Evolution of the olfactory code in the Drosophila melanogaster subgroup

The Drosophila melanogaster subgroup has been the focus of numerous studies about evolution. We address the question of how the olfactory code has evolved among the nine sister species. By using in vivo electrophysiological measurements, so called single-cell recordings, we have established the ligand affinity of a defined subset of olfactory receptor neurons (ORNs) across all nine species. We show that the olfactory code as relayed by the investigated subset of ORNs is conserved to a striking degree. Distinct shifts in the code have occurred only within the simulans clade. However, these shifts are restricted to an altered tuning profile of the same single ORN type in all three of the simulans siblings and a more drastic change unique to D. sechellia, involving a complete loss of one sensillum type in favour of another. The alterations observed in D. sechellia may represent a novel host-specific adaptation to its sole host, morinda fruit (Morinda citrifolia). The overall high degree of similarity of the code within the subgroup is intriguing when considering the great variety in distributions as well as in habitat and host choice of the siblings, factors that could greatly affect the olfactory system.     

Evolution of the Gypsy endogenous retrovirus in the Drosophila melanogaster subgroup

We conducted a phylogenetic survey of the endogenous retrovirus Gypsy in the eight species of the Drosophila melanogaster subgroup. A 362-bp fragment from the integrase gene (int) was amplified, cloned, and sequenced. Phylogenetic relationships of the elements isolated from independent clones were compared with the host phylogeny. Our results indicate that two main lineages of Gypsy exist in the melanogaster subgroup and that vertical and horizontal transmission have played a crucial role in the evolution of this insect endogenous retrovirus.     

Evolutionary pattern of the gtwin retrotransposon in the Drosophila melanogaster subgroup

The gtwin retrotransposon was recently discovered in the Drosophila melanogaster genome and it is evolutionarily closer to gypsy endogenous retrovirus. This study has identified gtwin homologous sequences in the genome of D. simulans, D. sechellia, D. erecta and D. yakuba by performing homology searches against the public genome database of Drosophila species. The phylogenetic analyses of the gtwin env gene sequences of these species have shown some incongruities with the host species phylogeny, suggesting some horizontal transfer events for this retroelement. Moreover, we reported the existence of DNA sequences putatively encoding full-length Env proteins in the genomes of Drosophila species other than D. melanogaster. The results suggest that the gtwin element may be an infectious retrovirus able to invade the genome of new species, supporting the gtwin evolutionary picture shown in this work.     

On the maximum likelihood method for estimating molecular trees: Uniqueness of the likelihood point

Summary Studies are carried out on the uniqueness of the stationary point on the likelihood function for estimating molecular phylogenetic trees, yielding proof that there exists at most one stationary point, i.e., the maximum point, in the parameter range for the one parameter model of nucleotide substitution. The proof is simple yet applicable to any type of tree topology with an arbitrary number of operational taxonomic units (OTUs). The proof ensures that any valid approximation algorithm be able to reach the unique maximum point under the conditions mentioned above. An algorithm developed incorporating Newton's approximation method is then compared with the conventional one by means of computers simulation. The results show that the newly developed algorithm always requires less CPU time than the conventional one, whereas both algorithms lead to identical molecular phylogenetic trees in accordance with the proof. Contribution No. 1780 from the National Institute of Genetics, Mishima 411, Japan     

Evolution of the LINE-like I element in the Drosophila melanogaster species subgroup

LINE-like retrotransposons, the so-called I elements, control the system of I-R (inducer-reactive) hybrid dysgenesis in Drosophila melanogaster. I elements are present in many Drosophila species. It has been suggested that active, complete I elements, located at different sites on the chromosomes, invaded natural populations of D. melanogaster recently (1920–1970). But old strains lacking active I elements have only defective I elements located in the chromocenter. We have cloned I elements from D. melanogaster and the melanogaster subgroup. In D. melanogaster, the nucleotide sequences of chromocentral I elements differed from those on chromosome arms by as much as 7%. All the I elements of D. mauritiana and D. sechellia are more closely related to the chromosomal I elements of D. melanogaster than to the chromocentral I elements in any species. No sequence difference was observed in the surveyed region between two chromosomal I elements isolated from D. melanogaster and one from D. simulans. These findings strongly support the idea that the defective chromocentral I elements of D. melanogaster originated before the species diverged and the chromosomal I elements were eliminated. The chromosomal I elements reinvaded natural populations of D. melanogaster recently, and were possibly introduced from D. simulans by horizontal transmission.     

Evolution of sexual dimorphism of wing shape in the Drosophila melanogaster subgroup

Background  

Sexual dimorphism of body size has been the subject of numerous studies, but few have examined sexual shape dimorphism (SShD) and its evolution. Allometry, the shape change associated with size variation, has been suggested to be a main component of SShD. Yet little is known about the relative importance of the allometric and non-allometric components for the evolution of SShD.     

Light wavelength dependency of mating activity in the Drosophila melanogaster species subgroup

The action spectra of mating activity among the six species of the Drosophila melanogaster species subgroup were compared to understand how light wavelength affects mating activity. The species fell into three groups with respect to the action spectrum of mating activity. We chose one representative species from each of the three types for detailed study: D. melanogaster, D. sechellia and D. yakuba. The mating activities were investigated under three different light intensities of three monochromatic lights stimulus. Each species showed a unique spectral and intensity response. To know the evolutionary meaning of the light wavelength dependency of mating activity, we superimposed the type of action spectrum of mating activity in these six species on a cladogram. Mating inhibition under UV was conserved in evolution among these species. Furthermore we clarified that D. melanogaster showed low mating activity under UV because males courted less under UV.     

