A polynucleotide expression system is provided comprising at least one heterologous polynucleotide coding sequence for a functional protein and/or an interference RNA (RNAi), and a splice control sequence which is capable of mediating the splicing of an RNA transcript of the coding sequence to yield a first spliced mRNA product, and is capable of mediating at least one alternative splicing of the RNA transcript to yield an alternative mRNA product. The mediation of the splicing is selected from sex-specific, germline-specific or tissue-specific mediation, and combinations thereof. In a preferred embodiment the alternative splicing is sex-specific and the splice control sequence is derived from a tra introl, preferably Cctra, or it may be derived from the alternative splicing mechanism of the Actin-4 gene. The system may be used in the control of insect populations.
