Maternal provision of transformer-2 is required for female development and embryo viability in the wasp Nasonia vitripennis

Elzemiek Geuverink,Anna H Rensink,Inge Rondeel,Leo W Beukeboom,Louis Van De Zande,Eveline C Verhulst

doi:10.1016/j.ibmb.2017.09.007

Elzemiek Geuverink, Anna H Rensink + Show 4 more

Open Access

https://doi.org/10.1016/j.ibmb.2017.09.007

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Abstract

In insect sex determination a primary signal starts the genetic sex determination cascade that, in most insect orders, is subsequently transduced down the cascade by a transformer (tra) ortholog. Only a female-specifically spliced tra mRNA yields a functional TRA-protein that forms a complex with TRA2, encoded by a transformer-2 (tra2) ortholog, to act as a sex specific splicing regulator of the downstream transcription factors doublesex (dsx) and fruitless (fru). Here, we identify the tra2 ortholog of the haplodiploid parasitoid wasp N. vitripennis (Nv-tra2) and confirm its function in N. vitripennis sex determination. Knock down of Nv-tra2 by parental RNA interference (pRNAi) results in complete sex reversal of diploid offspring from female to male, indicating the requirement of Nv-tra2 for female sex determination. As Nv-tra2 pRNAi leads to frequent lethality in early developmental stages, maternal provision of Nv-tra2 transcripts is apparently also required for another, non-sex determining function during embryogenesis. In addition, lethality following Nv-tra2 pRNAi appears more pronounced in diploid than in haploid offspring. This diploid lethal effect was also observed following Nv-tra pRNAi, which served as a positive control in our experiments. As diploid embryos from fertilized eggs have a paternal chromosome set in addition to the maternal one, this suggests that either the presence of this paternal chromosome set or the dosage effect resulting from the diploid state is incompatible with the induced male development in N. vitripennis caused by either Nv-tra2 or Nv-tra pRNAi. The role of Nv-tra2 in activating the female sex determination pathway yields more insight into the sex determination mechanism of Nasonia.

Highlights

Insect sex determination involves a cascade of regulatory genes that is evolving bottom-up (Wilkins, 1995)
We showed that an ortholog of tra2 is present in the genome of N. vitripennis, and that it is not sex- spliced
The tra2 gene is an important component of the Nasonia sex-determining cascade, as prevention of Nv-tra2 maternal transcript provision by parental RNA interference (pRNAi) leads to male-specific splicing of both Nv-tra and Nv-dsx pre-mRNA in diploid fertilized eggs and subsequent differentiation into functional males

Summary

Introduction

Insect sex determination involves a cascade of regulatory genes that is evolving bottom-up (Wilkins, 1995). The primary signal at the top of the cascade directs sex determination into the female or male mode and is highly variable throughout the insect class (Sanchez, 2008). The regulation of sex determination resembles an hourglass model in which the variable primary signal is considered as the instruction, and the multiple actions of dsx as the execution phase (Bopp et al, 2014). The center of the hourglass is represented by transformer (tra), a fast-evolving gene that transduces the primary signal through sex-specific splicing (Sanchez, 2008; Verhulst et al, 2010b). Tra maintains its own female-specific splicing mode through an auto-regulatory loop that functions as a “memory” to ensure proper sexual differentiation (Bopp et al, 2014)

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