Karyotype variation in Drosophila birchii

Drosophila birchii, a member of the melanogaster species group of the subgenus Sophophora, is common in the tropical rain forests of the Australia-New Guinea areas. Chromosome squashes are easily prepared from the larval ganglion cells and the sex chromosomes are readily recognizable. The species exhibits a remarkable karyotype variation. The metaphase plate figures, in general, show two pairs of V's, one pair of dots and one pair of sex chromosomes. Variations in metaphase chromosome morphology are found in the X (with four types), the Y (with three types) and chromosome IV (with two types). Chromosomal interchanges between X- and Y-chromosomes Type I are postulated to be involved in the differentiation of sex chromosome morphology while the modification of chromosome IV seems likely to be a result of the acquisition of extra heterochromatin. These chromosome types form seven distinct metaphase plate figures, all encountered in wild populations, thus giving D. birchii the most variable karyotype in the genus Drosophila.     

Molecular phylogeny of black flies (Diptera: Simuliidae) from Thailand, using ITS2 rDNA

The sequences of the second internal transcribed spacer (ITS2) of ribosomal DNA (rDNA) were determined for 40 black fly species from Thailand, belonging to 4 subgenera of the genus Simulium, namely Gomphostilbia (12 species), Nevermannia (5 species), Montisimulium (1 species), Simulium sensu stricto (21 species), and an unknown subgenus with one species (Simulium baimaii). The length of the ITS2 ranged from 247 to 308 bp. All black fly species had high AT content, ranging from 71 to 83.8%. Intraindividual variation (clonal variation) occurred in 13 species, ranging from 0.3 to 1.1%. Large intrapopulation and interpopulation heterogeneities exist in S. feuerboni from the same and different locations in Doi Inthanon National Park, northern Thailand. Phylogenetic relationships among 40 black fly species were examined using PAUP (version 4.0b10) and MrBAYS (version 3.0B4). The topology of the trees revealed two major monophyletic clades. The subgenus Simulium and Simulium baimaii were placed in the first monophyletic clade, whereas the subgenera Nevermannia + Montisimulium were placed as the sister group to the subgenus Gomphostilbia in the second monophyletic clade. Our results suggest that S. baimaii belongs to the malyschevi-group or variegatum-group in the subgenus Simulium. The molecular phylogeny generally agrees with existing morphology-based phylogenies.     

Isozyme evidence for three sibling species in the Anopheles sundaicus complex from Indonesia

Electrophoretic studies of isoenzymes in three chromosomally distinct forms (A, B and C) of the mosquito Anopheles sundaicus Theobald (Diptera: Culiciae) were undertaken on wild samples collected from six geographically isolated populations in Indonesia. Analyses of 12 enzyme systems comprising 15 loci revealed significant allelic variations, genetic polymorphism, within and among the populations of the An. sundaicus complex. Phylogenetic dendrograms produced by analysis using the Biosys-1 program based on UPGMA methods show that all the populations of form A fall into one cluster, which is closely related to the form C cluster, whereas the populations of form B belong to a more distinct cluster. Allelic frequency and Wright's F statistics of Mpi (mannose phosphate isomerase) are sufficient to identify individuals of each cytological form. This isozyme data correlates with our previous cytological evidence for the existence of three isomorphic species within the taxon An. sundaicus in Indonesia. These three species of the An. sundaicus complex were found together sympatrically at one locality, Asahan in North Sumatra.     

Genetic population structure and introgression in Anopheles dirus mosquitoes in South-east Asia

Genetic structure and species relationships were studied in three closely related mosquito species, Anopheles dirus A, C and D in Thailand using 11 microsatellite loci and compared with previous mitochondrial DNA (mtDNA) data on the same populations. All three species were well differentiated from each other at the microsatellite loci. Given the almost complete absence of mtDNA differentiation between An. dirus A and D, this endorses the previous suggestion of mtDNA introgression between these species. The high degree of differentiation between the northern and southern population of An. dirus C (RST = 0.401), in agreement with mtDNA data, is suggestive of incipient species. The lack of genetic structure indicated by microsatellites in four populations of An. dirus A across northern Thailand also concurs with mtDNA data. However, in An. dirus D a limited but significant level of structure was detected by microsatellites over ~400 km in northern Thailand, whereas the mtDNA detected no population differentiation over a much larger area (>1200 km). There is prior evidence for population expansion in the mtDNA. If this is due to a selective sweep originating in An. dirus D, the microsatellite data may indicate greater barriers to gene flow within An. dirus D than in species A. Alternatively, there may have been historical introgression of mtDNA and subsequent demographic expansion which occurred first in An. dirus D so enabling it to accumulate some population differentiation. In the latter case the lack of migration-drift equilibrium precludes the inference of absolute or relative values of gene flow in An. dirus A and D.     