Evolutionary relationships to parasitism by seven species of the Drosophila melanogaster subgroup

Parasitic wasps are an important component of the niche of Drosophila species. The susceptibility to the Cynipid Leptopilina boulardi was estimated in the seven sibling species of Drosophila belonging to the melanogaster subgroup. Three categories of Hies can be distinguished, according to the level of cellular immune reaction and success of parasitism. Drosophila melanogaster and D. mauritiana belong to the category 1, specified by no encapsulative reaction and a high rate of successful parasitism. Category 2, characterized by a moderate encapsulation rate and a high mortality include D. simulans.5, D. erecta and D. orena . Category 3, with D. yakuba and D. tcissien , is specified by a very low rate or absence of successful parasitism due to a highly efficient immune cellular reaction. This classification parallels the phylogenic relationship based upon polytene chromosome banding sequences. Such specific ditferences in susceptibility to parasites may plan an important role in the competition between these species in Africa.     

Evolutionary relationships to parasitism by seven species of the Drosophila melanogaster subgroup

Parasitic wasps are an important component of the niche of Drosophila species. The susceptibility to the Cynipid Leptopilina boulardi was estimated in the seven sibling species of Drosophila belonging to the melanogaster subgroup. Three categories of Hies can be distinguished, according to the level of cellular immune reaction and success of parasitism. Drosophila melanogaster and D. mauritiana belong to the category 1, specified by no encapsulative reaction and a high rate of successful parasitism. Category 2, characterized by a moderate encapsulation rate and a high mortality include D. simulans.5, D. erecta and D. orena. Category 3, with D. yakuba and D. tcissien, is specified by a very low rate or absence of successful parasitism due to a highly efficient immune cellular reaction. This classification parallels the phylogenic relationship based upon polytene chromosome banding sequences. Such specific ditferences in susceptibility to parasites may plan an important role in the competition between these species in Africa.     

On the maximum likelihood method in molecular phylogenetics

Summary The efficiency of obtaining the correct tree by the maximum likelihood method (Felsenstein 1981) for inferring trees from DNA sequence data was compared with trees obtained by distance methods. It was shown that the maximum likelihood method is superior to distance methods in the efficiency particularly when the evolutionary rate differs among lineages.     

Relationships within the melanogaster species subgroup of the genus Drosophila (Sophophora)

The polytene chromosomes of two new species of Drosophila, D. sechellia and D. orena, both members of the melanogaster species subgroup, are described. The chromosomes of D. sechellia, a species endemic to certain islands in the Seychelles, are homosequential with those of D. simulans and D. mauritiana. The chromosomes of D. orena, a species from the mountains of west Africa, are very similar to those of D. erecta. We discuss the interrelationships of the eight known species of the melanogaster species subgroup, based upon an analysis of their chromosome banding patterns.     

Evolution of eye morphology and rhodopsin expression in the Drosophila melanogaster species subgroup

A striking diversity of compound eye size and shape has evolved among insects. The number of ommatidia and their size are major determinants of the visual sensitivity and acuity of the compound eye. Each ommatidium is composed of eight photoreceptor cells that facilitate the discrimination of different colours via the expression of various light sensitive Rhodopsin proteins. It follows that variation in eye size, shape, and opsin composition is likely to directly influence vision. We analyzed variation in these three traits in D. melanogaster, D. simulans and D. mauritiana. We show that D. mauritiana generally has larger eyes than its sibling species, which is due to a combination of larger ommatidia and more ommatidia. In addition, intra- and inter-specific differences in eye size among D. simulans and D. melanogaster strains are mainly caused by variation in ommatidia number. By applying a geometric morphometrics approach to assess whether the formation of larger eyes influences other parts of the head capsule, we found that an increase in eye size is associated with a reduction in the adjacent face cuticle. Our shape analysis also demonstrates that D. mauritiana eyes are specifically enlarged in the dorsal region. Intriguingly, this dorsal enlargement is associated with enhanced expression of rhodopsin 3 in D. mauritiana. In summary, our data suggests that the morphology and functional properties of the compound eyes vary considerably within and among these closely related Drosophila species and may be part of coordinated morphological changes affecting the head capsule.     

The estimation of epistasis in components of fitness in experimental populations of Drosophila melanogaster I. A two-stage maximum likelihood model

Laboratory populations of Drosophila melanogaster bearing the Curly and Plum marked second chromosome inversions were observed in selection experiments for ten discrete generations. Maximum likelihood estimates of the relative fitnesses of Curly, Plum, Curly-Plum, and wild phenotypes were obtained from selection trajectories. Using these estimates, measures of multiplicative and additive epistasis were calculated. These were partitioned into pre-sampling and post-sampling components, and both were found to be significant. In several cases the sign of the epistasis of the two components was reversed, and the direction of net epistasis depended on the particular inversion. The significance of partitioning epistasis into components is discussed in the light of two locus population genetic theory.