Genetic basis of female-limited polymorphism of the damselfly Ischnura senegalensis

概要

Understanding the maintenance and mating strategy, life history, genetic mechanism of conspecific color polymorphisms (CPs) is evolutionally and ecologically important. Odonata (dragonfly and damselfly) species display a remarkable diversity of color patterns, including intrasexual CPs. Most of intrasexual CPs in Odonata are female-limited CPs, which is consisted of andromorphs (male-like color) and gynomorphs (female-specific color). Although female-limited CP in Odonata has been studied about the origin and maintenance for the CP and the associated behavior and mating strategy, what gene(s) controlling female-limited CP is unclear. The purpose of the present study is to understand the genetic basis of female-limited CPs in the damselfly Ischnura senegalensis by estimating candidate gene(s) of which the expression(s) associated with the color morphs using RNA-seq (Chapter 1), and by conducting the functional analysis of candidate gene(s) using RNA interference (RNAi) (Chapter 2).

In Chapter 1, I investigated the differentially-expressed transcripts (DETs) among males and two female morphs (gynomorphs and andromorphs) using RNA-seq to identify candidate transcripts involved in female-limited CPs in the damselfly I. senegalensis. Seven DETs that had significantly different expression levels between males and gynomorphs, but not between males and andromorphs, were detected. Reverse transcription PCR and sequence analysis of the doublesex (Isdsx) gene revealed that this gene produced at least three transcripts. Two short transcripts were mainly expressed in males and andromorphs, while the long transcript was specifically expressed in both morph female groups; that is, the expression pattern of the Isdsx splice variants in andromorphs was an intermediate between that of males and gynomorphs (Fig. 1).

In Chapter 2, the function analysis of the Isdsx gene on thoracic coloration was conducted by electroporation-mediated RNAi. RNAi of common region of the Isdsx long and short transcripts in males (14/17; 82%) and andromorphs (7/9; 78%) reduced melanization and thus changed the color pattern into that of gynomorphs, while the gynomorphic color pattern was not affected. Whereas, RNAi against the Isdsx long transcript produced no changes. As a result of examining the expression levels of five genes with differences between sexes and female morphs in Chapter 1, two melanin-suppressing genes, black and ebony, were expressed at higher levels in the Isdsx RNAi body area than a control area.

The results of my study suggest that the Isdsx short transcripts are important for body color masculinization in both males and andromorphs. Because the dsx gene functions as a transcription factor controlling the expression of downstream genes, the Isdsx short isoform may induce thoracic color differentiation by suppressing black and ebony, thereby generating female CP in I. senegalensis. These findings contribute to the understanding of the molecular and evolutionary mechanisms underlying female CP in Odonata.