Horizontal transmission versus vertical inheritance of P elements in Drosophila and Scaptomyza: has the M-type subfamily spread from East Asia?

Polymerase chain reaction (PCR) screening for P elements was carried out in 77 species with a primer set highly specific for the M-type subfamily. In the course of this search M-type elements were detected in 29 species: In the melanogaster (subgroups montium and rhopaloa ) and obscura species groups of Drosophila (25 out of 71 species examined), and in the genus Scaptomyza (four out of six species). M-type elements are present in all species of the montium subgroup investigated so far (21), but occur only sporadically in other groups. Within the montium subgroup 20 species possess only incomplete copies, only one species ( D. lacteicornis ) harbours apparently full-sized elements. In contrast, outside the montium subgroup almost all species with M-type elements carry full-sized copies suggesting transpositional activity, at least in the recent past. The interior section of the full-sized M-type element of D. lacteicornis was partially sequenced (936 bp). In addition, the complete sequences of four internally deleted M-type elements of D. lacteicornis, D. rufa, D. quadraria , and D. triauraria were determined. Sequence comparisons (including sequence data from previous investigations) revealed striking discrepancies between P element phylogeny and the cladogenesis of their host species. Among several possible pathways for interspecific transfers of M-type elements, we favour the hypothesis assuming the invasion of Scaptomyza as well as the obscura group species of Drosophila via independent transmission routes originating from Asian species of the montium subgroup of Drosophila . The logical geographic scenario for these events would be East-Asia.     

Herbivory by leaf miners in response to experimental shading of a native crucifer

Summary We tested the hypothesis that light intensity was the direct, proximal mechanism causing significantly higher vulnerability of Bittercress (Cardamine cordifolia A. Gray) clones in the sun to herbivory by a leaf-mining fly (Scaptomyza nigrita Wheeler). Clones in the sun were experimentally shaded. Plant performance and losses to leaf miners were compared to controls in the sun and natural willow shade. Leaf-mining damage was significantly higher on artificially-shaded plants (P<0.01), opposite of our expectation. Shading sun plants shifted their growth pattern toward that of naturally-shaded plants. No significant differences were detected in leaf water status or glucosinolate concentrations, eliminating water stress and variation in defensive posture for mediating the between habitat differences in levels of herbivory. Although soluble sugars varied significantly, they were higher in sun than either shade treatment. Total and free amino nitrogen concentrations were highest in the artificially-shaded plants and lowest in naturally-occurring sun plants. Adult flies were more abundant on sun and on artificially-shaded plants than on naturally-shaded plants. Thus, relative abundance of ovipositing flies in the sun-exposed area, combined with the higher nitrogen availability in artificially-shaded plants, form the most plausible hypothesis for factors mediating the experimentally documented pattern of herbivory.     

Species composition and population dynamics of leafmining flies and their parasitoids in Victoria

Abstract  Mined leaves were sampled from unsprayed sites in Victoria to record the range of leafminers and their parasitoids. Three agromyzid leafminers, Liriomyza brassicae (Riley), Liriomyza chenopodii (Watt) and Chromatomyia syngenesiae Hardy, and one drosophilid leafminer, Scaptomyza flava (Fallén), were collected, along with 15 parasitoids, mainly Eulophidae. The most common parasitoids were Hemiptarsenus varicornis (Girault) (42.5%), Diglyphus isaea (Walker), 14.6%, Closterocerus mirabilis Edwards & La Salle (10.5%), and Opius cinerariae Fisher (8.5%). Most parasitoids were collected from two or more leafminer hosts. Weekly collections from Chinese cabbage ( Brassica rapa var. pekinensis ) infested with L. brassicae and S. flava , and beetroot ( Beta vulgaris var. crassa ) infested with L. chenopodii were made over two seasons at Knoxfield, Victoria to assess the relative impact of these parasitoids on agromyzid fly populations in crops. A further two parasitoid species were identified at low densities. Hemiptarsenus varicornis and D. isaea were the most numerous parasitoids collected in both crops. A different sampling method in the second year showed that O. cinerariae made up 25% of the sample from Chinese cabbage and was probably more common than estimated in the first season. Control exerted by local parasitoids was high, with 100% control of L. chenopodii reached in beets within 1–3 weeks of mines appearing and 100% control of L. brassicae within 6 weeks.     

