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Transformer in ceratitis

Development 129, 3715-3725 (2002) Printed in Great Britain The Company of Biologists Limited 2002DEV7952 The transformer gene in Ceratitis capitata provides a genetic basis for
selecting and remembering the sexual fate
Attilio Pane, Marco Salvemini, Pasquale Delli Bovi, Catello Polito and Giuseppe Saccone*
Dipartimento di Genetica, Biologia Generale e Molecolare, Università degli Studi di Napoli ‘Federico II', Via Mezzocannone 8,80134 Napoli, Italy*Author for correspondence (e-mail: [email protected]) Accepted 30 April 2002 The medfly Ceratitis capitata contains a gene (Cctra) with
expression in XX embryos by RNAi treatment can cause
structural and functional homology to the Drosophila
complete sexual transformation of both germline and soma
in adult flies, resulting in a fertile male XX phenotype. The
Similar to tra in Drosophila, Cctra is regulated by
male pathway seems to result when Cctra autoregulation is
alternative splicing such that only females can encode a
prevented and instead splice variants with truncated open
full-length protein. In contrast to Drosophila, however,
reading frames are produced. We propose that this
where tra is a subordinate target of Sex-lethal (Sxl), Cctra
repression is achieved by the Y-linked male-determining
seems to initiate an autoregulatory mechanism in XX
factor (M).
embryos that provides continuous tra female-specific
function and act as a cellular memory maintaining the
Key words: Ceratitis capitata, Sex determination, transformer, female pathway. Indeed, a transient interference with Cctra
all aspects of somatic sexual dimorphism via a short cascadeof subordinate regulatory genes (Nagoshi et al., 1988). When A broad variety of genetic cues that determine the sexual fate the gene is active, it dictates female development; when it is of a developing individual are known. Even within a minor inactive, male development follows. Once the gene is activated taxonomic group, for example, dipteran insects (Marin and in females, its products initiate a positive autoregulatory Baker, 1998; Schutt and Nöthiger, 2000), we find male mechanism that guarantees the continuous production of SXL, heterogamety with a male-determining Y chromosome thus forming a cell memory of the sex and maintaining the cells (Musca, Ceratitis) or with a single autosomal factor on the female pathway throughout development (Bell et al., (Megaselia, Culex), female heterogamety (Musca, 1991). In males, however, where Sxl is not activated, the gene Chironomus), chromosomal balance systems (Drosophila, will remain functionally OFF. Sxl produces sex-specific Sciara), and maternal effects (Chrysomya). This variety raises mRNAs by alternative splicing: the female-specific mRNAs the issues of how these different mechanisms have evolved and encode full-length functional Sxl protein, while the male- how much they differ at the genetic and molecular level.
specific ones have an additional stop-containing exon and Comparative analyses of different species can be used to encode a truncated non-functional Sxl peptide. The ON/OFF address these. We have chosen the economically important state of Sxl activity is set early during embryogenesis by medfly (Mediterranean fruitfly), Ceratitis capitata complex combination of transcriptional and post- (Tephritidae). In this species a Y-linked factor, M, determines transcriptional gene regulation (Bell et al., 1991; Keyes et al., maleness, and absence of M leads to female development 1992). The initial activation of Sxl in XX embryos relies on the (Willhoeft and Franz, 1996). However, how this signal is use of an alternative XX-embryo-specific promoter that relayed to genes responsible for expressing dimorphic traits is responds to the genes signaling the X:A ratio (Parkhurst et al., completely unknown. 1990). Sxl pre-mRNAs produced from this promoter have such The genetic cascade regulating sexual development in a structure that they are spliced in a female-specific mode by Drosophila is well known down to molecular details (Cline, the spliceosome independently of additional trans-acting 1993; Cline and Meyer, 1996). In contrast to Ceratitis, the factors, such as the Sxl protein itself (Horabin and Schedl, primary signal in Drosophila is polygenic and is formed by the 1996; Zhu et al., 1997). The RNA-binding Sxl proteins ratio of X chromosomes to sets of autosomes, the so-called translated from these early mRNAs then initiate the X:A ratio. When this ratio is 1.0 (XX:AA), the gene Sex-lethal autoregulatory loop by directing the female-specific processing (Sxl) is activated; with a ratio of 0.5 (X:AA), Sxl remains of the pre-mRNAs produced from the late Sxl promoter. The inactive. Sxl now acts as the key ON/OFF switch that controls late pre-mRNAs, in contrast to the early Sxl pre-mRNAs, can 3716 A. Pane and others be spliced in the female-specific mode only in the presence of First-Strand Synthesis System for RT-PCR (Gibco BRL). RT-PCR Sxl protein.
