered an ‘antique’. (Jon) What is the difference between hard and soft selection? Pg. 63 (Amy) “soft”-populations are regulated independently in each environment and each niche contributes a constant fraction to the total population “hard”=contribution of each niche is weighted by its mean fitness and therefore the population is regulated globally (Via and Lande, pg 508)

Dodson -What possible constraints may be acting on terrestrial organisms (and not aquatic ones) that limit the evolution of cyclomorphosis? (Justin/Willow)

-If cyclomorphosis is predictable then could a predator take advantage of this and reduce the benefits of plasticity such that less plastic phenotypes become advantageous? (Amber)

-When maternal exposure to predators causes changes in offspring traits (as in Daphnia, pg 448) is that phenotypic plasticity? (Justin)

-If Daphnia were exposed to multiple predator types then would a generic cue be more beneficial for detecting predators then the Chaoborus-specific cue? Are they exposed to multiple predators? Pg 448 (Greg)

Full text DQ’s

Tara 1) pg 61-62 - When looking at additivity you seem to be left with the classic argument of nature vs nurture. What are some practical methods of determining if additivity can be explained by environmental factors (nurture) or if it is just a specific genotypes (nature)? 2) pg 63- Concerning Via and Lande's model- Do you think most organisms reach optimum phenotype in each environment, or do the exceptions occur more often then optimal conditions?

Marty 1) Schmalhausen gives equal importance to the effects of both internal and external environments (p 51). Pigliucci further enforces the importance of this idea later on in the paragraph as a major component of current studies of plasticity. Given the difficulty of controlling an internal environment for experimental purposes, how does this limit our ability to understand the plasticity we see? 2) Lewontin states that the real object of studies of phenotypic plasticity should be the reaction norm (p 61) as ANOVA is flawed. However, in Chapter 1, ANOVA and reaction norms are described as 'tightly linked' (p13 and following discussion). How is it that ANOVA is a flawed form of analysis while the reaction norm (a visual representation of ANOVA) should be the central tool in studies of phenotypic plasticity?

Amber 1. The specific type of plasticity that Dodson seems to focus on in his review is cyclomorphosis, in which the morphology of an organism can vary in cycles. This suggests to me that cyclomorphosis might often be predictable. Dodson also emphasizes the usefulness of cyclomorphosis in avoiding predation. However, how useful would it be if it's predictable? Wouldn't this lead to coevolution or an evolutionary arms race between the predator and the prey? I suppose this would be the case whether the plasticity was predictable or not. So, in predator-prey interactions, could a single organism that had an unusually small amount (in comparison to the rest of the population) of plasticity for a given trait (i.e. Daphnia's shape) actually be selected for because it doesn't really enter the race? Not to mention that it doesn't have the extra cost of maintaining the plastic response....? 2. On page 58 of the Pigliucci, stabilizing selection and its affect on the role of plasticity is discussed as part of the summary of Bradshaw's contribution to the field. Bradshaw apparently pointed out that under stabilizing selection, plasticity could actually cause different genotypes to result in the same phenotype. Is this idea hinted at somewhat by the debate (page 50) between Johannsen and Woltereck? Apparently, this situation in Daphnia was part of what caused Woltereck's confusion about the relationship between genotype and phenotype. How often might a similar situation come up in modern science? Could a situation in which the plasticity of two different genotypes converges on one level of expression be passed over altogether, and the plasticity not even noticed?

Willow 1. In the Pigliucci chapter, organisms are often referred to as having a particular genotype. Given that most of the papers discussed were published before molecular techniques were much in use, what does Pigliucci mean by genotype? I assume that the genotype is something they are inferring from mating crosses, or some other indirect technique, but it does not seem to follow that much about an organism's total genetic makeup can be inferred by these methods. On page 56, it is said that Bradshaw came to the conclusion that "plasticity of a given trait can be completely independent of the plasticity of another trait for the same genotype." When using techniques for genotyping which only give you information about a few loci, how can he know whether the individuals have the same overall genetic makeup, and does this not make the above conclusion somewhat invalid? 2. On page 55 of Pigliucci, he states that "developmental mechanisms are clearly more conserved across species than plasticity." This seems to me to be a bias imposed by the fact that it was written by someone who specialized in the field of phenotypic plasticity. Given the incredible diversity of developmental mechanisms in nature how can he quantify the degree of conservation of either development or plasticity in a way that makes them comparable? 3. In the Dodson paper, he mentions that an aquatic habitat appears to be a requirement for a cyclomorphic response to occur in response to predators (in animals, at least.) Even though transmission of chemical signals is not as efficient through air as through water, it does occur. Are there any other reasons that we would not see this type of adaptation in terrestrial animals?

Justin 1. At the end of the second column on page 447, Dodson states "Curiosly, the great majority of examples of cyclomorphosis occur in aquatic or intertidal habitats." What is special about these types of environments that this would be the case? Is it perhaps due to great heterogeneity within these environments? I thought this might be a good topic for discussion. 2. At the end of the second column on page 448, Dodson describes a study he performed in 1989. "In a survey of seven Daphnia species exposed to three predators, [he] found that when mothers were exposed to predation the offspring often showed changes as neonates and adults in body length, helmet length, tail spine length, egg length, and clutch size." I found that to be a particularly interesting result. Basically, mothers can detect environmental conditions and pass that information on to offspring. Is this phenotypic plasticity as we've defined it? Or is this an example of GxE? I'll have more to say about this one in class.

Greg 1) On page 448 Dodson discusses the chemical cues that induce morphological changes in Daphnia. These chemical cues seem to be very specific; are the cues Dodson refers to only produced by the C. americanus predator? Having no knowledge of the daphnia system and what has and has not been worked out, it seems like daphnia could potentially be exposed to many different taxa of predators, in which case having a less specific cue (chemical or otherwise) would be much more advantageous. 2) On page 60 to 62 (Pigliucci), is Lewontin's criticism of ANOVA still valid today? I think most researchers understand the limits of ANOVA (i.e.interactions limit interpretations of main effects) and that it can be used to identify interactions but not to characterize them.

Will 1. The discussion of Bradshaw's thoughts on how different types of selection should affect plasticity was interesting (p. 57-58). This question has two parts. First, I have forgotten the exact definition of "genetic assimilation," but I don't think it's too important for the rest of the question. The theory that plasticity might be allow a population to persist in an environment where directional selection is acting in the same direction as the plasticity is interesting. Is this one of the areas in which plasticity could play a role in evolution, and is it even possible to study this situation in the field or even the lab? 2. Regarding Lewontin's argument that gene by environment interactions invalidate the ANOVA as a technique for analyzing how genes affect phenotypes (p. 59-63), can't this type of argument be applied to any study that looks for patterns (not processes) and, if so, isn't it just a restatement that correlation is not causation? Isn't it better to use the ANOVA first, figure out what the variance is,and investigate all potential sources that violate the assumptions of ANOVA? If some of the assumptions of quantitative genetics and ANOVA are violated in a system, is the partitioning of variance totally invalidated or can the model be corrected to take these violations into account?

Amy 1. On page 56, P briefly mentions some "long standing debate" about heterozygosity and plasticity. What is this all about? I am not sure I see the connection at all.

2. Hard selection and soft selection....difference not defined (probably means I should know this already!), but it obviously makes a difference. How, and why is this difference important?