The Drosophila Bruton's tyrosine kinase (Btk) homolog is required for adult survival and male genital formation

We isolated a Drosophila fickle(P) (fic(P)) mutant with a shortened copulat ory duration and reduced adult-stage life span. The reduced copulatory dura tion is ascribable to incomplete fusion of the left and right halves of the apodeme that holds the penis during copulation. fic(P) is an intronic muta tion occurring in the Btk gene, a gene which encodes two forms (type 1 and type 2) of a Bruton's tyrosine kinase (Btk) family cytoplasmic tyrosine kin ase as a result of alternative exon usage. The fic(P) mutation prevents the formation of the type 2 isoform but leaves expression of the type 1 transc ript intact. Ubiquitous overexpression of the wild-type cDNA by using a hea t shock 70 promoter during the late larval or pupal stages rescued the life span and genital defects in the mutant, respectively, establishing the cau sal relationship between the fic(P) phenotypes and the Btk gene mutation. T he stage specificity of the rescuing ability suggests that the Btk gene is required for the development of male genitalia and substrates required for adult survival.