O. Siddiqi

Genetic analysis of chemosensory pathway

Some olfactory and gustatory mutants ofDrosophila melanogaster are described. Theolf mutants are insensitive to the repellent, benzaldehyde or the attractant ethyl acetate or simultaneously to both. Thegust mutants are unable to taste quinine or NaCl or sucrose. Electrophysiological tests show that one of the sugar non-tasters has an altered primary chemoreceptor response to sucrose.

Genetics of Olfactory Behavior in Drosophila Melanogaster

We have used a behavioral genetic approach to identify six X-linked loci which specify olfaction in Drosophila melanogaster. Mutations in five of these genes lead to partial anosmias affecting responses either to aldehydes (olfA, olfB, olfE and olfF) or to acetate esters (olfC). Only one of the mutants obtained in our screening (olfD) resulted in a insensitivity to several different odorants. olfA, olfE and olfC map close together in a small region of the chromosome between 7C and 7D. The alleles at the olfC locus fall into two phenotypic classes according to their responses to different acetate esters. The two groups of olfC alleles interact in-trans.

Neurogenetics of olfaction in Drosophila melanogaster

A number of genes involved in olfactory behaviour of Drosophila have been identified. The understanding of the physiology and anatomy of the first few stages of the olfactory pathway promises a reliable diagnosis of genetic lesions. The stage is thus set for studies on the molecular biology of olfaction.

A gustatory mutant of Drosophila defective in pyranose receptors

SummaryMutations in an X-linked gene, gust-A, block the responses of Drosophila melanogaster to a group of pyranose sugars. It is shown that the behavioural effects of this mutation are correlated with a loss of electrical responses in taste receptors. The mutation affects the chemoacceptors for pyranose sugars leaving the furanose acceptors intact.

Genetic analysis of a complex chemoreceptor.

Taste responses of Dipterans have been extensively investigated for more than three decades. V.G. Dethier (1955) used the proboscis extention test to measure the natural preferences of the flies to different chemicals. He found that blowflies accepted sugars and rejected common salt or quinine. If a mixture of an attractant and a repellent was presented, the fly’s response was determined by the ratio of the two substances. Hodgson, Lettvin and Roeder (1955) first recorded the electrical responses of the Dipteran chemoreceptors. Since then a number of workers have attempted to correlate the electrophysiological and behavioral responses of a variety of flies. (See review by Hodgson, 1974.) It is evident that taste discrimination in insects, as in mammals, is based upon a small set of categoric distinctions, but the exact nature of the sensory code remains unknown (Perkel and Bullock, 1969).

CLOSELY LINKED LESIONS IN A REGION OF THE X CHROMOSOME AFFECT CENTRAL AND PERIPHERAL STEPS IN GUSTATORY PROCESSING IN DROSOPHILA

SummaryWe have analyzed a set of closely linked mutations on the X chromosome of Drosophila that lead to defects in gustatory behavior. The mutations map to a small region of the X chromosome between 10E1–4. Two distinct complementation groups, gustB and gustD, map to the ends of this region. These groups show complex complementation patterns with the mutations gustC and GT-1, which also map to this region. We describe the behavioral and electrophysiological properties of the mutants. These mutations affect peripheral receptor properties as well as more central processing steps in the gustatory pathway.

DNA synthesis during bacterial conjugation: II. Is DNA replication in the Hfr obligatory for chromosome transfer ?

F-mediated transfer replication in a thymine-requiring Hfr of Eseherichia coli containing a thermosensitive dna B gene has been investigated. Ordinarily, DNA synthesis in the Hfr accompanies chromosome transfer at the non-permissive temperature. When the donors are, however, deprived of thymine, transfer occurs without any DNA synthesis.

Integration of donor DNA in bacterial conjugation

Abstract Conjugation between 13 C 15 N- and 3 H-labelled hybrid donors and 13 C 15 N-labelled hybrid recipients of Escherichia coli gives rise to recombinant radioactive DNA of density greater than labelled hybrid. The donor radioactivity is present, in these molecules, in discrete heavy segments covalently attached to the light strand. When light radioactive Hfr cells are mated to heavy F − cells in light medium, the donor label appears, in DNA extracted from the F − cells, in labelled hybrid molecules. The radioactivity in these molecules is exclusively in the light strand. The insertion of donor material is thus restricted to a single newly formed strand of the recipient DNA and double-strand integrations do not occur. A temperature-sensitive recipient containing the dna B mutation ts 43 accumulates single-stranded Hfr DNA if mating is carried out at the nonpermissive temperature. The formation of a complementary strand in the recipient does not, therefore, appear to be necessary for continued transfer of Hfr DNA.

DNA synthesis during bacterial conjugation: I. Effect of mating on DNA replication in escherichia coli Hfr

Abstract Conjugation in Escherichia coli involves an oriented transfer of DNA from the Hfr to the F−. We have examined the course of DNA replication in a donor cell while it is transferring its DNA. Using isotopic density shift for estimating replication, we have shown that mating is accompanied by initiation of a new round of DNA replication in Hfr cells. With the onset of F-mediated transfer replication, the normal vegetative replication in the Hfr appears to be suppressed. Experiments with F′ donors indicate that the transfer of the chromosome is necessary for switching off vegetative replication.

Olfaction in invertebrates

Olfactory transduction in invertebrates seems to be similar to that in vertebrates. Three signalling systems involving activation of adenylate cyclase, phospholipase C and guanylate cyclase are present. A variety of second messengers, including cAMP, inositol 1,4,5-trisphosphate, diacylglycerol, nitric oxide and Ca2+, have been identified but their target sites and mode of action are not yet fully understood. The central projections of olfactory signals in invertebrates are relatively simple and perhaps more hard-wired than in vertebrates. Information about circuitry and functional mapping in the olfactory pathway is lacking. This is essential for understanding the sensory code and higher olfactory functions. Neurogenetic analysis has provided useful insights into olfaction and olfactory learning.