Background

To understand which reproductive barriers initiate speciation is a major question in evolutionary research. Despite their high species numbers and specific biology, there are only few studies on speciation in Hymenoptera. This study aims to identify very early reproductive barriers in a local, sympatric population of Nasonia vitripennis (Walker 1836), a hymenopterous parasitoid of fly pupae. We studied ecological barriers, sexual barriers, and the reduction in F1-female offspring as a postmating barrier, as well as the population structure using microsatellites.

Results

We found considerable inbreeding within female strains and a population structure with either three or five subpopulation clusters defined by microsatellites. In addition, there are two ecotypes, one parasitizing fly pupae in bird nests and the other on carrion. The nest ecotype is mainly formed from one of the microsatellite clusters, the two or four remaining microsatellite clusters form the carrion ecotype. There was slight sexual isolation and a reduction in F1-female offspring between inbreeding strains from the same microsatellite clusters and the same ecotypes. Strains from different microsatellite clusters are separated by a reduction in F1-female offspring. Ecotypes are separated only by ecological barriers.

Conclusions

This is the first demonstration of very early reproductive barriers within a sympatric population of Hymenoptera. It demonstrates that sexual and premating barriers can precede ecological separation. This indicates the complexity of ecotype formation and highlights the general need for more studies within homogenous populations for the identification of the earliest barriers in the speciation process.

To study phenology and to establish laboratory cultures of N. vitripennis, wasps were collected using bait bags with fly pupae in bird nests,  next to carrion, and without any associated odour in the park of the University of Hohenheim (Stuttgart, Germany). 

In a four-chamber olfactometer we tested for the presence of a positive reaction to the host habitat odour of bird nests, and for the presence of a positive reaction to carrion odour in strains collected in bird nests or next to carrion.  

To study host acceptance of Lucilia carrion flies, single wasps females were placed in Petri dishes with five fly pupae each for 6 h. The numbers of fly pupae that had been parasitized by each female and the offspring of each female were counted after 3 weeks. 

To study sexual isolation, mating experiments were performed with females from three strains originating from bird nests and three strains originating from carrion in all possible combinations.

To study postmating isolation, females were mated as described above and transferred into Petri dishes with 10 pupae of L. sericata for parasitization and kept there until their death. After 4 weeks the offspring was sexed and counted.

A population genetic analysis was performed with 103 females collected in bird nests or next to carrion using eight microsatellite markers distributed all over the five chromosomes.