Comparative Analysis of Transcriptomes among Bombyx mori Strains and Sexes Reveals the Genes Regulating Melanic Morph and the Related Phenotypes

Abstract

As a source of insect polymorphism, melanism plays an important role in ecological adaption and usually endows advantageous phenotypic-effects on insects. However, due to the mechanistic diversity, there are knowledge gaps in the molecular mechanisms underlying melanism and the related phenotypes. In silk moths, a recessive melanic mutant (sex-controlled melanism, sml) strain exhibits extended adult longevity. We took a transcriptome approach to perform a comparative analysis between this sml strain and a wild-type strain (Dazao). Our analysis resulted in the identification of 59 unique differentially expressed genes in the melanic mutant. Two key genes (laccase2 and yellow) involved in melanin formation were significantly up-regulated in melanic individuals. The laccase2 B-type isoform (BGIBMGA006746) was found to likely participate in the silkworm cuticular melanism process at late pupal stage. Moreover, we discovered 22 cuticular protein encoding genes with the possible function in melanin transport and/or maintenance. Based on our findings, we presume that the longer survival of the melanic sml male moths might be associated with the enhanced antioxidant defense systems and a reduction in the insulin/IGF-1 signaling pathway (IIS). These findings will facilitate the understanding of the molecular basis underlying melanism and the derived phenotypic-effects.

Melanism patches up the defective cuticular morphological traits through promoting the up-regulation of cuticular protein-coding genes in Bombyx mori

“…The expression patterns of BmLcp17, BmLcp18, BmLcp22 and BmLcp30 in melanic or non-melanic epidermal regions are shown in Figure 1. These results, together with earlier studies (Futahashi et al 2012; He et al 2016; Wu et al 2016), revealed that the gene expressions were significantly higher in melanic parts of the cuticle than in non-melanic parts. Moreover, micro protrusions were more intensive in the melanic regions than in the non-melanic regions, and accompanied by a higher chitin content (another important cuticle component (Hopkins and Kramer 1992; Moussian et al 2006; Andersen 2010; Moussian 2010; Chaudhari et al 2011; Qiao et al 2014), and the reduction chitin content was reported to impede cuticle melanism (Moussian et al 2005)) (Figure S1A).…”

“…The expression profile of cuticular protein-encoding genes in melanic silkworm mutants and the black markings of Papilio larvae supported the hypothesis that up-regulated cuticular proteins probably participate in the transport or maintenance of the corresponding pigments in a specifically colored cuticles (Figures 1, 2 and Figure S1) (Futahashi et al . 2012; He et al . 2016; Wu et al .…”

“…Recently, several high throughput expression surveys showed that the abundance of cuticular protein-encoding genes in different colored integuments varied in some insect species, especially with evidently up-regulated in the melanic regions (Futahashi et al . 2012; He et al . 2016; Wu et al .…”

“…Recently, several high throughput expression surveys showed that the abundance of cuticular protein-encoding genes in different colored integuments varied in some insect species, especially with evidently up-regulated in the melanic regions (Futahashi et al 2012; He et al 2016; Wu et al 2016; Tajiri 2017), and some of those shared very similar expression patterns and functions (Nakato et al 1994; Nakato et al 1997; Shofuda et al 1999; Okamoto et al 2008; Liang et al 2010; Tang et al 2010; Qiao et al 2014). These studies implied that there are probably some relationships between the promotion of melanism and the expression of cuticular protein-encoding genes.…”

Comparative analysis of integument transcriptomes identifies genes that participate in marking pattern formation in three allelic mutants of silkworm, Bombyx mori

“…Dai et al found that the lack of a low-complexity epidermal protein BmorCPH24 in silkworm causes abnormal markings, which suggests that coloration in silkworm larvae is influenced by cuticular proteins (Xiong et al 2017). Previous transcriptomes analyses have been identified a number of cuticular proteins in the pigmented integument of silkworm strains, including quail (q) (Nie et al 2014), quail-like (q-l p ) (Wang et al 2017), black dilute (bd) (Wu et al 2016), sexcontrolled melanism (sml) (He et al 2016), and the epidermis of larvae and wings of adult in butterfly (Zhang et al 2017), suggesting that cuticular proteins are important for epidermal construction and pigmentation. However, the identified Fig.…”