reported in Fig. 1B was performed with the following primers: To execute the correct developmental program, Sxl transmits 164+ (5′-CAGTGGTTCGGTTCGGAAG-3′) located in Cctra the determined state to transformer (tra) (Boggs et al., 1997), the next gene in the cascade. At this level, Sxl regulates the 900– (5′-TCCATGATGTCGATATTGTCC-3′) located in Cctra choice between two alternative 3′ splice sites in the pre-mRNA Cctra male specific cDNA M1 and M2 were amplified by RT-PCR of tra (Inoue et al., 1990; Valcárcel et al., 1993). In absence of using the following oligonucleotides: SXL, the more proximal site is used resulting in a tra mRNA F+ (5′-CATGAACATGAATATTACAAAGGC-3′) that encodes a truncated inactive protein. When SXL is present, E– (5′- TCGCGTTCTCTAATCTCGTC-3′) it will bind to the tra pre-mRNA and enforce the use of the These primers were derived from female-specific CctraF1 cDNA.
distal 3′ splice site to produce an mRNA with a full-length ORF RT-PCR was performed on RNA from unfertlized eggs, using (Sosnowski et al., 1989). The state of activity of tra is then 164+/900- primers. Cycling conditions were denaturation at 94°C for transmitted to doublesex (dsx) (Burtis and Baker, 1989), the last 5 minutes, followed by 35 cycles of 94°C for 1 minutes, annealing at component of the pathway. In females, TRA, together with 60°C for 1 minute and extension at 72°C for 2.5 minutes, with a final the constitutively expressed TRA-2, binds to dsx pre-mRNA 5 minute extension at 72°C. The PCR products were gel-purified,cloned using the Sure Clone Ligation Kit (Amersham Pharmacia directing its female-specific splicing, such that a mature mRNA Biotech) and sequenced by T7 Sequencing Kit (Amersham Pharmacia encoding the DSXF protein is generated (Hoshijima et al., Biotech). Y-specific repetitive elements were amplified from genomic 1991; Tian and Maniatis, 1993). In males, absence of TRA DNA by PCR using the following oligonucleotides: causes male-specific splicing and the production of a DSXM Y-spec1 (5′-TACGCTACGAATAACGAATTGG-3′) protein. The two proteins, DSXF and DSXM, are transcription Y-spec2 (5′-GCGTTTAAATATACAAATGTGTG-3′) factors that regulate the activity of sex-specific differentiation To perform positive control experiment Cctra-specific primers genes (Burtis and Baker, 1989).
164+ (described above) and 481– (5′-CTGGAATGGCACTGGTAT- Previous studies have indicated that control of sexual TG-3′) were used. development in the medfly follows a different route. In RT-PCR experiments to analyze Ccdsx expression pattern were particular, the Ceratitis homolog of Sxl does not appear to have performed using a mix of the following Ccdsx-specific primers: Non-sex-specific 1400+ (5′-GGCATCAAGGCGTATAGAAGA-3′) a switch function: the gene is expressed in both sexes, irrespective of whether the male-determining Y is present or absent (Saccone et al., 1998), which is inconsistent with a main For the negative RT-PCR controls reverse transcriptase was not sex-determining function. However, preliminary data suggest included in the first strand cDNA synthesis reaction. that the bottom-most component of the pathway, dsx, is notonly present in Ceratitis (Ccdsx), but has conserved a role in Northern blot analysis
sexual differentiation (Saccone et al., 2000). The pre-mRNA of We separated 2 µg polyA(+) RNA by formaldehyde gel this gene is also alternatively spliced giving rise to sex-specific electrophoresis and transferred RNA onto a Hybond NX membrane products that show a remarkable structural conservation when filter (Amersham Pharmacia Biotech). For hybridization, we compared with the corresponding male and female products in incubated filters at 42°C overnight in a buffer of 50% formamide,5×SSPE buffer, 5×Denhardt solution and 1% SDS. A Cctra probe was Drosophila. Sequence analysis of Ccdsx revealed the presence prepared by nick-translation labeling of full-length CctraF1 cDNA in of putative TRA/TRA-2-binding sites close to the regulated the presence of [α32P]dCTP (NEN; 3,000 Ci/mmol). splice site, suggesting that the underlying mechanism of sex-specific splicing is conserved and under the control of proteins homologous to TRA and TRA-2 (Saccone et al., 2000; Saccone Cctra dsRNA was obtained and injected as described for Drosophila and Polito, 2002). To extend our comparative analysis, we (Kennerdel and Carthew, 1998). A CctraF1 fragment from positions isolated the Ceratitis homolog of the Drosophila transformer 164 to 900 was amplified with primers that introduced a T7 promoter sequence at each of the product ends. In vitro RNA transcriptions were In this report, we demonstrate that a homolog of this gene performed with the Megascript Kit (Ambion). Sense and antisense (Cctra), although highly diverged in sequence, is indeed RNAs were separately obtained and equal amounts of the two ssRNAwere mixed together, ethanol precipitated and resuspended in the present in the genome of Ceratitis and that, as in Drosophila, injection buffer (Rubin and Spradling, 1982). Embryos were collected Cctra has a female-determining master function. However, in 1 hour AEL (after egg laying), hand dechorionated and microinjected contrast to the Drosophila tra, Cctra plays an essential role in with either 5 µM or 15 µM dsRNA solutions. We set up 27 cages, Ceratitis sex determination by maintaining the female sexual each containing single apparently normal males chosen from the cell state through a positive feedback loop and by forming an injected flies and three Benakeion females. Twenty cages produced epigenetic memory of the sex of the organism (analogous to bisexual progenies, each consisting of a number of flies ranging from Sxl in Drosophila) (Jablonka and Lamb, 1995). two to 51 individuals. Seven cages gave female-only progenies, eachconsisting of a number of flies ranging from seven to 66 individuals(7, 17, 20, 28, 33, 52 and 66). MATERIALS AND METHODS
Genomic and cDNA library screening
To identify Ceratitis l(3)73Ah genomic clones, we screened a
PCR and RT-PCR
Ceratitis genomic library in the EMBL3 vector using standard Total RNA was extracted, as described elsewhere (Andres and methods. A probe was obtained from a 500 bp RT-PCR product Thummel, 1994), from adult individuals and from unfertilized eggs.
