Richard. VOGT, Marie-dominique FRANCO. Matthew ROGERS.
Department of Biological Sciences. University of South Carolina. Columbia, SC 2920)8 USA.
lnsects detect and respond to a broad range of odors through multiple classes (phenotypes) of olfactory sensilla distinguishable by morplology, odor/ligand sensitivity and behavioral modality. We see studying the differentially regulated expression of Odorant Binding Proteins (OBP) to understand the genetic basis of these phenotypic specificities. The antennae of Munduca sexta antennae express three homologous OBP genes: Pheromone Binding Proteins (P131's) are expressed in long trichoid sensilla and the General Odorant Binding Proteins GOBP1 and G0BP2 are expressed in different classes of basiconic sensilla. Each olfactory sensillum consist of a cluster of cells including one or more sensory neurons plus three support cells, all deriving from a common sensory precursor cell (SPC). In the mature sensillum OBP's are expressed in the trichogen support cells and secreted into the extraecllular space surrounding the olfactory dendrites. Presumably, the specific phenotype of each sensillum and the subsequent seIection of OBP gene is determined at the time of SPC division; in some cases sensillar phenotype may be spatially determined by extrinsic positional cues while in other cases phenotype may be determined by a stochastic process which appears to be independent of extrinsic spatial information. The result is a highly organized olfactory system in which the expression of odor receptor and O)BP genes are coordinated between two different but sister cells such that a distinct sensillar phenotype emerges.
Previous studies identified the three M. sexta OBPs; cDNA clones of all three were obtained and expression of all three was shown to be initiated late in adult development by the decline in pupal ecdysteroids. Current in situ hybridizatioin studies in male antennae show PBP and G0BP2 express in non-overlapping spatial domains in each annulus. GOBPl expression is observed in both the PBP and GOBP2 domains; GOBPl may compress with PBP but appears to express in a different class of sensilla than GOBP2 among the basiconic sensilla. In female antennae, G0BP2 expression appears to predominate with GOBP1 and PBP expression intermingling with GOBP2 sensilla; GOBP1 appears to express in far fewer cells than GOPB2 and PB expresses in a relatively small subset of sensilla. Recent genomic analysis of OBPs suggests that regulation of expression of respective genes is highly interrelated.
These patterns of OBP expression appear to mirror patterns established at the time of SPC division. Cell birthdating studies employing BrdU show stripes and patches of cell division corresponding respectively to spatial doinains defined by PBP and GOPB expression. Immunocytochemical staining for the neuronal marker ELAV confirm that these cell divisions give rise to, sensory neurons. Interestingly, SPC divisions for different sensilla classes do not coincide: long trichoid divisions proceed basiconic divisions in the male antenna; basiconic divisions in male and female antennae occur at different times.
Taken together, these findings suggest a model for phenotype determination and expression where ( I ) extrinsic and intrinsic cues determine phenotypic fate at the time of SPC division, (2) neuronal and support cell gene expression is coordinated likely through inductive signals, (3) gene expression is temporally coordinated through dynamic shifts in ecdysteroid levels (and likely ecdysone receptor dynamics), and (4) expression of a given OBP likely involves the active silencing of the other respective OBP genes.
Support: NIH NIDCD DC-00588; USDA CSRS 94-37302-0615.