G. R. Platner

The utility of internally transcribed spacer 2 DNA sequences of the nuclear ribosomal gene for distinguishing sibling species of Trichogramma

The usefulness of the internally transcribed spacer 2 (ITS2) of the nuclear ribosomal gene complex is tested for providing taxonomic characters to identify Trichogramma species. The ITS2 sequences of a group of sibling species of the T. deion/T. pretiosum complexes were determined. A simple and precise identification key to the species of these assemblages was constructed using as taxonomic characters the size of the ITS2 and the difference in restriction length polymorphism of species with similarly sized ITS2. Individual wasps can be identified by amplification of their ITS2 with general primers, determining the size of the PCR product using standard agarose electrophoresis, followed in some species by a DNA-digestion with a restriction enzyme. Because this system works well for a number of closely related species we are hopeful that similar PCR-based identification can be extended to all species of the genus once their ITS2 sequences have been determined. The advantage of this identification system over the morphology-based system is that non-specialists are able to quickly and cheaply identify individual specimens. In addition, species specific primers were tested for the two most common species of these groups (i.e. T. pretiosum and T. deion). These primers can be used either as a direct identification tool or as a method to confirm the identification using the general key. The phylogeny of this group of wasps was also analyzed based on the ITS2 sequence.

Crossing Incompatibility Between Trichogramma minutum and T. platneri (Hymenoptera: Trichogrammatidae): Implications for Application in Biological Control

Abstract The two egg parasitoid species Trichogramma minutum Riley and T. platneri Nagarkatti are closely related. No morphological or molecular characters are known to distinguish them. Their treatment as distinct species rests on the absence of female offspring in between-species crosses. The current species definitions, which must be considered tentative, assume their natural ranges to differ, with T. minutum occurring east of the Rocky Mountains and T. platneri occurring to the west. Both species are used throughout North America for the biological control of moths in orchards and forests. They are available to growers and researchers through a large number of biological control suppliers. Because the species can only be identified after crosses with known cultures, it is likely that producers often cannot be sure of the identity of their wasps. Here we studied the causes of the lack of female offspring in the between-species crosses and what happened when one species is released for biological control in the native area of the other. Our results show that females in between-species crosses are inseminated and use the sperm to fertilize their eggs, but that these fertilized eggs die. In addition, females do not preferentially mate with males of their own species when exposed to conspecific and nonconspecific males. These results are used in a model to predict the effect of releasing the non-native species in the native area of the other species. This model shows that such introductions can result in a prolonged and substantial reduction of intended biological control.

The systematics of the Trichogramma minutum species complex (Hymenoptera: Trichogrammatidae), a group of important North American biological control agents: the evidence from reproductive compatibility and allozymes

Abstract Two North American Trichogramma , Trichogramma minutum Riley, and Trichogramma platneri Nagarkatti (the T. minutum complex), cannot be distinguished morphologically and their species status has been questioned. Both are commercially available for biological control and are important parasitoids of the eggs of Lepidoptera in agricultural ecosystems. Results are reported of an extensive survey of reproductive compatibility and allozymes at the phosphoglucomutase ( Pgm ) locus in collections representing 85 localities throughout much of the geographical range of this species complex. In total, 103 reproductive crosses were performed and 161 collections were analyzed electrophoretically. Results support the recognition of T. minutum and T. platneri as distinct species. The two are distinguishable electrophoretically at the Pgm locus. High levels of reproductive incompatibility are correlated with the allozymic differences. Although T. platneri is western in distribution (W of 115° longitude) and T. minutum is primarily eastern, the two are sympatric in the Pacific Northwest. A low level of interspecific hybridization in laboratory studies notwithstanding, there is no evidence that introgression occurs in the zone of sympatry. This study continues to stress the importance of species recognition and the inadvisability of using T. minutum and T. platneri interchangeably in biological control.

Non-reciprocal cross-incompatibility in Trichogramma deion

In non‐reciprocal cross‐incompatibility (NRCI), the crossing of a female of a strain A with a male of a strain B results in hybrid offspring, whereas the reciprocal cross produces few or no hybrids. Only females are of hybrid origin in Hymenoptera because they arise from fertilized eggs; males arise from unfertilized (haploid) eggs. Crosses between many strains of Trichogramma deion showed some degree of NRCI. Crosses between a T. deion culture collected in Seven Pines, California (SVP) with one from Marysville, California (MRY) showed an extreme form of NRCI in which practically no female offspring was produced when MRY females were crossed with SVP males. The reciprocal cross produced a close to normal proportion of female and male offspring. Detailed studied of this cross indicated that 1) the female offspring produced in the compatible interstrain cross were not the result of parthenogenesis but were true hybrids, 2) the incompatible interstrain cross did not produce female offspring because fertilized eggs died during development, 3) the death of these eggs could not be prevented by either antibiotic or temperature treatment, 4) cytoplasmically inherited factors causing NRCI could be discounted because backcrossed females with the genome of MRY and the cytoplasm of SVP, exhibit the NRCI relationship characteristic of their genome. Therefore the NRCI between these strains appears to be caused by a modification coded for by the nuclear genes of MRY that results in incompatibility when SVP sperm fertilizes MRY eggs. In addition the level of incompatibility in crosses between the SVP females and MRY males is temperature sensitive, the higher the rearing temperature the lower the level of compatibility.