Oligo-dT-primed cDNA was made from DNaseI-treated total RNA of and l(3)1581–, 5′-TTGGCCACCAGCTTCTTGAG-3′) corresponding unfertilized eggs, male and female flies using the SuperScriptTM to a conserved region of Drosophila melanogaster l(3)73Ah gene

transformer in Ceratitis 3717 (GenBank Accession Number, X84372). Genomic inserts weresubcloned in pBluescript (Stratagene) and sequenced using the T7Sequencing Kit (Amersham Pharmacia Biotech). To clone the female-specific Cctra F1 cDNA we screened an adult female cDNA libraryin Lambda-Zap vector (Stratagene), using a probe obtained from aCctra 400bp HincII genomic fragment corresponding to a region ofthe common exon 2. Sequence analysis
Protein alignment was performed by MACAW clustalw/) with default settings (NCBI, NIH, Bethesda, USA). The
TRA/TRA-2 binding sites were identified in Cctra, by MACAW
and by DNA Fasta sequence comparison between Ceratitis and
Drosophila sequences.
GenBank Accession Numbers
Ccl(3)73Ah cDNA, AF436077; Cctra F1 cDNA, AF434936; Cctra
Fig. 1. Analyses of Cctra transcripts. (A) Northern blot analysis on
M1 cDNA, AF434937; Cctra M2 cDNA, AF4349378; Ccdsx F poly A+ RNA from embryos (E), larvae (L), and adult males (M) and cDNA, AF 435087; and Ccdsx M, AF434935. females (F), using as probe the F1 cDNA clone. In males, twopredominant transcripts 1.9 kb and 2 kb long are detected, while twodifferent transcripts 1.6 kb and 3 kb long are present in females. In embryos, two transcripts are detected and in larvae all fourtranscripts are detected. (B) RT-PCR amplification of Cctra on adultmales (M) and females (F) total mRNA samples. Three main Isolation of tra in Ceratitis by synteny
products are present in the female lane (F), which are 0.7 kb, 1.3 kb Given the unusually high degree of sequence divergence and 2.1 kb long. In the male lane (M) four bands are detectable among tra homologs in Drosophila (O'Neil and Belote, 1992), which are 1 kb, 1.1 kb, 1.3 kb and 2.1 kb long. Male (cM) and we decided to attempt the isolation of the tra gene in the medfly female (cF) RT-PCR negative controls (reactions without reverse by exploiting its close linkage in Drosophila to a well- transcriptase) are shown. conserved gene, l(3)73Ah (Irminger-Finger and Nöthiger,1995). Hence, as a first step towards the isolation of tra, weisolated cDNA and genomic Ceratitis sequences that cross- only in female adults, while two mRNAs, of 1.9 kb and 2 kb hybridized to a 500 bp Drosophila cDNA fragment of l(3)73Ah in size, appear only in male adults. In embryos two mRNAs at reduced stringency. These isolates indeed contained a are detected having sizes similar to those of the adult female- structurally well conserved homolog of l(3)73Ah as confirmed specific transcripts. In RNA sample extracted by larvae of by sequencing and comparison (Ccl(3)73Ah). We then mixed sexes all four transcripts can be detected suggesting that, continued to sequence a 4 kb long genomic region downstream as in Drosophila, Cctra sex-specific processing may already of the l(3)73Ah homolog and identified a putative ORF that operate early in Ceratitis development.
showed by Blast search significant sequence similarity at the A female-specific cDNA corresponding in size to the 1.6 kb amino acid level to tra in Drosophila (ranging from 32% to transcript was isolated and entirely sequenced; a comparison 40% identity scattered over 120 amino acids) and contained with partial genomic sequences revealed that it is composed of an arginine-serine-rich domain (SR-rich region) commonly three exons (Fig. 2). Using RT-PCR with various pairs of found in splicing regulators (Manley and Tacke, 1996). As in Cctra-specific primers, sex-specific amplification products Drosophila, the two genes are transcribed in opposite were recovered from RNA samples of adult flies (Fig. 1B). The orientation and sequence analysis of corresponding cDNA 164+/900– pair amplified (only in males) an abundant fragment clones revealed that they overlap by about 200 bp (data not of 1.1 kb and three minor bands of 1 (faint), 1.3 and 1.5 kb, shown). We conclude that this gene arrangement must have whereas in females, they amplified a prominent 0.7 kb long already existed in the common ancestor of these fly species.
fragment, and three minor bands of 1 (faint), 1.3 and 2.1 kb Though the significance of this synteny is unknown, it provided (Fig. 1B). The size of the female-specific 0.7 kb cDNA product an ideal entry point to the molecular identification of the tra corresponds to the one expected on the basis of the F1 cDNA homolog in Ceratitis (Cctra).
structure. The non-sex-specific fragments of 1 and 1.3 kb, inother RT-PCR experiments, were sometimes undetectable.
