This means that the Temozolomide MGC may be created through a budding process, where new, satellite glomeruli have been added over evolutionary time. Such a process is suggested by the finding that OSNs carrying genetically similar ORs project to adjacent glomeruli in the antennal lobe of the vinegar fly ( Couto et al., 2005), and a similar arrangement could be envisaged in the moth MGC. Specific factors determining glomerulus formation
has been identified both morphologically (e.g., Oland and Tolbert, 1996) and molecularly (e.g., Rodrigues and Hummel, 2008). These do, however, still not provide a conclusive picture of how the glomerular array might change over evolutionary time. Interestingly, in the hawk moth and the American cockroach Periplaneta americana (Blattaria: Blattidae) a subdivision of the major glomerulus (the cumulus) has been
observed ( Christensen et al., 1995 and Hösl, 1990). find more In both species differential innervation patterns seem to be connected to topographical representation of the antennal length axis. Sexual dimorphism in the AL is not only restricted to the Lepidoptera. Also in drosophilid flies, sexual dimorphism with respect to specific glomeruli has been observed (Figure 6B). An investigation across 37 species of drosophilids from the Hawaiian Islands found two glomeruli enlarged in males across several of the investigated species (Kondoh et al., 2003). The homologous glomeruli
in D. melanogaster (the DA1 and DL3) have also been shown to receive pheromonal input ( van der Goes van Naters and Carlson, 2007). A phylogenetic comparison further revealed that the noted sexual dimorphism has evolved independently in two of the lineages. Male-specific macroglomerulus/macroglomeruli have also been found in several other insect groups, such as, e.g., cockroaches, wasps (Hymenoptera: Vespidae), and bees (Hymenoptera: Apidae) ( Jawlovski, 1948), but is probably a much more widespread phenomenon, from having evolved wherever a need for long-distance detection of female produced volatile pheromones is present. Other environmental selection pressures beyond pheromones, including food and oviposition site-associated odors, can also shape glomerular organization and structure. For example, the two glomeruli (DM2 and VM5d) in the fly D. sechellia, targeted by OSNs tuned to its singular food source, the noni-fruit, are 200% larger in both sexes relative to D. melanogaster ( Dekker et al., 2006) ( Figure 6C). Interestingly, the expansion of the noni-fruit specific detection system in D. sechellia not only provides higher sensitivity to the fruit odors, but it also makes the fly tolerant to much higher odor concentrations that would inhibit attraction in all other fruit flies. The mechanisms underlying this dual function are still unclear.