Complex genetic architecture of Drosophila aggressive behavior

Abstract

Epistasis and pleiotropy feature prominently in the genetic architecture of quantitative traits but are difficult to assess in outbred populations. We performed a diallel cross among coisogenic Drosophila P-element mutations associated with hyperaggressive behavior and showed extensive epistatic and pleiotropic effects on aggression, brain morphology, and genome-wide transcript abundance in head tissues. Epistatic interactions were often of greater magnitude than homozygous effects, and the topology of epistatic networks varied among these phenotypes. The transcriptional signatures of homozygous and double heterozygous genotypes derived from the six mutations imply a large mutational target for aggressive behavior and point to evolutionarily conserved genetic mechanisms and neural signaling pathways affecting this universal fitness trait.

Fig. 1.

Aggressive behavior of P-element insert lines. (A) Mean aggression scores (MAS) for the control strain (CSB; gray bar) and 10 hyperaggressive homozygous mutant lines (light blue bars). (B) MAS for CSB (gray bar) and 45 double heterozygotes constructed by a half diallel cross of the 10 homozygous mutants. Light blue bars indicate double heterozygotes for which the estimate of SCA was not significant. Dark red and dark blue bars indicate, respectively, significant positive and negative SCA values. Error bars in A and B are SEM. *P < 0.05; **P < 0.01; ***P < 0.001; ***P < 0.0001. (C) Pairwise epistatic interactions among six P-element mutations affecting aggression. The lines indicate the interacting mutations; dark red denotes positive SCA values, and dark blue denotes negative SCA values.

Fig. 2.

Pleiotropic effects on brain morphology of P-element mutations affecting aggression. (A–D) Measurements of brain morphology for the control (CSB; gray bar), 6 hyperaggressive homozygous mutants (light blue bars), and 15 double heterozygotes constructed by a half diallel cross of the six mutant lines. Dark red and dark blue bars indicate, respectively, significant positive and negative SCA values. Error bars are SEM. Asterisks above the homozygotes indicate significant differences from the control. Asterisks above the double heterozygotes indicate significant SCA values. *P < 0.05; **P < 0.01; ***P < 0.001. (A) α-Lobe length, (B) α-lobe width, (C) β-lobe width, and (D) ellipsoid body area. (E–H) Pairwise epistatic interactions for brain morphology. The lines indicate the interacting mutations; dark red denotes positive SCA values, and dark blue denotes negative SCA values. (E) α-Lobe length, (F) α-lobe width, (G) β-lobe width, and (H) ellipsoid body area.

Fig. 3.

Diallel cross analysis of six homozygous hyperaggressive mutants. (A) Numbers of transcripts with significant (P < 0.05) GCA values. Gray bars denote the total number of transcripts for each genotype, and red and blue bars show the numbers of transcripts with partially dominant and recessive effects, respectively. (B) Numbers of transcripts with significant (P < 0.05) SCA values for each of the double heterozygotes. Gray bars indicate the total number of transcripts with significant epistasis, and red and blue bars show the numbers of transcripts with positive and negative SCA estimates, respectively. (C–H) Examples of epistasis for gene expression. The lines indicate the interacting mutations; dark red denotes positive SCA values, and dark blue denotes negative SCA values. (C) CG32368, (D) CG4593, (E) CG14050, (F) CHOp24, (G) CG4962, and (H) CG1667.

Fig. 4.

Functional tests. Error bars are SEM. *P < 0.05; **P < 0.01. (A) MAS for homozygous mutant alleles of candidate genes (light blue bars) and the control (CSB; gray bar). (B) MAS after treatment with DABA (light blue bars), DAPA (dark blue bars), and H₂O (gray bars). (C) MAS after treatment with lithium (light blue bars), valproic acid (dark blue bars), and H₂O (gray bars).