Naturally occuring polymorphisms in behavior are difficult to map genetically and thus are refractory to molecular characterization. An exception is the foraging gene (for), a gene that has two naturally occurring variants in Drosophila melanogaster food-search behavior: rover and sitter. Molecular mapping placed for mutations in the dg2 gene, which encodes a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG). Rovers had higher PKG activity than sitters, and transgenic sitters expressing a dg2 complementary DNA from rover showed transformation of behavior to rover. Thus, PKG levels affected food-search behavior, and natural variation in PKG activity accounted for a behavioral polymorphism.

We describe here for the first time the site of retention within the nucleus of pre-mRNA processing mutants unable to be exported to the cytoplasm. Fluorescence in situ hybridization was used to detect transcripts from human β-globin genes that are either normal or defective in splicing or 3Ј end formation. Nuclear transcripts of both wild-type and mutant RNAs are detected only as intranuclear foci that colocalize with the template gene locus. The kinetics of transcript release from the site of transcription was assessed by treatment of cells with the transcriptional inhibitors actinomycin D, α-amanitin and DRB. These drugs induce the rapid disappearance of nuclear foci corresponding to wild-type human β-globin RNA. In contrast, pre-mRNA mutants defective in either splicing or 3Ј end formation and which fail to be transported to the cytoplasm, are retained at the site of transcription. Therefore, 3Ј end processing and splicing appear to be rate limiting for release of mRNA from the site of transcription.

One of the rare examples of a single major gene underlying a naturally occurring behavioral polymorphism is the foraging locus of Drosophila melanogaster. Larvae with the rover allele, for R , have significantly longer foraging path lengths on a yeast paste than do those homozygous for the sitter allele, for s . These variants do not differ in general activity in the absence of food. The evolutionary significance of this polymorphism is not as yet understood. Here we examine the effect of high and low animal rearing densities on the larval foraging path-length phenotype and show that density-dependent natural selection produces changes in this trait. In three unrelated base populations the long path (rover) phenotype was selected for under high-density rearing conditions, whereas the short path (sitter) phenotype was selected for under low-density conditions. Genetic crosses suggested that these changes resulted from alterations in the frequency of the for s allele in the low-density-selected lines. Further experiments showed that density-dependent selection during the larval stage rather than the adult stage of development was sufficient to explain these results. Densitydependent mechanisms may be sufficient to maintain variation in rover and sitter behavior in laboratory populations.Studies of population dynamics have for the most part mistakenly considered the behavior of organisms within a population to be homogeneous (1). However, individual differences in behavior are common and have consequences for the ecology and evolution of populations. A special case of individual differences is behavioral polymorphism, where individuals within a population can be categorized into morphs (phenotypes or strategists) according to their behavior(s). When the polymorphism has a heritable component, the relative selective advantages of the morphs under different environmental conditions can be measured (2). One condition, population density, varies over time and space and plays a significant role in the evolution of characteristics within populations (3). Population density affects a number of processes, for example, predator-prey (4) and parasite-host interactions (5), the spread of disease (6), competition (7), population regulation (8), and territoriality (9). There have been many theoretical studies of density-dependent selection, including those that model its effect on competitive ability (10-13). However, empirical studies that address whether behavioral polymorphisms affect fitness in a density-dependent manner are rare. The fruit fly Drosophila melanogaster is one of the few systems in which we are beginning to have a detailed empirical understanding of density-dependent selection (14-22). It is an ideal model system to study how a genetically characterized polymorphism in behavior responds to density-dependent selection.The rover͞sitter polymorphism in D. melanogaster is a model system that has been used to address both the mechanistic and evolutionary significance of behavioral phenotypes. The locomotio...

Previous (3); it is expressed only when food is present in the environment and when larvae are feeding (4).In the present paper, we examine adult fly walking behavior after feeding to determine whether the foraging locus affects adult behavior in a similar fashion to larval behavior. We found that adults homozygous for the forR allele walk farther from a drop of sucrose after feeding per unit time thanThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement"in accordance with 18 U.S.C. §1734 solely to indicate this fact.do adults homozygous for the fors alleles. This difference in adult behavior is not found when flies are walking on a nonnutritive substrate. Neither larvae nor flies of the two morphs differ in their general activity or in muscle usage (4). We think that the foraging gene affects how larvae and adult flies perceive and/or evaluate food in their environment. Thus, understanding foraging may help elucidate processes important to the development of complex behavior in insects.Here we use a genetic approach to show (i) that the foraging gene of D. melanogaster, originally identified through its effect on larval behavior, influences adult behavior, and (ii) that induced mutations in for affect both larval and adult locomotion when food is present in the environment. METHODSStrains. Strains were maintained in 250-ml plastic culture bottles on 45 ml of a dead yeast/sucrose/agar medium at 25°C ± PC, 15 ± 1 mbar (1 bar = 100 kPa) vapor pressure deficit, and a 12-h light/12-h dark photocycle with lights on at 0800 h.Rover-larval (R) and sitter-larval (S) behaving D. melanogaster strains were obtained by collecting 500 adult flies from an orchard in the Toronto area. The population was reared in the laboratory and not allowed to go through bottle necks. After 1 year, the lengths of the foraging trails of 500 thirdinstar larvae were measured (as described in ref. 5; also see below). Individual rover and sitter behaving male larvae were used to produce homozygousforR/forR andfors/fors strains. Since for had been localized to chromosome 2 at cytological position 24A3-5 (6, 7), we crossed the sampled flies to a chromosome-2 balancer stock [In(2LR)SMJ,aF Cy cn2 sp2/ In(2LR)bwVl, ds33k bwVl; described in ref. 8] that had been repeatedly backcrossed (10 times) to the orchard population. The resulting lines had heterogeneous genetic backgrounds from the orchard population and were homozygous for either the forR or the fors allele. We verified this with crosses to laboratory rover and sitter larval strains and to the deficiency Df(2L)edSz, which uncovers for (6).de Belle et al. (6) produced two strains, fors(R)136 and fors(R)'", by irradiating a forR/forR strain (called BB) and selecting for sitter behaving larvae. These strains carried second-site lethal chromosome-2 mutations. We used genetic recombination with a lethal for mutation (made on a BB genetic background) to cross off these second-site lethal mutations. This...