Cctra produces sex-specific transcripts
They most probably represent partially spliced and/or unstable If this tra homologous gene indeed corresponds to the tra Cctra RNAs. The 2.1 kb female-specific cDNA was isolated switch gene in Drosophila, we expect it to be regulated sex and entirely sequenced; a comparison with genomic sequences specifically. A Northern blot containing poly(A)+ RNA from revealed that it is an unspliced product (data not shown). The different developmental stages of the medfly was probed with size of this cDNA product suggests that it is derived from the a genomic fragment derived from the Cctra locus. We find that 3 kb female-specific transcript. Cctra transcripts are continuously present from embryonic The F+/Z1– pair of primers (Fig. 2A) amplified a male- stages until adulthood (Fig. 1A). Furthermore, this probe specific 1.7 kb cDNA product, named CctraM1 (data not detects sex-specific transcripts in samples from adult flies (Fig.
shown). The nucleotide sequence alignment of CctraM1 and 1A). The Ceratitis tra locus expresses four different mRNA CctraF1 revealed that they are colinear with the exception of variants: two products, of 1.6 kb and 3 kb in size, are found two additional exons present in the male-specific cDNA. The 3718 A. Pane and others Tra/Tra2 putativebinding sites TC C ATCAACA Drosophila Tra/Tra2 binding sites Fig. 2. Genomic organization of the Ceratitis capitata tra gene. (A) The top line represents the genomic DNA encompassing the Cctra locus.
The positions of exons in the Cctra mRNAs are shown above the line, with Ex1, Ex2 and Ex3 representing exons in common between the male
and the female mRNAs, the blue boxes representing male-specific exons, the yellow box indicating a male-specific exon in the M1 mRNA, and
the red box representing a male-specific exon included in the M2 mRNA. Numbered green ovals indicate TRA/TRA-2-binding sites (see B).
Introns are represented by solid lines. Open boxes represent the ORF of the female-specific 1.6 kb long mRNA (Female F1) encoding the
putative 429 amino acid TRA protein (see Fig. 3). Gray boxes indicate 5′ and 3′ untranslated regions. Arrows above the first line represent the
positions of the oligonucleotides used in the RT-PCR experiments. The bar indicates the scale of the figure. (B) Sequence alignment of eight
putative TRA/TRA-2 binding sites found in the Cctra genomic sequence (see A). Conserved positions between Ceratitis and Drosophila are
indicated in bold.
male-specific exons are located between the first and the prematurely the protein translation. Indeed partially different second exon of CctraF1 and they are 40 bp (ME1a) and 203 intronic sequences are retained in the M1 and M2 cDNA bp (ME1b) in length (Fig. 2A). Another pair of primers, F+/E– clones, adding stop codons in different positions (Fig. 2A).
(Fig. 2A), amplified a male-specific fragment of 0.9 kb (data This finding suggests that a functional full-length TRA is only not shown), named CctraM2, that was cloned and sequenced, encoded by the female-specific transcripts. This mode of sex- showing with respect of CctraF1 two additional exon specific regulation at the level of splicing is well documented sequences of 210 bp (ME2a) and 176 bp (ME2b). ME2a is an for the tra gene in Drosophila (Boggs et al., 1997). Different alternative exon including the previously described exons 1 and from Drosophila, however, where sex-specific regulation is ME1a, plus the intervening intronic region (Fig. 2A). This based on the alternative use of two 3′ splice acceptor sites, sex- ‘composed' new exon is produced by skipping the first 5′ splice specific regulation in Ceratitis appears more complex and is donor site. ME2b has an identical sequence to ME1b but it achieved by a combination of exon skipping and differential lacks the first 27 bp because of the usage of a downstream 3′ use of 5′ donor and 3′ acceptor sites. alternative splice site (Fig. 2A). The long ORF in the female-specific CctraF1 encodes a putative protein of 429 amino acids. The CcTRA protein Cctra female-specific transcript encodes a SR-rich
exhibits a low degree of similarity to TRA proteins in Drosophila species and it is significantly larger in size in both An alignment of CctraF1, CctraM1 and CctraM2 cDNA N and C termini. Sequence processing tools of MACAW led sequences with the genomic sequence exposes the organization to the identification of five small blocks of sequence similarity of tra in Ceratitis (Fig. 2A). The gene is composed of five dispersed throughout the longest ORF of the female-specific exons. The first, fourth and fifth exons are included in the transcripts (Fig. 3). The regions with highest similarity mature transcripts of both sexes, while the second and the third (identified also by FastA analysis) are located between CcTRA exons are male specific. The most important finding is that the positions 150-230, 286-292 and 332-342 (Fig. 3). The SR-rich female-specific transcript has a long open reading frame, while region in Ceratitis TRA and possibly the other conserved the male-specific mRNAs contain stop codons that abort domains may confer specific RNA binding and protein-protein transformer in Ceratitis 3719 interactions consistent with a proposed role in splicing suggested to us that this gene had an essential role in female regulation (Manley and Tacke, 1996). The male-specific development of the medfly. To test its function, we employed truncated protein isoforms lack the conserved boxes, the SR- the RNAi technique that permits functional studies of genes in rich region and do not show significant similarity with other genetically less amenable organisms (Kennerdel and Carthew, 1998; Hunter, 1999). A 900 bp fragment of CctraF1 was usedas a template to produce dsRNA that was then injected as a 15 tra is essential for female development in C. capitata
µM solution into either the anterior or the posterior poles of The confinement of transcripts with a long ORF to females embryos of two different laboratory strains (Benakeion and Fig. 3. Multiple sequence alignment of TRA proteins. Ceratitis capitata (Cc), D. melanogaster (Dm), D. erecta (De), D. simulans (Ds), D.
