Gene expression system using alternative splicing in insects

A polynucleotide expression system capable of expressing a functional protein in an insect, comprising:

• a heterologous polynucleotide sequence encoding a functional protein, the coding sequence of which is defined between a start codon and a stop codon;
• a promoter capable of initiating transcription in the insect operably linked to the heterologous polynucleotide sequence;
• and
• a splice control sequence, which, in cooperation with a spliceosome in the insect or its offspring, is capable of sex-specifically mediating in the insect or its offspring (i) a first splicing of an RNA transcript of the polynucleotide sequence to produce a first spliced mRNA product, which does not comprise a continuous open reading frame extending from the start codon to the stop codon, and (ii) an alternative splicing of said RNA transcript to yield an alternatively spliced mRNA product, which comprises a continuous open reading frame extending from the start codon to the stop codon, wherein
• the splice control sequence comprises a core consensus sequence of WWCRAT, wherein W=A or T and R=A or G, or its RNA equivalent and said protein has a lethal effect.