Species and sexual differences in antennal lobe architecture and glomerular organization in two parasitoids with different degree of host specificity, Microplitis croceipes and Cotesia marginiventris

Abstract

The endoparasitic wasps (Hymenoptera: Braconidae), Microplitis croceipes (specialist) and Cotesia marginiventris (generalist), are parasitoids of lepidopteran larvae and differ in their degree of host specificity. Recent studies have reported key differences between the two species in the abundance of antennal olfactory sensilla and their response to host-related volatiles. Here, we have compared antennal lobe architecture and glomerular organization in the two parasitoid species by using a combination of axonal tract tracing techniques and confocal microscopy. In M. croceipes, the medial half of the antennal lobe is larger with a greater number of glomeruli compared with the lateral half, whereas in C. marginiventris, the lateral half is larger than the median half. The volume of the antennal lobe is approximately 2.5 times greater in M. croceipes than in C. marginiventris. However, the number of glomeruli per antennal lobe is only slightly higher in M. croceipes (females: 219-222; males: 220-224) than in C. marginiventris (females: 192-194; males: 193-196). A comparison of males and females within each species demonstrated a striking sexual difference in terms of an enlarged glomerulus (macroglomerulus or MG) at the entrance of the antennal nerve and of a complex of 3-4 MG (CMG) in the posterior region of the antennal lobe of males of both species. Being specific to males, both the MG and CMG might be involved in the detection of female-related odor.

Olfactory pathway of the hornet Vespa velutina: New insights into the evolution of the hymenopteran antennal lobe

“…Whereas female macroglomeruli are adaptations related to the colony life style of eusocial insects, male macroglomeruli are found throughout insects because they are involved in mating (Arnold et al, ; Hansson et al, ; Nishikawa et al, ; Nishino et al, ). In many species, such as moths or cockroaches, male‐specific macroglomeruli are located in the caudal region of the AL, close to the entrance of the antennal nerve (Rospars and Chambille, ; Skiri et al, ; Nishino et al, ; Das and Faddamiro, ). In the ant Camponotus japonicus , two macroglomeruli at similar locations were attributed to the T3 tract and were assumed to have a sex pheromone‐receptive function (Nakanishi et al, ).…”

“…These examples demonstrate that essential knowledge on the evolution of the insect olfactory system can be obtained from its comparative analysis in well‐chosen taxa. Thus, comparing the organization of the AL and its specific adaptations in terms of glomerular number, size, and input–output connectivity may allow one to understand adaptations linked to changes in diet, host preference, or social life style in Hymenoptera (Rospars, ; Zube and Rössler, ; Kelber et al, ; Dacks et al, ; Ibba et al, ; Das and Fadamiro, ; Guidobaldi et al, ).…”

Antennal Proteome of the Solenopsis invicta (Hymenoptera: Formicidae): Caste Differences in Olfactory Receptors and Chemosensory Support Proteins

“…The number of antennal lobe glomeruli is generally considered to be a good estimate for the number of different OR genes expressed. Typical social insect workers are known to have many more glomeruli than males ( Arnold et al 1985 , Zube and Rossler 2008 , Nakanishi et al 2010 , McKenzie et al 2016 ), in contrast to non-social hymenoptera, where the numbers of antennal lobe glomeruli in males and females are about equal ( Das and Fadamiro 2013 ). Considering the high worker-to-male Orco ratio that we measured, and the high worker-to-male antennal lobe glomeruli ratios previously measured in ants, it is surprising that the diversity ratio is so much lower for antenna-specific olfactory binding/transport proteins ( Tables 1 – 3 ) and deactivation enzymes ( Tables 4 – 5 ).…”

The art of being small : brain-body size scaling in minute parasitic wasps

“…Within hymenopteran insects, however, the 100 glomeruli in T. evanescens is the lowest number found so far. Hymenopterans generally have large numbers of glomeruli, for example approximately 165 in honeybee workers (Arnold et al, 1985), and approximately 190 in parasitic wasps of the genus Cotesia (Smid et al, 2003;Das and Fadamiro, 2013). Even larger numbers of glomeruli are found in ants; e.g.…”

“…Depending on the size of the wasps, there is a 10-100-fold difference in total volume of glomeruli inside the antennal lobes of the two wasps. However, there is only a 2-fold difference in antennal lobe complexity: Cotesia wasps have almost 200 glomeruli in the antennal lobe (Smid et al, 2003;Das and Fadamiro, 2013), whereas T. evanescens wasps have 100 glomeruli (Chapter 3).…”