virilis (Dv) and D. hydei (Dh). Asterisks indicate amino acid identity in all species. Intron/exon boundaries are indicated by vertical arrows.
Amino acid residues occurring in the conserved regions are indicated by capital letters.
3720 A. Pane and others Fig. 4. Phenotypic analysis of RNAi intersexes. (A) Wild-
type female has long pigmented bristles on the femur
pointing towards the coxa of the foreleg (arrow in E) and
the ovopositor (A,I). (B) Wild-type male exhibits two
spatulated bristles on the head (B), a row of non-pigmented
bristles on the ventral part of the femur towards the coxa of
the foreleg, short pigmented bristles grouped on the dorsal
part of the femur (arrow in F) close to the coxa of the
foreleg (F) and male genitalia (B,O). (C,D) Intersexes
obtained by dsRNA injection into the anterior pole of the
embryos exhibit male-specific spatulated bristles on the
head (arrow in C), male-specific bristles (upper arrow in H)
and female-specific bristles (arrow in G; lower arrow in H)
mixed together on the femur of the foreleg (G,H) and
female genitalia (C,D). Some intersexes show various
degrees of abnormal gonadal development exhibiting bent
(arrow in D), deformed (L-N) or completely absent (arrow
in P) genitalia. Scale bar in D applies at A-D; scale bar in
H applies to E-H; scale bar in I applies to I; scale bar in L
applies to L; and scale bar in P applies to M-P.
anterior pole resulted in the formation of male-specific spatulated bristles on the head of intersexes(Fig. 4C,D), male-specific blue eye reflections (datanot shown), male-like bristles mixed with female-likebristles on the femur toward the coxa of the foreleg(Fig. 4G,H), but the genitalia at the posteriorremained female-like (Fig. 4C). Conversely, injectioninto the posterior pole gave rise to mosaic adults withmale genitalia but with female bristles on the head andfemale-specific green eye reflections (data notshown). The intersexes showed also various degreesof abnormal gonadal development, with abnormallybent (Fig. 4D) or deformed ovopositor (Fig. 4L) andwith mixed male-like and female-like tissues (Fig.
4M,N). A few intersexes apparently lacked genitalia(Fig. 4P). Karyotypic analyses of RNAi-treated adults
To assess the sexual karyotype of affected flies, we
performed a PCR amplification of genomic DNA
using Ceratitis Y-specific primers (Anleitner and
Haymer, 1992). No products were detected in single
preparations of 10 randomly chosen intersexes (data
not shown) and six out of 10 phenotypic males did
not reveal the presence of a Y chromosome by this
test, indicating that all these animals have a female
XX karyotype (Fig. 5). These results are in agreement
with the expected loss of female-promoting activity
when tra function is impaired by RNAi. On the
contrary, male development of XY flies seems not to
be affected by RNAi of tra, suggesting that the gene,
white-eye). From a total of 900 injected embryos, 272 adult as in Drosophila, is dispensable in this sex. The occurrence of flies were recovered and grouped by their sexual phenotype. A intersexes and of few females is most likely due to incomplete strong sex ratio bias was observed in favor of males. Out of penetrance of the RNAi effect. Indeed, when a lower 272, 231 flies (84.9%) showed a normal male morphology, 37 concentration of dsRNA (5 µM versus 15 µM) was injected flies (13.6%) exhibited various degrees of intersexuality (Fig.
into the anterior embryonic region, we obtained 64 intersexes, 4) and the remaining four (1.4%) were the only flies recovered 76 males and four females out of 144 adult flies. Therefore the with a normal female phenotype. All of the 37 intersexes percentage of intersexes increased from 14% to 44%, while the exhibited an anteroposterior pattern of intersexuality. More percentage of males decreased from 84% to 52%, suggesting tellingly, the position of male tissues correlated exactly with that XX individuals were only partially masculinized. From the initial injection site in the embryo: injection into the these results, we conclude that tra is required for female

transformer in Ceratitis 3721 Fig. 5. Karyotypic analysis of RNAi treated individuals.
