Sex and the single embryo: early deveopment in the Mediterranean fruit fly, Ceratitis capitata

Abstract

BackgroundIn embryos the maternal-to-zygotic transition (MTZ) integrates post-transcriptional regulation of maternal transcripts with transcriptional activation of the zygotic genome. Although the molecular mechanisms underlying this event are being clarified in Drosophila melanogaster, little is know about the embryogenic processes in other insect species. The recent publication of expressed sequence tags (ESTs) from embryos of the global pest species Ceratitis capitata (medfly) has enabled the investigation of embryogenesis in this species and has allowed a comparison of the embryogenic processes in these two related dipteran species, C. capitata and D. melanogaster, that shared a common ancestor 80-100 mya.ResultsUsing a novel PCR-based sexing method, which takes advantage of a putative LTR retrotransposon MITE insertion on the medfly Y chromosome, the transcriptomes of individual early male and female embryos were analysed using RT-PCR. This study is focused on two crucial aspects of the onset of embryonic development: sex determination and cellular blastoderm formation. Together with the three known medfly genes (Cctransformer, Cctransformer2 and Ccdoublesex), the expression patterns of other medfly genes that are similar to the D. melanogaster sex-determination genes (sisterlessA, groucho, deadpan, Sex-lethal, female lethal d, sans fille and intersex) and four cellular blastoderm formation genes (Rho1, spaghetti squash, slow-as-molasses and serendipity-α) were analyzed, allowing us to sketch a preliminary outline of the embryonic process in the medfly. Furthermore, a putative homologue of the Zelda gene has been considered, which in D. melanogaster encodes a DNA-binding factor responsible for the maternal-to-zygotic transition.ConclusionsOur novel sexing method facilitates the study of i) when the MTZ transition occurs in males and females of C. capitata, ii) when and how the maternal information of "female-development" is reprogrammed in the embryos and iii) similarities and differences in the regulation of gene expression in C. capitata and D. melanogaster. We suggest a new model for the onset of the sex determination cascade in the medfly: the maternally inherited Cctra transcripts in the female embryos are insufficient to produce enough active protein to inhibit the male mode of Cctra splicing. The slow rate of development and the inefficiency of the splicing mechanism in the pre-cellular blastoderm facilitates the male-determining factor (M) activity, which probably acts by inhibiting CcTRA protein activity.

The transformer gene of Ceratitis capitata: a paradigm for a conserved epigenetic master regulator of sex determination in insects

“…These molecular data are consistent with previous very elegant genetic data which demonstrated that the female-promoting zygotic activity of Musca F gene (now Mdtra) depends on its maternal activity (Dubendorfer and Hediger 1998). Gabrieli et al (2010) showed that Cctra ep mRNAs are present in XX and in XY embryos, suggesting that there is no sex-specific early Cctra ep transcription. As the authors cannot discriminate between maternal and zygotic Cctra ep contribution, this issue is still open.…”

“…Indeed in Drosophila it has been previously shown that early female-specific activation of the Sxl is achieved by a concentration threshold which will enable X-linked signalling elements to form active transcription complexes (Estes et al 1995). Gabrieli et al (2010) apparently proposed to transpose this model of Drosophila Sxl dose-sensitive transcriptional activation to the threshold-sensitive Ceratitis splicing activation of Cctra ep , based on their preliminary embryonal expression data. The most recent and in part novel sex determination model for Ceratitis capitata has been proposed by Salvemini et al (2009).…”

“…As the authors cannot discriminate between maternal and zygotic Cctra ep contribution, this issue is still open. Gabrieli et al (2010) proposed that in Ceratitis ''the M-factor acts by inhibiting the CcTRA protein activity, so that it does not reach the threshold concentration required for female-specific splicing of all the Cctra ep transcripts''. This model of Ceratitis sex determination is not novel in its essence (inhibition of the CcTRA protein function) but overlapping to the model 2 of Pane et al (2002); interestingly Gabrieli and colleagues added to this model the integrative novel idea of a threshold effect, which could be however expected in the light o the previous studies performed in the model system Drosophila.…”

Isolation and characterization of Doublesex homologues in the Bactrocera species: B. dorsalis (Hendel) and B. correcta (Bezzi) and their putative promoter regulatory regions

“…At a very early stage of embryogenesis, sex determination is one of the main events in the Mother-to-Zygotic Transition (MZT) (Gouw et al 2009;Gabrieli et al 2010). …”

Sex Determination Mechanisms in the Calliphoridae (Blow Flies)

“…It was concluded that the Ag mutation prevents the production of a sufficient supply of maternal tra F needed to activate the female mode of tra splicing. However, studies of precisely timed C. capitata embryos suggest that the presence of maternal tra RNA is not sufficient to lock female embryos into the female splicing mode [Gabrieli et al, 2010]. Taking advantage of the much slower development of medfly embryos (cellularization at 9-11 h compared to 2.10-2.35 h in D. melanogaster ), it was found that both male and female embryos contained several tra transcripts during the critical pre-cellular phase, including tra M and tra F .…”