Mortality in Interspecific Hybrids of Nasonia vitripennis and Nasonia giraulti (Hymenoptera: Pteromalidae)1

Barriers to gene flow can result in populations evolving into separate species, and understanding how these barriers evolve is a key step in understanding the process of speciation. This study investigates a major barrier to gene flow, mortality during development, of two parasitoid wasp species, Nasonia vitripennis Walker and N. giraulti Darling. Previous work has demonstrated high mortality in haploid hybrid males of these species and has shown potential rescue from this mortality in diploid hybrid females through backcrossing. However, this previous work did not directly measure egg clutch sizes in hybrids and also did not account for male eggs in these clutches. Here, we measure female-only egg and adult clutch sizes of both parent species and F2 hybrids, as well as F2 male egg and adult clutch sizes, in order to determine the impact of backcrossing on mortality in these hybrids. We find significant egg-to-adult mortality in diploid F2 hybrid females with a N. giraulti cytotype when backcrossed to N. vitripennis males; yet, these hybrids also experience less mortality than haploid F2 males. These results confirm the mortality and rescue in these F2 hybrid females and indicate that there is likely a combination of nuclear-cytoplasmic and nuclear-nuclear incompatibilities in these hybrids that lead to this mortality. This work provides a foundation for future studies to investigate the genetic basis of this mortality.