Superb images illustrate families and higher groupings. [@ ABRS & CBIT]
Report from an ABRS grantee
Systematics of the parasitic wasp subfamily Agathidinae (Hymenoptera: Braconidae) in Australia
The Agathidinae are a large group of sometimes brightly patterned parasitic wasps that attack lepidopteran caterpillars. Over 1200 species are described worldwide. The Australian fauna, though, is particularly poorly studied, with only 35 species described. Some species probably play an important role in suppressing the number of larvae of native pest moths such as the lucerne seed web moth (Etiella behrii) and light brown apple moth (Epiphyas postvittana).
Nicholas Stevens at the University of Adelaide has spent the last three years working on the taxonomy and phylogenetics of these parasitoids to provide a more detailed picture of their diversity, distribution and systematics in Australia. Supervised by Prof. Andrew Austin and Drs John Jennings and Nick Murphy, the project has focused on the genus Bassus, which is the most speciose element of the Australian fauna.
Of particular interest to braconid wasp systematists around the world is whether Bassus is monophyletic, and what is the nature of the distinctive colour pattern found in many species that have a pronounced black, red-orange and white body (referred to below as the BROW colour pattern), which is thought to be part of a putative mimicry pattern found in several Australian insect groups.
Using sequence data obtained from mitochondrial (COI, 16S) and nuclear (28S, LW) genetic markers, as well as morphological characters, the study has confirmed the results of a previous, less comprehensive study, that Bassus is indeed polyphyletic and comprises at least four independent lineages. Interestingly, the Australian Bassus has representatives in all of the lineages present elsewhere in the world. However, most Australian species fall out in one lineage that appears to represent a relatively recent and rapid radiation, with a number of Palaearctic and Oriental species being placed basally within this lineage.
The main factor contributing to the polyphyletic nature of Bassus is the difficulty of defining the group reliably on morphological criteria. The level of morphological variation and the degree of homoplasy present in the genus is extreme and has confounded the classification of this and closely related groups. The results of this study highlight the importance of molecular data in helping to untangle the evolutionary relationships within highly speciose groups, in the absence of reliable morphological evidence.
Unfortunately, because the genus has a world-wide distribution, to understand fully the relationships among its various elements, a much more detailed study on a global basis, with a denser taxon sampling, will be required to delineate a natural classification comprising monophyletic genera.
An intriguing aspect to Nicholas’s study has been an investigation of the BROW colour pattern which is more common in species from the southern temperate zone of Australia. The pattern is thought to be aposematic (warning signal) and part of an extensive putative mimicry complex that includes species from other Australian braconid wasp subfamilies, as well as other insect orders such as Coleoptera, Diptera, Hemiptera and Lepidoptera (see Figs Z to ZD, colour plate 6, Insects of Australia, 1990). Within the Hymenoptera the BROW complex appears to be confined mostly to the Australian region.
Within the clade that comprises most of the Australian Bassus are several intricate and, as yet, unresolved species complexes that are dominated by species in the BROW mimicry complex. In addition, three of the four divergent Bassus clades contain Australian species exhibiting the BROW colouration, clearly demonstrating that the pattern has evolved multiple times.
Some questions that our research might answer are: Is the BROW colour pattern aposematic, and if so is the warning signal directed at avian predators? What part do the agathidine wasps play in this putative mimicry complex? Are they models strengthening the Mullerian signal, or are they Batesian mimics and so are sheep in wolves’ clothing?
Female Bassus do possess a long ovipositor, which can be as long as the entire body, and so potentially could deliver a painful sting to a would-be predator. Thus, if the BROW pattern is aposematic, female Bassus could be models in this system with males theoretically weakening the signal. However, the colour pattern may also warn potential predators that the wasp contains distasteful compounds, possibly retained from their host. Therefore, both males and females would serve as models in a complex mimicry system involving other wasps and other insects. Although at present this is largely conjecture, it does highlight an important first step in unravelling mimicry complexes, which is to develop a robust phylogeny for the groups involved.
Nicholas’s PhD was partially funded by a grant from ABRS to John Jennings and Nicholas Stevens.