(A) PCR with Y-specific oligonucleotides carried on medfly
genomic DNA. From lane 1 to 10, PCR on single males
developed from dsRNA-injected embryos; lanes 11 and 12,
PCR on single wild-type females; lanes 13 and 14, PCR on
single wild-type males. PCR on flies of mixed sexes and a
negative control are shown, respectively, in lanes 15 and 16.
The PCR amplification patterns in lanes 1,2,6 and 7
correspond to those of wild-type males, indicating that the
analysed adults have an XY karyotype. By contrast, no
bands are detected in lanes 3-5,8-10 indicating that these
males lack a Y chromosome and therefore are XX sexually
transformed males. (B) Positive PCR control with Cctra
specific primers (Cctra164+ and Cctra481–) showing that
medfly genomic DNA is present in all samples. Lane M
(A,B) presents the molecular weight marker.
development in Ceratitis. Moreover, it is conceivable that male fertility, as previously suggested by others (Willhoeft and absence of tra activity constitutes a signal that triggers the male fate. Thus, as in Drosophila, Ceratitis tra may act as a geneticswitch between female (when functionally ON) and male Activity of tra is maintained by autoregulation
(when functionally OFF) development. The male-specific short Next, we wanted to investigate the mechanisms which control peptides encoded by the alternatively spliced male-specific the activity of tra in Ceratitis. In Drosophila, regulation of tra transcripts seem to be non-functional, at least at early activity is achieved at the post-transcriptional level based on 3′ embryonic stages, because the RNAi has no evident effects on splice site selection (Boggs et al., 1997). When SXL protein is the development of XY males. We cannot determine, however, present, it prevents the use of a distal acceptor site, thereby whether they play a function at later stages, when the RNAi promoting the use of the next downstream available 3′ splice starts to lose its efficiency. site, and it shifts about 50% of the pre-mRNA molecules froma non-sex-specific splicing to a productive female-specific Adult XX males developed from RNAi-injected
mRNA (Sosnowski et al., 1989). It has been shown that this embryos are fertile
regulation requires the direct binding of SXL to a poly (U)8 To investigate the fertility of the RNAi-treated adults, 27 males stretch upstream of the regulated splice site (Kanaar et al., obtained from embryos injected with 15 µM dsRNA solution 1995). Several findings argue against a similar mechanism for were individually crossed with wild-type females. We conferring sex-specific splicing of tra in Ceratitis (Saccone et predicted that if XX males are fertile than they should give a al., 1998). First, Cctra splicing is based on a combination of female-only progeny when crossed with wild-type virgin exon skipping and 5′ and 3′ splice site regulation, rather than females. Indeed out of 27, seven crosses gave a unisexual on 3′ splice site selection. Second, CcSXL protein is present female-only progeny. The karyotype of these seven males was in both sexes of Ceratitis. However, upon close inspection of then analyzed by PCR, as previously described, confirming the Cctra sequence, we made an important discovery: within that they were XX fertile males. As expected, PCR karyotypic the two male-specific exons and the male-specifically retained analyses of those males giving a bisexual progeny revealed that intron, eight repeats were found by DNA sequence comparison they were XY males (data not shown). Our data demonstrate that are structurally related to the TRA/TRA-2 binding sites that the Y-chromosome does not carry genes necessary for (13 nucleotides long) in the dsx gene of Drosophila (Tian and Fig. 6. Analysis of Cctra and Ccdsx splicing
patterns in adult individuals. (A) RT-PCR with
Cctra specific primers Cctra164+ and Cctra900–
on XY and XX males from dsRNA-injected
embryos (lanes 1 and 2) and on wild-type males
(lane 3) and females (lane 4). Lanes c1-c4 show RT-
PCR negative controls. The dsRNA injection in XX
embryos induces a permanent shift in the splicing
pattern of Cctra that turns from a female to a male
mode. (B) RT-PCR with Ccdsx-specific primers
(Ccdsx1400+, Ccdsx1130– and Ccdsx2000–) on the
same cDNA samples used in A. The 0.6 kb
fragment corresponds to a region of Ccdsx female-
specific transcript, while the 0.3 kb fragment
represents a region of Ccdsx male-specific
transcript. A consequence of the Cctra-specific RNAi is a persistent change in Ccdsx regulation that turns from a female-specific to a male-specific splicing mode. A molecular weight marker is also shown in lane M (A,B). 3722 A. Pane and others Maniatis, 1993) (Fig. 2A,B). Similar repeats are alsodetected in the female-specific exon of the dsx homolog in Ceratitis (Saccone et al., 2000). Their highsequence similarity to Drosophila binding sites (Fig. 2B) and peculiar localization withinthe Cctra gene led us to believe that these sequences are involved in the sex-specific splicing regulation ofCctra itself. In Drosophila, dsx and fru genes these cis-elements act as, respectively, 3′ and 5′ splice enhancers by recruiting the TRA/TRA-2 complex topromote the use of the regulated splice site (Tian andManiatis, 1993; Heinrichs et al., 1998). The presence of potential TRA/TRA-2-binding sites in and aroundthe male-specific exons suggests that the female-specific CcTRA could inhibit their usage and led us toinvestigate whether an autoregulatory function of Cctra is involved in the process of sex-specificsplicing. If female-specific splicing of tra pre-mRNA indeed depends on tra activity, we reasoned that a transientdepletion of tra activity should no longer be able tosustain the female mode of splicing. To test thissupposition, we analysed sex-reversed XX males Fig. 7. Model for sex determination in Ceratitis capitata. (A) In XX
recovered from Cctra dsRNA injections. By RT-PCR embryos, a maternal Cctra mRNA provides full-length CcTRA protein thatinitiates a positive feedback regulation. This protein drives a female-specific analysis, only male-specific tra products were splicing of the zygotically transcribed Cctra pre-mRNA so that new CcTRA detected in adult tissues of injected XX and XY protein can be produced. The newly synthesized protein controls the individuals, but no female-specific products (Fig. 6A).
