The evolutionary maintenance of sex is one of the big unresolved puzzles in biology . All else being equal, all-female asexual populations should enjoy a two-fold reproductive advantage over sexual relatives consisting of male and female individuals . However, the ‘‘all else being equal’’ assumption rarely holds in real organisms because asexuality tends to be confounded with altered genomic constitutions such as hybridization  and polyploidization  or to be associated with parthenogenesis-inducing microbes [4, 5]. This limits the ability to draw general conclusions from any particular system. Here we describe a new system that permits unbiased comparisons of sexual and asexual reproduction: the parasitic wasp Lysiphlebus fabarum. Crossing experiments demonstrated that asexual reproduction has a simple genetic basis in this species and is consistently inherited as a single-locus recessive trait. We further show that the asexuality-inducing allele exhibits complete linkage to a specific allele at a microsatellite marker: all
asexual lines in the field were homozygous for this allele,
and the allele cosegregated perfectly with asexual reproduction in our experimental crossings. This novel system of contagious asexuality allows the production of closely related individuals with different reproductive modes, as well as the monitoring of the asexuality-inducing allele in natural and experimental populations.