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Solitary parasitoid insects usually lay only one egg per host and reject already parasitized hosts, because only one offspring can successfully develop (1). Despite the constraints, superparasitism is commonly observed. A body of theoretical works has explained that the decision of a parasitoid to lay extra eggs can be advantageous and selected for when hosts are rare (2). However, superparasitism in a solitary Drosophila parasitoid was not determined by parasitoid nuclear genes but caused by an infectious extrachromosomal factor. This microparasite takes advantage of the wasp's superparasitism behavior for its own transmission. This leads one to reconsider the evolutionary interpretation of this behavior.
A comparison of seven laboratory strains of Leptopilina boulardi (3) showed clear between-population variation in superparasitism behavior (a mean of 1.00 to 3.56 eggs/parasitized host, P < 0.0001; table S1). This trait was also highly variable within strains, even the Sienna strain that was initiated from a single female. To investigate the origin of such variability, 20 inbred lines were established from the Sienna strain (eight generations of sibmating, homozygosity > 0.996). Stable lines were obtained, some of which never caused superparasitism (NS lines), whereas others laid up to 15 eggs in the same host [S lines, see two typical S and NS lines in Fig. 1A; see also fig. S1 and supporting online material (SOM) Text]. Crosses between S and NS inbred lines (3) revealed strict maternal transmission of the phenotypes (Fig....