Comparative analysis of the nonA region in Drosophila identifies a highly diverged 5' gene that may constrain nonA promoter evolution

Sequence polymorphism of candidate behavioural genes in Drosophila melanogaster flies from 'Evolution canyon'

This study focuses on the molecular features of three candidate behavioural genes in Drosophila melanogaster from the opposite slopes of Nahal Oren Canyon, Mount Carmel, Israel. These slopes display dramatic physical and biotic contrasts. Spatial variation of microclimatic conditions leads to adaptive differentiation and partial sexual isolation of populations, as suggested by our previous studies. The chosen candidate genes presumably contributing to genetic variation in sexual behaviour of Drosophila in the Canyon were desaturase, period, and no-on-transient A. These genes are known to include polymorphic repeated sequences, insertions/deletions, or nucleotide substitutions. The idea was that their polymorphism might be one of the determinants of behavioural peculiarities of flies derived from the opposite slopes. Indeed, interslope differences in the sequence encoding the (Thr-Gly)n repeat (exon 5) of the period gene were established, suggesting evolutionary functional importance. In particular, we unraveled variation in the length and composition of this region in different NFS (north-facing slope) and SFS (south-facing slope) lines. The 'European' allele (n = 20) was a 2.6-fold more abundant on the NFS compared to the SFS. This predominance probably gives some advantages for flies inhabiting wet and less warm conditions of the NFS. We suggest that repeat length/composition may influence the functional features of flies, i.e. habitat choice, nonrandom mating, and temperature adaptation. A series of female single-mate-choice tests show that females derived from NFS distinguish between males with specific per alleles (n = 17 vs. n = 20), as well as between males originated from the opposing slopes. Females from SFS were less discriminating and did not manifest significant deviation from random mating.

Population genetics of the developmental gene optomotor-blind (omb) in Drosophila polymorpha: evidence for a role in abdominal pigmentation variation

The developmental gene optomotor-blind (omb) encodes a T-box-containing transcription factor that has multiple roles in Drosophila development. Previous genetic analyses established that omb plays a key role in establishing the abdominal pigmentation pattern of Drosophila melanogaster. In this report we examine patterns of omb nucleotide variation in D. polymorpha, a species that is highly polymorphic for the phenotype of abdominal pigmentation. Haplotypes at this locus fall into two classes that are separated by six mutational steps; five of these mutational events result in amino acid changes. Two lines of evidence are consistent with a role for omb in the abdominal pigmentation polymorphism of D. polymorpha. First, we find that haplotype classes of omb are correlated with abdominal pigmentation phenotypes, as are microsatellite repeat numbers in the region. Second, tests of selection reveal that the two haplotype classes have been maintained by balancing selection. Within each class there is a significantly low amount of diversity, indicative of previous selective sweeps. An analysis including D. polymorpha's closest relatives (members of the cardini group) provides evidence for directional selection across species. Selection at this locus is expected if omb contributes to variation in abdominal pigmentation, since this trait is likely of ecological importance.

Mutations and Natural Genetic Variation in the Courtship Song of Drosophila

All Drosophila species have a courtship repertoire by which a male stimulates a female to mate with him. In many species, males vibrate their wings to produce courtship song, an element of courtship that plays an important role in female choice. Each species has a unique courtship song, with the major differences among species songs being in timing and/or structure. Analysis of genetic mutations has revealed 17 genes that affect courtship song in Drosophila melanogaster. Most of the genes were first identified as affecting another trait and were subsequently shown to affect song. Quantitative genetic studies have demonstrated a polygenetic additive genetic architecture for many song traits. Few candidate genes, identified through the classical genetic approach, coincide with the regions implicated as affecting natural variation. With many new tools in genetic analysis and the multiple Drosophila genome projects currently underway, the ability to relate mutational and quantitative analyses will improve.

Courtship and other behaviors affected by a heat-sensitive, molecularly novel mutation in the cacophony calcium-channel gene of Drosophila

The cacophony (cac) locus of Drosophila melanogaster, which encodes a calcium-channel subunit, has been mutated to cause courtship-song defects or abnormal responses to visual stimuli. However, the most recently isolated cac mutant was identified as an enhancer of a comatose mutation's effects on general locomotion. We analyzed the cac(TS2) mutation in terms of its intragenic molecular change and its effects on behaviors more complex than the fly's elementary ability to move. The molecular etiology of this mutation is a nucleotide substitution that causes a proline-to-serine change in a region of the polypeptide near its EF hand. Given that this motif is involved in channel inactivation, it was intriguing that cac(TS2) males generate song pulses containing larger-than-normal numbers of cycles--provided that such males are exposed to an elevated temperature. Similar treatments caused only mild visual-response abnormalities and generic locomotor sluggishness. These results are discussed in the context of calcium-channel functions that subserve certain behaviors and of defects exhibited by the original cacophony mutant. Despite its different kind of amino-acid substitution, compared with that of cac(TS2), cac(S) males sing abnormally in a manner that mimics the new mutant's heat-sensitive song anomaly.

