Understanding the genetics of sex determination in insects and its relevance to genetic pest management

Abstract

Sex determination pathways regulate male and female-specific development and differentiation and offer potential targets for genetic pest management methods. Insect sex determination pathways are comprised of primary signals, relay genes and terminal genes. Primary signals of coleopteran, dipteran, hymenopteran and lepidopteran species are highly diverse and regulate the sex-specific splicing of relay genes based on the primary signal dosage, amino acid composition or the interaction with paternally inherited genes. In coleopterans, hymenopterans and some dipterans, relay genes are Transformer orthologs from the serine-arginine protein family that regulate sex-specific splicing of the terminal genes. Alternative genes regulate the splicing of the terminal genes in dipterans that lack Transformer orthologs and lepidopterans. Doublesex and Fruitless orthologs are the terminal genes. Doublesex and Fruitless orthologs are highly conserved zinc-finger proteins that regulate the expression of downstream proteins influencing physical traits and courtship behaviours in a sex-specific manner. Genetic pest management methods can use different mechanisms to exploit or disrupt female-specific regions of different sex determination genes. Female-specific regions of sex determination genes can be exploited to produce a lethal gene only in females or disrupted to impede female development or fertility. Reducing the number of fertile females in pest populations creates a male-biased sex ratio and eventually leads to the local elimination of the pest population. Knowledge on the genetic basis of sex determination is important to enable these sex determination pathways to be exploited for genetic pest management.

Keywords: complementary sex determiner; femaleless; masculinizer; sex‐lethal; wasp overruler of masculinization.

FIGURE 1

Sex determination pathway of the dipteran species Drosophila melanogaster, Musca domestica and Anopheles gambiae; the coleopteran species Tribolium castaneum, the hymenopteran species Apis mellifera and Nasonia vitripennis; and the lepidopteran species Bombyx mori. In dipteran, coleopteran and hymenopteran females, the primary signals, functional Sex‐lethal ortholog, Transformer ortholog or Transformer paralog proteins, regulate female‐specific splicing of Transformer ortholog pre‐mRNA, the relay genes. Functional Transformer ortholog proteins in dipteran, coleopteran and hymenopteran females regulate sex‐specific splicing of Doublesex pre‐mRNA and Fruitless‐P1 mRNA to produce a female‐specific Doublesex isoform and a non‐functional Fruitless‐P1 protein. In dipteran, except for mosquitos, coleopteran and hymenopteran males, functional Transformer ortholog or Transformer paralog proteins are not produced. Resulting in the default splicing of Transformer ortholog pre‐mRNA, Doublesex pre‐mRNA and Fruitless‐P1 pre‐mRNA in dipteran, coleopteran and hymenopteran males. Males of dipteran, coleopteran and hymenopteran species produce non‐functional Transformer ortholog proteins, male‐specific Doublesex isoforms and functional Fruitless‐P1 proteins. The sex determination pathway of An. gambiae is mostly unknown. The Yob male‐determining factor and potential primary signal Femaleless mRNA were identified, but the exact interaction of the components remains to be determined. Unknown relay genes ensure male and female‐specific Doublesex proteins are produced in An. gambiae males and females. Additionally, unknown factors regulate the production of functional and non‐functional Fruitless‐P1 proteins in An. gambiae males and females. Contrastingly, in the lepidopteran B. mori, a Masculinizer protein increases the expression of an RNA‐binding protein that interacts with other RNA‐binding proteins to regulate male‐specific splicing of Doublesex pre‐mRNA. In B. mori females, a female‐specific piRNA inhibits the production of Masculinizer proteins through an RNA‐silencing feedback loop. AGO3.P, Argonaute‐3 protein; CSD gene, Complementary sex determiner gene; CSD.P, Complementary sex determiner protein; Dsx, Doublesex; Fem, Feminizer; Fem.P, Feminizer protein; Fru‐P0, Fruitless derived from promotor 0; Fru‐P0.P, Fruitless protein derived from the promotor 0; Fru‐P1, Fruitless derived from promotor 1; Fru‐P1.P, Fruitless protein derived from promotor 1; IMP.P, Insulin‐like growth factor mRNA‐binding protein; Masc, Masculinizer; PSI.P, P‐element somatic inhibitor protein; RBP1.P, RNA‐binding protein 1 protein; RBP3.B, RNA‐binding protein 3.B; RBP3.B.P, RNA‐binding protein 3.B protein; siwi.P, silkworm PIWI protein; Sxl, Sex‐lethal; Sxl.P, Sex‐lethal protein; Tra, Transformer; Tra.P, Transformer protein; Tra‐2.P, Transformer‐2 protein; Wom, Wasp overruler of masculinization; Wom.P, Wasp overruler of masculinization protein; XSE, X‐linked signal elements.

FIGURE 2

Genomic architecture of Transformer and Transformer‐2 genes. The number and position of autoregulation domains and arginine/serine‐rich domains differ between dipteran, coleopteran and hymenopteran species (Adapted from Geuverink and Beukeboom (2014) and Nguantad et al. (2020)). A, autoregulation domain; Dip., dipteran specific domain; Hym., hymenopteran specific domain; Pro, proline‐rich domain; RRM, RNA‐recognition motif; RS.1/2, arginine/serine‐rich domain.

FIGURE 3

Genomic architecture of the Doublesex and Fruitless genes. The Doublesex and Male‐abnormal‐3 domain in the N‐terminal of the Doublesex gene contains a DNA‐binding domain (DBD) and an Oligomerization domain 1 (OD1). The Oligomerization domain 2 (OD2) in the C‐terminal of the Doublesex gene contains a unisex and sex‐specific region (Adapted from Geuverink and Beukeboom (2014)). Fruitless isoforms contain an N‐terminal bric à brac, tramtrack and Broad‐Complex (BTB) domain and a C‐terminal zinc‐finger motif that can differ between Fruitless isoforms. Male‐specific Fruitless‐P1 isoforms also contain a male‐specific amino acid sequence upstream of the BTB domain. Similarly, female‐specific Fruitless‐P0 isoforms unique to N. vitripennis were predicted to contain amino acid sequence upstream of the BTB domain (Adapted from Bertossa et al. (2009), Salvemini et al. (2010) and Sato and Yamamoto (2020).