maintenance of Cctra autoregulation and the female-specific splicing of In addition, the same males contained predominantly Ccdsx pre-mRNA. Therefore a CcDSXF protein is produced that induces, at male-specific splice variants of dsx, a probable least in part, female development. (B) In XY embryos, Cctra autoregulation downstream target of tra also in Ceratitis (Fig. 6B).
is impaired by the male determining M factor. The M factor could prevent We infer from these results that early application of the translation of the maternal Cctra transcript (1) or inhibit the function of RNAi transiently eliminates Cctra mRNAs and, thus, the protein that is produced by this mRNA (2). It is also conceivable that the prevents continued production of TRA protein. Once M could interact with the spliceosome or repress Cctra transcription tra pre-mRNA production is resumed at a later stage initiation in the zygote (3). In any case, the result is always that a full-lengthCcTRA protein is not produced in XY embryos and, thus, the autoregulatory in development, the unproductive male mode of tra loop can not initiate. In absence of CcTRA protein, Ccdsx is expressed by splicing is launched because of the absence of default to produce the CcDSXM isoform, which induces, in turn, male functional TRA. Likewise, absence of TRA causes its direct target dsx to be spliced in the male mode. Theseresults are compatible with our postulate that Cctrasustains the productive mode of its splicing by anautoregulatory feedback loop and mediates female processing its own mRNA, thus initiating an autoregulatory differentiation, at least in part, by the control of its target gene mechanism to continuously produce a full-length protein.
dsx. The initiation of the autoregulatory loop in XX embryos Interference at this level, for example, by injection of Cctra could be based on maternal Cctra mRNAs that have been dsRNA, leads to a breakdown of the regulatory loop and to the detected in unfertilized eggs by RT-PCR experiments (data not production of male-specific mRNAs encoding truncated shown). These mRNAs are spliced in the female mode and peptides. Thus, Cctra can be regarded as (1) an early binary hence could provide a source of CcTRA activity that allows switch in the sex-determining pathway of Ceratitis: when ON, female-specific splicing of zygotic Cctra pre-mRNA. female development ensues, when OFF, male developmentfollows; (2) a key gene controlling an epigenetic cell ‘memory'system of Ceratitis sex determination with evident analogies DISCUSSION
with the Drosophila Sxl gene. We have isolated a gene, Cctra, which is an ortholog of A comparison between Ceratitis tra gene and its
Drosophila tra and acts as key regulator in sex determination homolog in Drosophila: parallels and differences
of the medfly Ceratitis capitata. Cctra is regulated, as in Our results show that Ceratitis and Drosophila sex- Drosophila, by sex-specific splicing and encodes a protein determining cascades share a conserved tra>dsx genetic showing, as expected, low sequence conservation, when module to control sex determination and sexual differentiation compared with TRA proteins of Drosophila species (O'Neil, as well as that tra sex-specific splicing regulation differs in the and Belote, 1992). We present evidence that female two species. In Drosophila, TRA protein, together with TRA- development depends on an active Cctra that, in XX 2, binds to the TRA/TRA-2 recognition sequences on the individuals, seems to promote the productive mode of Drosophila dsx pre-mRNA and promotes the use of a nearby transformer in Ceratitis 3723 female-specific acceptor site. We show that Cctra is needed to As zygotes that carry a Y chromosome do not activate Cctra impose the female-specific splicing of Ccdsx, most probably female-specific splicing and autoregulation, we propose that by a similar mechanism as in Drosophila, invoking the the Y-linked male-determining M factor prevents this activation existence of a Cctra2 homolog (Saccone et al., 2000; Saccone (Fig. 7). It is conceivable that Cctra is a direct target of the M and Polito, 2002). This hypothesis is also supported by the factor. Presence of this M factor in the zygote may prevent the finding of TRA/TRA-2 recognition sequences located in close production of CcTRA protein. The Cctra positive feedback vicinity to the female-specific acceptor site in Ccdsx pre- loop is a probable target for regulation, because of its mRNA (Saccone et al., 2000). sensitivity (already shown by RNAi). An important question to In Drosophila, tra female-specific splicing is promoted by be addressed is how autoregulation of Cctra is initiated in XX SXL, which blocks the use of the non-sex-specific splice site embryos of C. capitata and how this is prevented in XY present in the tra pre-mRNA. In Ceratitis, the presence of embryos. A possible explanation is suggested by the Cctra multiple TRA/TRA-2-binding elements within the Cctra female-specific mRNAs encoding the full-length protein, male-specific exonic sequences strongly suggests that CcTRA which have been detected in unfertilized eggs. Depositing these and a hypothetical CcTRA-2 proteins could bind to them Cctra transcripts in eggs may provide a source of activity that mediating a direct autoregulation. The unusually strong can be used later for ‘female-specific' processing when Cctra phenotypic effects of the RNAi against this gene also support is zygotically transcribed (Fig. 7). Once zygotically activated this model of Cctra regulation. The localization of the putative in XX embryos, Cctra promotes its own female-specific regulatory elements within the Cctra gene indicates a splicing maintaining the female sex determination and the repression mode by which CcTRA in females prevents the female-specific splicing of the downstream Ccdsx gene. Taken recognition of male-specific splice sites. The mechanism by together, these events induce the female differentiation (Fig.