Evolutionary behavioral genetics in Drosophila

Behavioral genes have a special evolutionary interest because they are potentially involved in speciation and in many forms of adaptation. Dozens of loci affecting different aspects of behavior have been already identified and cloned in Drosophila. Some of these genes determine variation in such ethologically complex phenotypes as the male "love song" that is produced during courtship and the locomotor "sleep-wake" activity cycles that are controlled by the circadian clock. Although the evolutionary analysis of most behavioral genes in Drosophila is relatively new, it has already given important insights into the forces shaping the molecular variation at these loci and their functional consequences.

Nucleotide variation at the no-on-transient A gene in Drosophila littoralis

The no-on-transient A (nonA) gene encodes a putative RNA-binding protein, and mutations in this gene are known to affect vision, male courtship song and viability in Drosophila melanogaster. Here we have sequenced the coding region of the nonA gene of Drosophila littoralis and compared it with those of Drosophila virilis and D. melanogaster. All portions of nonA appeared to be conserved between D. littoralis and D. virilis, while the 5' region of the gene of these two species showed high divergence from that of a more distantly-related species, D. melanogaster. The same was true for the glycine repeat regions. No significant deviation from neutrality was observed in the analysis of intraspecific nucleotide variation in 5' or 3' region of the nonA gene in D. littoralis population. Also, comparison of D. littoralis sequences with homologous sequence of D. virilis suggests that the gene is evolving neutrally in D. virilis group. Divergence of the 5' regions between D. virilis group species and D. melanogaster could be a result of positive selection, but this finding is obscured by the long divergence time of the species groups.

The nonA gene in Drosophila conveys species-specific behavioral characteristics

The molecular basis of species-specific differences in courtship behavior, a critical factor in preserving species boundaries, is poorly understood. Genetic analysis of all but the most closely related species is usually impossible, given the inviability of hybrids. We have therefore applied interspecific transformation of a single candidate behavioral locus, no-on-transient A (nonA), between Drosophila virilis and D. melanogaster, to investigate whether nonA, like the period gene, might encode species-specific behavioral information. Mutations in nonA can disrupt both visual behavior and the courtship song in D. melanogaster. The lovesong of nonA(diss) mutant males superficially resembles that of D. virilis, a species that diverged from D. melanogaster 40-60 mya. Transformation of the cloned D. virilis nonA gene into D. melanogaster hosts carrying a synthetic deletion of the nonA locus restored normal visual function (the phenotype most sensitive to nonA mutation). However, the courtship song of transformant males showed several features characteristic of the corresponding D. virilis signal, indicating that nonA can act as a reservoir for species-specific information. This candidate gene approach, together with interspecific transformation, can therefore provide a direct avenue to explore potential speciation genes in genetically and molecularly tractable organisms such as Drosophila.

Molecular Dissection of the 5' Region of no-on-transientA of Drosophila melanogaster Reveals cis-Regulation by Adjacent dGpi1 Sequences

The nonA gene of Drosophila melanogaster is important for normal vision, courtship song, and viability and lies approximately 350 bp downstream of the dGpi1 gene. Full rescue of nonA mutant phenotypes can be achieved by transformation with a genomic clone that carries approximately 2 kb of 5' regulatory material and that encodes most of the coding sequence of dGpi1. We have analyzed this 5' region by making a series of deleted fragments, fusing them to yeast GAL4 sequences, and driving UAS-nonA expression in a mutant nonA background. Regions that both silence and enhance developmental tissue-specific expression of nonA and that are necessary for generating optomotor visual responses are identified. Some of these overlap the dGpi1 sequences, revealing cis-regulation by neighboring gene sequences. The largest 5' fragment was unable to rescue the normal electroretinogram (ERG) consistently, and no rescue at all was observed for the courtship song phenotype. We suggest that sequences within the nonA introns that were missing in the UAS-nonA cDNA may carry enhancer elements for these two phenotypes. Finally, we speculate on the striking observation that some of the cis-regulatory regions of nonA appear to be embedded within the coding regions of dGpi1.