The analysis of genetic behaviour within and between species provides important clues about the forces shaping the evolution of behavioural genes. Genes can affect natural behavioural variation in different ways. Allelic variation causes alternative behavioural phenotypes, whereas changes in gene expression can influence the initiation of behaviour at different ages. Identifying the genes involved in polygenic traits has been difficult. Chromosomal analysis has been widely used as a first step in elucidating the genetic architecture of several behaviours ofDrosophila. Behavioural genetic and molecular studies helped to reveal the genetic basis of circadian time keeping and rhythmic behaviours. InDrosophila, a number of key processes such as emergence from the pupal case, locomotor activity, feeding, olfaction and aspects of mating behaviour are under circadian regulation. Evolutionary biology considers migration behaviour as central in genetic structure of populations and speciation. Genetic loci that influence behaviour are often difficult to identify and localise in part due to the quantitative nature of behavioural phenotypes. Diapause is a hormonally mediated delayed response to future adverse conditions and can occur at any stage of development in an insect. Diapauseassociated gene expression was studied inDrosophila using subtractive hybridisation. Several approaches have been made to unravel the genetic complexity of the behaviour, which have provided information that may be useful in different ways. There is evidence that species do differ in genetic architecture of photoresponse and this may be related to their natural environment. The classical experiments by Jerry Hirsh and Th. Dobzhansky to know the nature of genetic basis for extreme selected geotactic behaviour in fruit flies constituted the first attempt at the genetic dissection of a complex, polygenic behaviour. Understanding the genetic differences between these selected lines would provide an important point of entry into the study of genetic mechanisms of sensing and responding to gravity, as well as clues to the origins of genetic flexibility and plasticity in an organism’s response.

Drosophila, a dipteran insect, has been found to be the best biological model for different kinds of studies. D. melanogaster was first described by Meigen in 1830, is most extensively studied species of the genus Drosophila and a number of investigations employing this species have been documented in areas such as genetics, behaviour, evolution, development, molecular biology, ecology, population biology, etc. Besides D. melanogaster, a number of other species of the genus Drosophila have also been used for different kinds of investigations. Among these, D. ananassae, a cosmopolitan and domestic species endowed with several unusual genetic features, is noteworthy. Described for the first time from Indonesia (Doleschall 1858), this species is commonly distributed in India. Extensive research work on D. ananassae has been done by numerous researchers pertaining to cytology, genetics, mutagenesis, gene mapping, crossing-over in both sexes, population and evolutionary genetics, behaviour genetics, ecological genetics, sexual isolation, fluctuating asymmetry, trade-offs etc. Genome of D. ananassae has also been sequenced. The status of research on D. ananassae at global level is briefly described in this review. Bibliography on this species from different countries worldwide reveals that maximum contribution is from India.

Drosophila pallidosa belongs to the D. ananassae complex, which includes a total of 10 species. Earlier D. pallidosa was known as light form of D. ananassae but later it was described as a new species, sibling of D. ananassae. Both these terms, light form and sibling species were used by Futch. This makes the taxonomic status of D. pallidosa confusing. In this review we have tried tounderstand the actual status of this sibling species pair. Considering the similarities and dissimilarities, we suggest that D. pallidosa does not have the full status of a species, rather it is in the process of speciation, statu-nascendi. Our suggestion is strengthened by the fact that male genitalia are identical in both the cases and they lack postmating reproductive isolation since hybrids between them are normal and fully fertile.