which Cctra seems to promote the female mode of processing 7A). In our model for sex determination of medfly, the M factor of its own pre-mRNA by TRA/TRA-2-binding elements is directly involved in the Cctra sex-specific regulation (Fig.
appears to be different also from the female-specific splicing 7B). Thus, in the presence of M Cctra, autoregulation is of dsx. Rather than activating a splice site nearby the regulated blocked and the gene produces male-specific transcripts exon, as in the case of dsx, inclusion of male-specific Cctra encoding short and possibly non-functional CcTRA peptides.
sequences is suppressed when CcTRA is present. Although The absence of CcTRA leads Ccdsx to produce male-specific this would be a novelty with the respect to known Drosophila transcripts by default, promoting male differentiation (Fig.
TRA/TRA-2 activities, it has been previously shown that the 7B). The control of the M factor upon Cctra expression could ‘behavior' of these cis elements is context dependent and that be exerted at different levels. The male determiner M could, changing the location of splicing enhancers can transform for example, act at the pre-translational level blocking the them into negative regulatory elements (Kanopka et al., 1996; production of CcTRA protein from the maternal transcripts. M Lopez, 1998).
could act at the post-translational level antagonising theformation of protein complexes necessary for the female A model for sex determination in Ceratitis capitata
splicing mode. Or M could act as a transient transcriptional In Drosophila, the presence of the Y chromosome is necessary repressor of Cctra to reduce the amount of active CcTRA for male fertility but not for male development (Hardy et al., below a threshold needed to maintain the feedback loop. The 1981). By contrast, RNAi-treated Ceratitis embryos with a proposed autoregulatory model of Cctra may also explain the female XX karyotype can develop into fertile males, which remarkable efficiency of sex reversal by Cctra RNAi: a indicates that transient repression of Cctra by RNAi is transient silencing of Cctra by injecting dsRNA is sufficient to sufficient to implement fully normal male development. The let the loop collapse. Furthermore, the sensitivity of this cases of complete sexual transformation of genetic Ceratitis positive autoregulation could be an evolutionary widely females (XX) into fertile males by RNAi demonstrate that the conserved pre-requisite to permit a ‘faster' recruitment/ Y chromosome, except for the dominant male determiner M, replacement of different upstream regulators and to easily does not supply any other contribution to both somatic and evolve different sex determining primary signals, as observed germline male development, as suggested by previous Y- in dipteran species.
chromosome deletion analysis (Willhoeft and Franz, 1996).
Sex can even be determined by a maternal effect in dipteran Other dipteran species, such as Musca domestica (Hilfiker- species such as Sciara coprophila (Crouse, 1960) and Kleiner et al., 1994) and Chrysomya rufifacies (Ullerich, 1984) Chrysomya rufifacies (Ullerich, 1984). Our hypothesis of a show a female and male germline sex determination that is Cctra maternal contribution to the activation of the zygotic completely dependent on the sexual fate of the soma. However, Cctra gene has similarities to the model of sex determination in Drosophila, the XX and XY germ cells seem to respond proposed for Musca domestica (Dübendorfer and Hediger, differently to sex determining somatic cues (Waterbury et al., 1998). In the common housefly, the maternal product of the 2000; Steinmann-Zwicky et al., 1989). Indeed the XY germ key switch gene F is needed to activate the zygotic function cells have also an autonomous stage-specific sex determination of F in females. Musca male development results whenever mechanism that probably integrates the somatic signal (Janzer F cannot become active in the zygote. This happens when the and Steinmann-Zwicky, 2001). In Ceratitis, Cctra could be male-determining M is present in the zygotic genome, or required in XX somatic cells to let them induce the XX germ when maternal F is not functional because of either the cells to differentiate as oogenic cells. Alternatively, Cctra presence of M or the mutational loss of function of F (Fman) could be required in XX germ cells to ‘feminize' them. This in the germline (Dübendorfer et al., 2002). More case would be a novelty with the respect of the known interestingly, embryonic RNAi against the Musca tra-2 Drosophila transformer gene functions. homolog caused sex reversion of Musca XX adults into 3724 A. Pane and others intersexes and fertile males, although this gene is not sex- specifically expressed (Dübendorfer et al., 2002). Theserecent data in Musca and our results in Ceratitis support Andres, A. J. and Thummel, C. S. (1994). Drosophila melanogaster:
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