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The genetic similarity of New and Old World samples of Spalangia spp. (Hymenoptera: Pteromalidae) was examined using two ribosomalDNAregions. The species examined were Spalangia cameroni Perkins, Spalangia endius Walker, Spalangia gemina Bouček, Spalangia nigra Latreille, Spalangia nigroaenea Curtis, and Spalangia slovaca Bouček. Two species of Muscidifurax, Muscidifurax raptor Girault & Sanders and Muscidifurax zaraptor Kogan & Legner (Hymenoptera: Pteromalidae) were included as outgroup taxa. The internal transcribed spacer (ITS)-1 region was highly variable among Spalangia species with many insertions/deletions making alignment of the sequences difÞcult. The D2-D3 region of the 28s ribosomal gene and the nuclear rDNA 18s gene were more conserved and enabled phylogenetic analysis. No genetic differentiation was observed among S. cameroni and S. endius samples from Kazakhstan, Russia, and North America. New World samples of S. nigroaenea are genetically distinct from S. slovaca, a morphologically similar Old World species that is newly recorded from Kazakhstan and Russia. The intact 920 bp ITS-1 amplicon of S. nigroaenea was much larger than the 780-bp amplicon of S. slovaca. Kimura two-parameter genetic distance between the two species was 0.015 for the 28s region. Otherwise, the smallest genetic distance among recognized Spalangia species was 0.037 between S. endius and S. nigra. The genetic distance between M. raptor and M. zaraptor was 0.004. Based upon these results, the utility of the D2-D3 region of the 28s ribosomal gene is substantiated for differentiating species of Spalangia. The molecular analysis of the six Spalangia species revealed two groupings: S. nigroaenea and S. slovaca and S. cameroni and S. gemina. A third clade, S. endius and S. nigra, was observed, but bootstrap support was weak. These relationships were compared with those indicated by morphology and all agreed except possibly between S. endius and S. nigra, for which morphological evidence is equivocal. Morphological features are described and illustrated to distinguish the morphologically most similar species, S. nigra, S. nigroaenea, and S. slovaca, from each other and from other Spalangia spp.