The genetic architecture underlying body-size traits plasticity over different temperatures and developmental stages in Caenorhabditis elegans

Most ectotherms obey the temperature-size rule, meaning they grow larger in a colder environment. This raises the question of how the interplay between genes and temperature affect the body size of ectotherms. Despite the growing body of literature on the physiological life-history and molecular genetic mechanism underlying the temperature-size rule, the overall genetic architecture orchestrating this complex phenotype is not yet fully understood. One approach to identify genetic regulators of complex phenotypes is Quantitative Trait Locus (QTL) mapping. Here, we explore the genetic architecture of body size phenotypes, and plasticity of body-size phenotypes in different temperatures using Caenorhabditis elegans as a model ectotherm. We used 40 recombinant inbred lines (RILs) derived from N2 and CB4856, which were reared at four different temperatures (16°C, 20°C, 24°C, and 26°C) and measured at two developmental stages (L4 and adult). The animals were measured for body length, width at vulva, body volume, length/width ratio, and seven other body-size traits. The genetically diverse RILs varied in their body-size phenotypes with heritabilities ranging from 0.0 to 0.99. We detected 18 QTL underlying the body-size traits across all treatment combinations, with the majority clustering on Chromosome X. We hypothesize that the Chromosome X QTL could result from a known pleiotropic regulator – npr-1 – known to affect the body size of C. elegans through behavioral changes. We also found five plasticity QTL of body-size which three of them colocalized with some body-size QTL at certain temperature. In conclusion, our findings shed more light on multiple loci affecting body size plasticity and the possibility of co-regulation of traits and traits plasticity by the same loci under different environment.

body-size such as body length, body volume, width at vulva, and surface area of the 2 5 9 nematodes of adult worms, we found that CB4856 did not completely follow the temperature-2 6 0 size rules (a decrease curve from 16-20 o C, followed by an increase from 20-24 o C, and 2 6 1 decreased from 24-26 o C) whereas Bristol N2 consistently grew bigger at lower temperatures. This gives a new insight to the finding from previous study (Gutteling, et al., 2007b; 2 6 3 Kammenga et al., 2007), where CB4856 found to deviate the temperature-size rule, which 2 6 4 that's not always the case. This results shows that N2 body size was less plastic than CB4856. To get insight into the relations between the traits measured, we performed a 2 6 6 correlation analysis for all pairs of traits at the two developmental stages. We found that the 2 6 7 level of between trait-correlation differed between L4 and adult stage, where temperature 2 6 8 seems to be the main driving factor ( Figure S4). Both in L4 and adult stage, the body-size 2 6 9 traits displayed a strong positive correlation within the same temperature, and strong negative 2 7 0 correlation between different temperatures, suggesting that the variation in the body-size traits 2 7 1 were temperature specific. Interestingly, both in L4 and adult stage, the body-size traits of indicated that there were more similar patterns of variation over RILs in temperature 20 o C and To explore the source of variation of the body-size traits in the RILs population, we 2 8 0 used principal component analysis (PCA) ( Figure S5). The PCAs describes the variation of while in 20 o C, they were distributed across the PC plot. Subsequently, the value of body-size show that there was a substantial variation in the RILs, suggesting that it was possible to 2 9 2 detect QTL controlling the traits. Upon inspecting the distribution of trait variation in the RILs compared to N2 and 2 9 7 CB4856, we observed high levels of variation exceeding those of the parental strains ( Figure   2 9 8 S6). This suggests transgressive segregation within the RIL population. Hence, we tested the trait values of each RIL versus the parents. We found transgression for almost all traits per 3 0 0 temperature-developmental stage combinations (t-test, p.adjust FDR < 0.05) (Table S3). Our leading to a more robust/stable phenotype over a broader temperature range. Using ANOVA, 3 0 8 we found that developmental stage was indeed the factor driving transgression (p = 0.0275; 3 0 9 R 2 = 0.073 Table 1) whereas temperature alone showed no relation to the transgression (p =    Next, to determine the proportion of variance in body-size traits that were caused by at 24 o C in L4) to 0.99 (length/width ratio at 16 o C in L4) ( Figure 2C; Table S4). Hence, for a 3 1 9 large fraction of traits we could detect a high contribution of genetic factors. In addition to H 2 , 3 2 0 we calculated the narrow-sense heritability (h 2 ) to identify how much of the variation could be    body-size traits plasticity showed significant H 2 , indicating a substantial effect of the genetic 3 9 8 background on the variation of these traits in this population.

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The previous results suggested that QTL affecting body-size traits can be located on  (Table S5). For all three conditions, we found five significant plasticity  Table S8). Hence, we 4 0 9 found less QTL than for the individual temperatures. However, this was to be expected since 4 1 0 the narrow-sense heritability of plasticity was lower and resulted in fewer significant values.

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We compared the plasticity QTL to the QTL mapped for the individual temperatures.

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Of the five plasticity QTL, three of them were colocalized with QTL for body-size traits We continued by investigating the direction of the plasticity QTL. In two of the QTL phenotype, while CB4856 genotype have an increased phenotype ( Figure 4C). In contrast, for  this strain is known to deviate from this rule (Gutteling et al., 2007;Kammenga et al., 2007).

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Interestingly, we also found that the type of reaction norm was affected by developmental indicates that variation in phenotypic plasticity was not common in juvenile animals 4 5 7 suggesting the absence of (or less significant) GxE for most genotypes as described in (Saltz Heritability is a population trait characteristic and highly depends on the type of population 5 0 5 used and environment. Therefore, the fact that we found similar heritability with previous 5 0 6 works indicate that the variation of these traits is quite stable between different mapping 5 0 7 population. This could also mean that the relative effect of the micro-environment as well as the stochasticity is small. Furthermore, similar patterns of heritability that changed over   while y-axis represents the corresponding body-size traits. Blue box visualizes CB4856 and 7 1 0 orange bos is N2. R 2 shows how much the variation of the body-size traits can be expalined 7 1 1 by genetic variation in the chromosome.