In chapters 2 to 6 of this thesis we describe the mutualism between Philanthus digger wasps (Hymenoptera, Crabronidae) and symbiotic Streptomyces bacteria. The streptomycetes are cultivated in specialized glands in the female digger wasps’ antennae and protect the beewolf offspring from pathogenic fungi.
The Philanthus-Streptomyces symbiosis is unique with regard to the location of the bacteria-cultivation organs within the beewolves antennae where space is limited and circulation of hemolymph is probably not optimal compared to antennae without the additional obstacle of glands.
To proliferate, the bacterial symbionts need nutrients which have to be transported inside the antennal segments via the hemolymph. In insects the dorsal vessel is not able to pump hemolymph in body appendages like antennae and therefore accessory pulsatile organs at the base of extremities function as auxiliary hearts (reviewed in Pass, 2000). Due to increased nutrition requirements inside antennae that result from the bacteria cultivation process we expect enlarged antennal pulsatile organs compared to other Hymenoptera (Matus and Pass, 1999; Pass, 2000). Such organs could also enhance the secreting of the reservoir content into the brood cells by increasing the hemolymph pressure in the antennae. However, this hypothesis has to be tested with detailed morphological investigations on antennal circulatory organs in the genus Philanthus.
Our studies on beewolves represent, to our knowledge, the first descriptions of insect antennal glands that are used for bacteria cultivation. Future studies should reveal, whether closely related taxa like the sister genus Trachypus (for phylogeny of Philanthinae see Alexander, 1992) also possess antennal glands. Particularly the discovery of less complex glands without bacteria could provide insights about the possible morphology of predecessors of Philanthus antennal glands and how they changed during evolution.
Streptomyces bacteria are commonly found in the soil (Kutzner, 1981; Dari et al., 1995) and insect antennae with their intersegmental gaps and overlapping segments have numerous cavities which microorganisms could invade and proliferate in – if a nutritional basis is present. Therefore it seems plausible that bacteria infected the antennal glands of beewolf ancestors as commensales or even parasites, possibly using the gland secretions as substrate. Future investigations should reveal the
chemical nature of the beewolves’ antennal gland cell secretions and their role for the bacteria as nutrition basis. Preliminary histological tests for lipids were negative (Goettler, unpubl. data).
However, we could identify proteinaceous vesicles within the antennal gland cells and also between the bacteria in the reservoirs (Fig. 11.1; Goettler, unpubl. data). The production of proteins is costly for beewolves, but it seems that the selective advantage of rearing symbiotic bacteria compensates this investment sufficiently.
During the vertical transmission from the beewolf mother to its daughters and at the bottlenecks during brood cell building the streptomycetes probably engage increased genetic drift and an accumulation of deleterious mutations, a process that is known as Muller’s ratchet (Lynch and Gabriel, 1990; Gabriel et al., 1993; Lynch et al., 1993; Andersson and Kurland, 1998). However, selection on the beewolf host probably limits the accumulation of detrimental mutations in obligate symbionts and thus counteracts this bias (Andersson and Kurland, 1998; Pettersson and Berg, 2007). Actually there are still many open questions about the beewolf symbionts. Future projects should e.g. reveal whether the mode of the bacteria transmission is strictly vertical or exhibits also horizontal transfer between different Philanthus species (Kaltenpoth, in prep.). Another topic is the search for antibiotics which are probably involved in the protection of the beewolf cocoon by the Streptomyces bacteria (Kaltenpoth, in prep.).
re
ba c3
c3
c3 c3
c3
Fig. 11.1 Semithin section of P. triangulum antennal glands. Histological staining with Coomassie brilliant blue revealed numerous proteinaceous droplets (some indicated by arrows) within gland cells (c3) and between bacteria (ba) inside the gland reservoir (re). scale bar = 20 µm.
of A. pubescens dig a relative short burrow with a single brood cell at the distal side in sandy soil and successively provision the larva with paralyzed caterpillars (Evans, 1965; Field, 2007). The last larval stage spins the cocoon in a way that the head of the pupa points towards the nest entrance (Honomichl, 1998). However, it is not known how the larva of this species obtains the directional cues for cocoon spinning. The larva could for example orientate itself on temperature differences within the substrate, gravitational cues or, like beewolves, on information provided by its mother. If the latter is true A.
pubescens could possess hitherto undescribed exocrine glands, which secretions act as information carriers. However, this hypothesis is pure speculation and has to be tested by behavioural and anatomical investigations in the future.
Comparable to the Philanthus-Streptomyces mutualism attine ants (Formicidae) evolved glands and cuticle crypts on their body surface where symbiotic Pseudonocardia bacteria proliferate (Currie et al., 2006). Both, Pseudonocardia and Streptomyces bacteria belong to the actinomycetes. This group is well known for its antimicrobial secondary metabolites which are frequently used as antibiotics in human medicine (e.g. Kutzner, 1981; Behal, 2000). So it appears that fungus-growing ants, beewolves and – only very recently on the evolutionary timescale – humans use actinomycetes as partners against microbial opponents.
Social insects like bees, wasps, ants and termites are known to produce various antibiotics in exocrine glands. The secretions of mandibular, metapleural, salivary, venom and Dufour’s glands have been frequently demonstrated to possess fungistatic and bacteriostatic properties (Hefetz, 1987; Veal et al., 1992; Schmid-Hempel, 1995; Rosengaus et al., 2000, 2004; Ayasse and Paxton, 2002). From a number of solitary insects parental care is known, e.g. earwigs (Dermaptera), burying beetles (Silphidae) or scarab beetles (Scarabaeidae) protect their offspring effectively against pathogenic microorganisms e.g. by eliminating fungal spores through licking and digestion (Tallamy, 1984;
Clutton-Brock, 1991; Rankin et al., 1995; Halffter et al., 1996, Eggert et al., 1998). Mass-provisioning of the progeny with perishable food occurs in various insect taxa without parental care, e.g. digger wasps (Crabronidae; Sphecidae) or dung beetles (Geotrupidae). It is unknown how these species deal with the risk of fungal infestation, but maybe symbiosis between insects and protective bacteria is more widespread than we hitherto recognized. The discovery of new effective antibiotics produced by bacteria involved in such symbioses would be an important step in the race of human medicine against multi-resistent pathogens.