BACKGROUND: In 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.
RESULTS: Using 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-alpha) 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.
CONCLUSIONS: Our 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.