On the Evolution of Dopa decarboxylase (Ddc) and Drosophila Systematics

We have sequenced most of the coding region of the gene Dopa decarboxylase (Ddc) in 24 fruitfly species. The Ddc gene is quite informative about Drosophila phylogeny. Several outstanding issues in Drosophila phylogeny are resolved by analysis of the Ddc sequences alone or in combination with three other genes, Sod, Adh, and Gpdh. The three species groups, melanogaster, obscura, and willistoni, are each monophyletic and all three combined form a monophyletic group, which corresponds to the subgenus Sophophora. The Sophophora subgenus is the sister group to all other Drosophila subgenera (including some named genera, previously considered outside the Drosophila genus, namely, Scaptomyza and Zaprionus, which are therefore downgraded to the category of subgenus). The Hawaiian Drosophila and Scaptomyza are a monophyletic group, which is the sister clade to the virilis and repleta groups of the subgenus Drosophila. The subgenus Drosophila appears to be paraphyletic, although this is not definitely resolved. The two genera Scaptodrosophila and Chymomyza are older than the genus Drosophila. The data favor the hypothesis that Chymomyza is older than Scaptodrosophila, although this issue is not definitely resolved. Molecular evolution is erratic. The rates of nucleotide substitution in 3rd codon position relative to positions 1 + 2 vary from one species lineage to another and from gene to gene. Received: 2 June 1998 / Accepted: 3 September 1998     

P-elements are old components of the Scaptomyza pallida genome

We report the cloning and analysis of a sample representative of all P-elements from Scaptomyza pallida. We have compared four independent stocks of this species, using Southern blot and in situ hybridization experiments to examine the number, structure, and distribution of P-elements. All four stocks give similar results: they contain about 15 P-elements including three to four full-length elements as well as shorter, deleted elements. All elements are divergent from one another and most of them appear to be immobile since they are located at identical positions in the genomes of independent stocks. These data indicate that P-elements are old components of the S. pallida genome. Moreover, the presence of P-sequences in species closely related to S. pallida suggests that they have had a long evolutionary history in the Scaptomyza genus. We have also found that most P-elements of S. pallida are located in the pericentromeric heterochromatin. This corroborates other studies which show that in the course of their evolution, transposable elements tend to accumulate into pericentromeric heterochromatin, where they become immobile and noncoding. Correspondence to: M. Simonelig     

Evolution of Olfactory Receptors Tuned to Mustard Oils in Herbivorous Drosophilidae

The diversity of herbivorous insects is attributed to their propensity to specialize on toxic plants. In an evolutionary twist, toxins betray the identity of their bearers when herbivores coopt them as cues for host-plant finding, but the evolutionary mechanisms underlying this phenomenon are poorly understood. We focused on Scaptomyza flava, an herbivorous drosophilid specialized on isothiocyanate (ITC)-producing (Brassicales) plants, and identified Or67b paralogs that were triplicated as mustard-specific herbivory evolved. Using in vivo heterologous systems for the expression of olfactory receptors, we found that S. flava Or67bs, but not the homologs from microbe-feeding relatives, responded selectively to ITCs, each paralog detecting different ITC subsets. Consistent with this, S. flava was attracted to ITCs, as was Drosophila melanogaster expressing S. flava Or67b3 in the homologous Or67b olfactory circuit. ITCs were likely coopted as olfactory attractants through gene duplication and functional specialization (neofunctionalization and subfunctionalization) in S. flava, a recently derived herbivore.     

A century of islands: From Darwin to the Hawaiian Drosophilidae

Darwin visited c. 50 islands during the voyage of the Beagle , and from this, and his reading, was impressed by the difficulty of dispersal to oceanic islands, and the descent with modification shown by forms on such islands. He did not regard isolation as being a normal requisite for speciation. The general importance of this was not shown until the turn of the century.
In recent years studies on the Drosophilidae of Hawaii have allowed intensive genetical work to be started on the process of speciation. A quarter to one-third of the world species of Drosophila are Hawaiian endemics. These have evolved both elaborate courtship patterns, with associated bizarre morphological changes, and an apparently simple courtship pattern. This second line has apparently evolved into Scaptomyza on Hawaii. From there, Scaptomyza appears to have spread round the world, in the first instance largely to other oceanic islands.
Genetical studies on species on isolated islands are one of the most promising areas for investigating the importance of founder principle, and the processes of speciation.