Nutritional conditions during early development influence the plastic expression of adult phenotypes. Among several body modules of animals, the development of sexually selected exaggerated traits exhibits striking nutrition sensitivity, resulting in positive allometry and hypervariability distinct from other traits. Using de novo RNA sequencing and comprehensive RNA interference (RNAi) for epigenetic modifying factors, we found that histone deacetylases (HDACs) and polycomb group (PcG) proteins preferentially influence the size of mandibles (exaggerated male weapon) and demonstrate nutrition-dependent hypervariability in the broad-horned flour beetle, Gnatocerus cornutus. RNAi-mediated HDAC1 knockdown (KD) in G. cornutus larvae caused specific curtailment of mandibles in adults, whereas HDAC3 KD led to hypertrophy. Notably, these KDs conferred opposite effects on wing size, but little effect on the size of the core body and genital modules. PcG RNAi also reduced adult mandible size. These results suggest that the plastic development of exaggerated traits is controlled in a module-specific manner by HDACs.phenotypic plasticity | HDAC | Gnatocerus cornutus | exaggerated male weapon | sexual dimorphism

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The songbird is an established model for comparative biology of human speech acquisition (Marler, 1970; Doupe & Kuhl, 1999). Many attempts to explore further parallels have been made to date: some described song syntax of Bengalese finches (e.g. Okanoya, 2004) and others trained zebra finches in artificial grammar composed of syllables (e.g. van Heijningen et al., 2013). Here they share a tacit assumption-songbirds consider syllables as tokens. It is a common practice for researchers in this field to isolate each syllable from birdsong for analysis. However, songbirds basically do not vocalize individual syllables apart from song bouts. In addition, Dooling & Searcy (1980) pointed out that auditory masking effects may not be ignorable when song notes are perceived. We hypothesized that if songbirds perceived syllables as tokens, they would detect a syllable regardless of temporal distance from another syllable. 6 female Bengalese finches were first trained to detect syllable [a] that were coupled with another syllable [p]. In p-t condition [p] preceded [a]; in t-p condition [p] followed [a]. The interval had 4 variants that were uncommon in Bengalese finch song (Figure 1A). After reaching a predetermined pass criterion, probe stimuli were introduced. They had shorter or longer interval than that of training stimuli (Figure 1A). Birds with odd ID went through p-t condition first, and birds with even ID went the other way around (Figure 1B). Difference in mean days each group took for the first condition was not significant (Welch's two sample ttest, t = 4.05, df = 2, P = 0.055). Once they finished either condition, they did not require so long days to get accustomed to new conditions (paired ttest, t = 5.87, df = 5, P = 0.0020).