Trichogramma

Trichogramma is a genus of minute polyphagous wasps that are endoparasitoids of insect eggs. Trichogramma is one of around 80 genera from the family Trichogrammatidae, with over 200 species worldwide. Although several groups of egg parasitoids are commonly employed for biological control throughout the world, Trichogramma spp. have been the most extensively studied. More than a thousand papers have been published on Trichogramma species, and they are the most used biological control agents in the world. Trichogramma spp. have less than 10,000 neurons, approaching the size limit of how small an insect can be, determined by how few neurons they can fit in their central nervous systems, yet exhibiting a complex behavior to sustain their lives. To locate host eggs, adult females use chemical and visual signals, such as egg shape and colour. After she finds a suitable egg, an experienced female attempts to determine if the egg has previously been parasitized, using her ovipositor and antennal drumming (tapping on the egg surface). Females also use antennal drumming to determine the size and quality of the target egg, which determines the number of eggs the female will insert. A single female can parasitize up to 10 host eggs a day. Trichogramma wasps are small and very uniform in structure, which causes difficulty in identifying the separate species. As females are all relatively similar, taxonomists rely upon examination of males to tell the different species apart, using features of their antennae and genitalia. The first description of a Trichogramma species was in North America in 1871, by Charles V. Riley. He described the tiny wasps that emerged from eggs of the viceroy butterfly as Trichogramma minutum. In taxonomy, original specimens are very important, as they are the basis of reference for subsequent descriptions of species. The original specimens, however, were lost. Riley also described a second species in 1879 as Trichogramma pretiosum, but these specimens were also lost. To correct these errors, entomologists returned to the areas where Riley originally found the species and obtained neotype specimens of T. minutum and T. pretiosum. These specimens are now preserved properly in the United States National Museum. Currently, the number of Trichogramma species is over 200, but as of 1960, only some 40 species of Trichogramma had been described. Wolbachia is a widespread bacterial genus that infects insects' organs, most commonly the reproductive organs. Wolbachia has been observed to alter the host’s reproductive success upon infection. Through a series of manipulations, Wolbachia-infected hosts transmit this intracellular bacterium to uninfected individuals. These manipulations include male killing (increasing ratio of infected females that can reproduce), feminization (males become fertile females), parthenogenesis, and cytoplasmic incompatibility. Horizontal transfer of parthenogenesis-inducing Wolbachia, which has been observed in Trichogramma wasps, causes infected females to asexually produce fertile females and nonfunctional males. The effects of this include potential speciation of Trichogramma, if Wolbachia is maintained long enough for genetic divergence to occur and for a new species of asexual wasps to become reproductively isolated. Transmission of the bacterium through horizontal transfer has been observed within the same species and among different species of Trichogramma, including T. kaykai, T. deion, T. pretiosum, and T. atopovirilia; however, limitations to transmission exist. In vitro successful horizontal transfer is uncommon within Trichogramma, which suggests that the density of Wolbachia must be relatively high inside of the hosts' ovaries. Cytoplasmic incompatibility of the host and bacterium can also be the source of this unsuccessful transfer in-vitro. These limitations in vitro suggest that in nature, horizontal transfer by parthenogenesis-inducing Wolbachia may be a difficult and rare phenomenon. However, when looking at the Wolbachia-host associations, the Trichogramma-Wolbachia form a monophyletic group based on several Wolbachia-specific genes, which may be explained by horizontal transfer of Wolbachia between different species. Therefore, although interspecific horizontal transfer of Wolbachia is limited in vitro, it is likely to occur quite frequently in nature and is not well understood yet. The effects of Wolbachia in Trichogramma have several evolutionary implications. Commonly, uninfected wasps are unable to breed with infected wasps. Many generations of reproductive isolation of these different groups may result in speciation. In addition, some hosts can evolve with a dependency on Wolbachia for core reproductive functions, such as oogenesis, so that eventually an infection is a requirement for successful reproduction. Finally, Wolbachia can influence gender determination in its hosts so that more females are successfully born. This results in a reversal in sexual selection, where females must compete for male mates, which has evolutionary implications as it exposes different phenotypes to natural selection.