DOI: 10.5061/dryad.zw3r2285b
title: Condition-dependence of phenotypic integration and the evolvability of genitalic traits in a neriid fly

description: The spectacular diversity of insect male genitalia, and their relative
 insensitivity to the environment, have long puzzled evolutionary
 biologists and taxonomists. We asked whether the unusual evolvability of
 male genitalia could be associated with low morphological integration of
 genitalic traits, by comparison with male somatic traits and female
 traits. We also asked whether this pattern was robust to variation in
 resource availability during development, which affects adult condition.
 To address these questions, we manipulated larval diet quality in a
 split-brood design and compared levels of integration of male and female
 genitalic and somatic traits in the neriid fly, Telostylinus
 angusticollis. We found that male genitalic traits were substantially less
 integrated than male somatic traits, and less integrated than female
 genitalic traits. Female genitalic traits were also less integrated than
 female somatic traits, but the difference was less pronounced than in
 males. However, integration of male genitalic traits was negatively
 condition-dependent, with high-condition males exhibiting lower trait
 integration than low-condition males. Finally, genitalic traits exhibited
 lower larval diet family interactions than somatic traits. These results
 could help explain the unusually high evolvability of male genitalic
 traits in insects.
description: We utilised a full-sib, split-family breeding design where
 randomly chosen individuals from these populations were paired to create
 17 mating pairs at 15 ± 2 days old. Each pair was allowed 48 hours to mate
 within 120 mL cages provided with a nutrient-rich oviposition medium and
 access to sugar, yeast and water ad libitum. Following the 48-hour period,
 from each mating pair we transferred 20 eggs to a nutrient-poor larval
 diet and 20 eggs to a nutrient-rich larval diet, also based on [33]. Upon
 emergence, virgin adult offspring were allowed 24 hours for their
 exoskeletons to sclerotize fully and then frozen at -80°C for dissection
 and morphological measurements. We quantified six genitalic and 12 somatic
 traits on each of 93 males, and four genitalic and 11 somatic traits on
 each of 96 females. All traits were quantified by measuring the lengths of
 the structures (<i>see electronic</i>
 <i>supplementary</i> <i>material</i>), except for
 testes (TE), for which we measured area in mm<sup>2</sup>. For
 both sexes we used thorax length as an index of body size [34]. To
 minimise the loss of samples for multivariate analyses, missing trait
 values were replaced with the mean value for the family × larval diet ×
 sex combination (where &gt; 3 individuals were available for that
 treatment combination). All analyses were carried out
 using R 3.5.3 [35]. For each set of traits (genitalic and somatic) and
 group combinations (sex and larval diet), we estimated morphological
 integration as the relative standard deviation of eigenvalues,
 SD<sub>rel</sub> (λ) [36]. The higher the value of
 SD<sub>rel</sub> (λ), the more variance is explained by the
 first few principal components of the trait matrix, and therefore the
 higher the integration. Integration was estimated from principal
 components analysis (PCA) performed separately on the correlation matrix
 for each sex × larval diet × trait type combination (Figures S3, S4).
 Standard errors for integration values were obtained by a Jackknife (n-1
 traits) procedure. As a measure of environmental effects (i.e., larval
 diet quality), we computed marginal effect sizes, which indicate the
 variance explained by fixed effects [37]. To estimate the maximum genotype
 × environment interaction (G × E), we estimated the family × larval diet
 interaction. We computed conditional effect sizes, which reflect the
 variance explained by both fixed and random effects [37], and estimated
 the family x larval diet interaction by comparing the magnitude of the
 marginal and conditional effect sizes (see <i>electronic
 supplementary material</i> for details). Median parameter values
 were compared between treatment groups using nonparametric Kruskal-Wallis
 or Wilcoxon tests, with trait as the unit of replication.
  
description: Male and female trait values are transformed using log10.
 Repeatabilities were obtained by re-positioning each sample/trait and
 taking multiple images of the same sample. There are missing values in
 datasets because some samples were either damaged or major outliers
 (&gt;3 sd) in the data and removed.