Phylogeography of the black fly Simulium tani (Diptera: Simuliidae) from Thailand as inferred from mtDNA sequences

Intraspecific phylogeography has been used widely as a tool to infer population history. However, little attention has been paid to Southeast Asia despite its importance in terms of biodiversity. Here we used the cytochrome oxidase I gene of mitochondrial DNA (mtDNA) for a phylogeographic study of 147 individuals of the black fly Simulium tani from Thailand. The mtDNA revealed high genetic differentiation between the major geographical regions of north, east and central/south Thailand. Mismatch distributions indicate population expansions during the mid-Pleistocene and the late Pleistocene suggesting that current population structure and diversity may be due in part to the species' response to Pleistocene climatic fluctuations. The genealogical structure of the haplotypes, high northern diversity and maximum-likelihood inference of historical migration rates, suggest that the eastern and central/southern populations originated from northern populations in the mid-Pleistocene. Subsequently, the eastern region had had a largely independent history but the central/southern population may be largely the result of recent (c. 100,000 years ago) expansion, either from the north again, or from a relictual population in the central region. Cytological investigation revealed that populations from the south and east have two overlapping fixed chromosomal inversions. Since these populations also share ecological characteristics it suggests that inversions are involved in ecological adaptation. In conclusion both contemporary and historical ecological conditions are playing an important role in determining population genetic structure and diversity.     

Protein polymorphism in natural populations of Diachasmimorpha longicaudata (Hymenoptera: Braconidae) in Thailand

Diachasmimorpha longicaudata (Ashmead) is a larval parasitoid of tephritid flies and is widely used as a classical biological control agent. We have used allozyme electrophoresis to evaluate the genetic relationships of six populations of D. longicaudata in Thailand. Twelve loci were examined of which 11 were polymorphic in at least some populations, especially that of Nakornratchasima. We observed a complete lack of heterozygotes for seven of the 10 polymorphic loci in the Nakornratchisima female population, and a significant deficiency of heterozygotes at a further two loci. We discuss possible hypotheses for these findings in light of the haplo-diploid sex determination system of these wasps. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wolbachia infections of tephritid fruit flies: molecular evidence for five distinct strains in a single host species

Endosymbiotic bacteria of the genus Wolbachia are widespread among arthropods and can induce cytoplasmic incompatibility, thelytokous parthenogenesis, male-killing or feminization in their hosts. Here, we report phylogenetic relationships of Wolbachia in tephritid fruit flies based on wsp gene sequences. We also report, for the first time, five distinct strains of Wolbachia in Bactrocera ascita sp. B. Four of the five Wolbachia strains found in this species were in the same groups as those found in other tephritid fruit flies, suggesting possible horizontal transmission of Wolbachia from other fruit flies into B. ascita sp. B. The unreliability of wsp-specific group primers demonstrated in this study suggests that these primers might be useful only for preliminary identification of Wolbachia. Final determination of group affiliation needs to be verified with wsp sequence data. Received: 12 December 2001 / Accepted: 11 January 2002     

Closely Related Wolbachia Strains within the Pumpkin Arthropod Community and the Potential for Horizontal Transmission via the Plant

Phylogenetic studies have implicated frequent horizontal transmission of Wolbachia among arthropod host lineages. However, the ecological routes for such lateral transfer are poorly known. We surveyed the species of two arthropod communities, one on pumpkin and the other on loofah plants, for Wolbachia, constructed wsp gene phylogenies of those Wolbachia strains found to infect community members, and established ecological links among infected members. Four taxonomically diverse insects in the pumpkin arthropod community contained very closely related Wolbachia wsp sequences (<1.5% divergence by Kimura-2-parameter distances). These insects, namely, the whitefly Bemisia tabaci, the planthopper Nisia nervosa, the flea beetle Phyllotreta sp., and the fleahopper Halticus minutus, were all collected from pumpkin leaves. They were ecologically linked through feeding on the same leaf substrate. Unlike other infected leaf insects, the whitefly population appeared to have a permanent breeding relationship with pumpkin plants, and high and stable, but not fixed, monthly Wolbachia infection rates. Our findings suggest potential roles for the plant in Wolbachia transmission and for whiteflies in being an infection source for other pumpkin leaf-feeding insects.