Mechanisms of Olfaction in Parasitic Wasps: Neurophysiological and Neuroanatomical Studies of Olfaction in a Specialist (Microplitis croceipes) and a Generalist (Cotesia marginiventris) Parasitoid

Abstract

The success of parasitic wasps (parasitoids) in controlling pest populations depends on their ability to locate their hosts in a complex olfactory environment. Parasitic wasps are known to utilize various types of host-related volatile signals as host location cues. These volatile signals could be green leaf volatiles (GLVs), herbivore induced plant volatiles (HIPVs) or host specific odors. The olfactory system plays a major role for odor detection, processing and relaying information to higher centers of the insect brain which helps in recognition and decision making. Despite the intense interest in host-parasitoid interactions, the underlying mechanism of olfactory communication in this group of insects is not well understood. This study was conducted to characterize mechanisms of olfaction and response to host-related odor in two parasitic wasps (Hymenoptera: Braconidae) with different degrees of host specificity, Microplitis croceipes (Cresson) (specialist) and Cotesia marginiventris (Cresson) (generalist), utilizing an integration of neurophysiological and neuroanatomical techniques. Specifically, this research characterized species and sexual differences in olfactory mechanisms in parasitic wasps by: (1) comparing abundance of olfactory sensilla in the antennae of M. croceipes and C. marginiventris ; (2) characterizing single sensillum responses of olfactory receptor neurons in the sensilla placodea of M. croceipes and C. marginiventristo host-related volatiles; (3) characterizing antennal lobe architecture and glomerular organization in M. croceipes and C. marginiventris; and (4) identifying glomerular projections of olfactory receptor neurons in the antennal lobes of both species. In chapter II, studies were conducted to compare the abundance of antennal sensilla types in both sexes of M. croceipes and C. marginiventris to determine if there is a correlation between abundance of olfactory sensilla and host specificity. Five major sensilla types were recorded in both species: sensilla chaetica (non-porous), s. trichodea (non-porous), s. placodea (multiporous), s. basiconica (two types, type 1 with terminal opening and type 2 with wall pores), and s. coeloconica (non-porous). The putative chemosensilla types, s. placodea and s. basiconica, were more abundant in M. croceipes (specialist) than in C. marginiventris (generalist), and this was true for both sexes. Comparing the sexes, s. placodea and s. trichodea were significantly more abundant in M. croceipes males compared with females. In contrast, s. placodea was relatively more abundant in female C. marginiventris than in males. In Chapter III, I characterized the responses of olfactory receptor neurons (ORNs) housed in the sensilla placodea of both parasitoid species to host-related plant volatiles. The extracellular activity showed presence of two neurons in the olfactory sensilla of these two parasitoids. In M. croceipes, single neuron elicited response to green leaf volatile (GLV) cis-3-hexenol and herbivore-induced plant volatile (HIPV) cis-3-hexenyl butyrate. The rest of the compounds elicited response in both ORNs in the sensilla. Mixtures of GLVs and HIPVs showed excitation in the ORNs, however, mixture of cis-3-hexenol & hexanal (GLVs) with linalool inhibited the ORNs. In C. marginiventris, cis-3-hexenol (GLV) and cis-3-hexenyl acetate (HIPV), elicited in single olfactory neuron. The other GLVs and HIPVs elicited responses in both neurons similar to M. croceipes. All mixtures of GLVs and HIPVs enhanced responses in the ORNs in C. maginiventris. The most significant finding is the inhibitiory effect of linalool in M. croceipes in a mixture with cis-3-hexenol and hexanal; however, ORNs in C. marginiventris showed enhanced response. The results show that ORNs in M. croceipes have specific responses to compounds as compared to C. marginiventris. This difference in neuronal activity might suggest that, olfactory system in M. croceipes (specialist) have evolved with specific response to host-related volatiles, which might provide an olfactory code for volatiles damaged by its hosts for specific host recognition and location. In Chapter IV, I reconstructed the antennal lobe morphology and glomerular organization of both parasitoid species. In M. croceipes, the medial half of the antennal lobe is larger with greater number of glomeruli compared to the lateral half, whereas in C. marginiventris the lateral half is larger than the median half. The most striking sexual difference was the presence of an enlarged glomerulus (macroglomerulus or MG) at the entrance of the antennal nerve in males of both species. In addition, a complex of 3-4 macro-glomeruli (complex of macro-glomeruli or CMG) was observed in the posterior region of the antennal lobe of males of both species. The average volume of the antennal lobe is similar between the sexes but ~ 2.5 greater in M. croceipes compared to C. marginiventris. In Chapter V, I conducted studies to understand how olfactory stimuli are processed in the brains of both species by characterizing glomerular projections of olfactory receptor neurons (ORNs) responding to single odors and mixtures of host-related plant volatiles in their antennal lobes. The ORNs responding to the tested host-related plant volatiles send projections to glomeruli in the medial half of the AL in M. croceipes, versus the lateral half of the AL in C. marginiventris. In M. croceipes, cis-3-hexenol (a green leaf volatile or GLV) and cis-3-hexenyl butyrate (a herbivore-induced plant volatile or HIPV) both activated a distinct glomerulus, whereas hexanal (HIPV) activated two adjacent glomeruli in the antero-median section. In contrast, cis-3-hexenol (GLV) and cis-3-hexenyl acetate (HIPV) activated distinct glomeruli in C. marginiventris. Hexanal (HIPV) activated one glomerulus and the ORN projects to another glomeruli in ventro-lateral region, suggesting a connection in the odor processing between these two glomeruli. In M. croceipes, an odor mixture (blend) of cis-3-hexenol (GLV) and cis-3-hexenyl butyrate (HIPV) showed enhanced activation in the same glomerulus activated by the single components. However, no activation was recorded with a mixture of cis-3-hexenol (GLV) and linalool (HIPV), suggesting inhibition by linalool. In C. marginiventris, a mixture of cis-3-hexenol (GLV) and cis-3-hexenyl acetate (HIPV) exhibited intense labeling in their respective glomeruli. These results suggest that odor mixtures activate the same glomeruli as their individual components. No remarkable sexual differences were recorded for both species in glomerular projections of ORNs. Like females, host-related plant volatiles are processed in ordinary glomeruli in males rather than in the male-specific complex of macro-glomeruli. The results of this research advance our understanding of the olfactory system and neurophysiological mechanisms of olfaction in parasitic wasps and may explain species and sexual differences in the response of M. croceipes and C. marginiventris to host-related plant volatiles.