Sexual Development in Lucilia cuprina (Diptera, Calliphoridae) Is Controlled by the Transformer Gene

Abstract

Insects use an amazing variety of genetic systems to control sexual development. A Y-linked male determining gene (M) controls sex in the Australian sheep blowfly Lucilia cuprina, an important pest insect. In this study, we isolated the L. cuprina transformer (Lctra) and transformer2 (Lctra2) genes, which are potential targets of M. The LCTRA and LCTRA2 proteins are significantly more similar to homologs from tephritid insects than Drosophila. The Lctra transcript is alternatively spliced such that only females make a full-length protein and the presence of six TRA/TRA2 binding sites in the female first intron suggest that Lctra splicing is autoregulated as in tephritids. LCTRA is essential for female development as RNAi knockdown of Lctra mRNA leads to the development of male genitalia in XX adults. Analysis of Lctra expression during development shows that early and midstage male and female embryos express the female form of Lctra and males express only the male form by the first instar larval stage. Our results suggest that an autoregulatory loop sustains female development and that expression of M inhibits Lctra autoregulation, switching its splicing to the male form. The conservation of tra function and regulation in a Calliphorid insect shows that this sex determination system is not confined to Tephritidae. Isolation of these genes is an important step toward the development of a strain of L. cuprina suitable for a genetic control program.

Molecular Characterization of the Key Switch F Provides a Basis for Understanding the Rapid Divergence of the Sex-Determining Pathway in the Housefly

“…Instead, all male-specific ORFs appear prematurely truncated and give rise to small and presumably nonfunctional peptides. The exon-intron organization of this gene largely coincides with that of tra orthologs found in Tephritidae and Calliphoridae (Pane et al 2002;Lagos et al 2007;Ruiz et al 2007;Concha and Scott 2009). Similarly to these tra orthologs, the sex-specific splicing regulation of Mdtra is mainly based on exon-skipping mechanisms and 59 alternative splicing, rather than 39 alternative splicing as observed for the Drosophila transformer gene.…”

“…Selection and maintenance of the female fate is based on a positive autoregulatory function of Cctra. The same mechanism might also be operational in other members of the Tephritidae (Lagos et al 2007;Ruiz et al 2007) and in Lucilia cuprina, a member of the Calliphoridae family (Concha and Scott 2009). Since Musca is phylogenetically more closely related to Tephritidae and Calliphoridae, transformer seems to be a more likely candidate for F.…”

About females and males: continuity and discontinuity in flies

“…This mode of operandi has been suggested for tra regulation in several species of Tephritidae and Calypatratae Ruiz et al 2007;Concha and Scott 2009;Hediger et al 2010). These findings led to the proposal that sex determination in the common ancestors of Acalyptratae and Calyptratae may have operated on the basis of an autoregulatory tra gene which is activated by its maternal activity.…”

“…Functional studies and structural analysis of mutant alleles proved that the Musca transformer gene (Mdtra) gene corre-sponds to F, the key switch gene in Musca sex determination (Hediger et al 2010). Likewise, expression studies of the tra orthologue in other Calyptratea species such as Lucilia cuprina (Concha and Scott 2009) and the tsetse fly, Glossina morsitans (Hediger et al 2010) point to a similar switch role in sex determination.…”

Agricultural production: assessment of the potential use of Cas9-mediated gene drive systems for agricultural pest control

“…In the absence of tra activity, XX females develop as phenotypic males. We don't know if XX males are fertile, but in the Australian sheep blowfly, a close relative of screwworm, the Y chromosome does not appear to encode any factors essential for male fertility (Concha and Scott 2009). Thus it is possible that transformed XX screwworm males will be fertile.…”