John A. Sved

Mitochondrial Control-Region Sequence Variation in Aboriginal Australians

The mitochondrial D-loop hypervariable segment 1 (mt HVS1) between nucleotides 15997 and 16377 has been examined in aboriginal Australian people from the Darling River region of New South Wales (riverine) and from Yuendumu in central Australia (desert). Forty-seven unique HVS1 types were identified, varying at 49 nucleotide positions. Pairwise analysis by calculation of BEPPI (between population proportion index) reveals statistically significant structure in the populations, although some identical HVS1 types are seen in the two contrasting regions. mt HVS1 types may reflect more-ancient distributions than do linguistic diversity and other culturally distinguishing attributes. Comparison with sequences from five published global studies reveals that these Australians demonstrate greatest divergence from some Africans, least from Papua New Guinea highlanders, and only slightly more from some Pacific groups (Indonesian, Asian, Samoan, and coastal Papua New Guinea), although the HVS1 types vary at different nucleotide sites. Construction of a median network, displaying three main groups, suggests that several hypervariable nucleotide sites within the HVS1 are likely to have undergone mutation independently, making phylogenetic comparison with global samples by conventional methods difficult. Specific nucleotide-site variants are major separators in median networks constructed from Australian HVS1 types alone and for one global selection. The distribution of these, requiring extended study, suggests that they may be signatures of different groups of prehistoric colonizers into Australia, for which the time of colonization remains elusive.

CLOSE GENETIC SIMILARITY BETWEEN TWO SYMPATRIC SPECIES OF TEPHRITID FRUIT FLY REPRODUCTIVELY ISOLATED BY MATING TIME

Abstract.— Two sibling species of tephritid fruit fly, Bactrocera tryoni and B. neohumeralis, occur sympatrically throughout the range of B. neohumeralis in Australia. Isolation between the two species appears to be maintained by a difference in mating time: B. tryoni mates at dusk, whereas B. neohumeralis mates during the middle of the day. A morphological difference in humeral callus color also distinguishes the two species. Despite clear phenotypic evidence that B. tryoni and B. neohumeralis are distinct species, genetic differentiation as measured by four markers–nuclear DNA sequences from the white gene and the ribosomal internal transcribed spacer (ITS2), and mitochondrial DNA sequences from the cytochrome b (cytb) and cytochrome oxidase subunit II (COII) genes–is very small. Minor fixed differences occur in the ITS2 sequence, however, in all other cases the two species exhibit a high level of shared polymorphic variation. The close genetic similarity suggests either that speciation has occurred very rapidly and recently in the absence of any mitochondrial DNA sorting or that the sharing of polymorphisms is due to hybridization or introgression. A third species within the tryoni complex, B. aquilonis, is geographically isolated. Bactrocera aquilonis is also genetically very similar, but in this case there is clear differentiation for the mitochondrial loci. The three species form a group of considerable interest for investigation of speciation mechanisms.

High levels of fitness modifiers induced by hybrid dysgenesis in Drosophila melanogaster

Wild-type chromosomes of D. melanogaster mutagenized by passage through a single generation of hybrid dysgenesis have been compared against identical chromosomes passed through a reciprocal, non-dysgenic cross. Fitness of the chromosome in homozygous condition has been examined in population cages using the technique of balancer chromosome equilibration. The results indicate that amongst chromosomes with no lethal or visible mutation, more than 50% have suffered a measurable decline in fitness. The magnitude of this decline is estimated to be in the range 10–20%.

Microsatellite analysis of the Queensland fruit fly Bactrocera tryoni (Diptera : Tephritidae) indicates spatial structuring : implications for population control

The population structure of a tephritid pest species, the Queensland fruit fly Bactrocera tryoni (Froggatt), has been analysed over a five year period (1994-1998), using six microsatellites. Adult fly samples were collected to cover most regions of eastern and central Australia where the flies are regularly found. Tests for heterogeneity indicated that flies within geographically defined regions were homogeneous. The samples were allocated into five regions, including one very large region, Queensland, which encompasses that portion of the flys range where breeding can occur year-round. With one exception, the collections from different regions were homogeneous between years, showing a fairly static distribution of the species. However, differences between regions were highly significant. The one case of a change in frequency between years indicated a gradual replacement of flies in a marginal region by flies from the main part of the range. The finding of stability in the distribution of a highly mobile insect is of interest, potentially also for other species which have expanded beyond their native range. It is argued that a contributing reason for this stability may be adaptation to different climatic regimes, and that strategies for control based on this hypothesis afford a reasonable chance of success.

Fitness of third chromosome homozygotes in Drosophila melanogaster.

A population cage experiment has been carried out to estimate fitness for a sample of fourteen non-lethal third chromosomes in D. melanogaster . This measurement, which should take into account all aspects of fitness, gives an estimated mean fitness of chromosome homozygotes of approximately ten percent.

Genetic Consequences of Domestication and Mass Rearing of Pest Fruit Fly Bactrocera tryoni (Diptera: Tephritidae)

ABSTRACT Tephritid fruit flies, an important pest of horticulture worldwide, are increasingly targeted for control or eradication by large-scale releases of sterile flies of the same species. For each species treated, strains must be domesticated for mass rearing to provide sufficiently large numbers of individuals for releases. Increases in productivity of domesticated tephritid strains are well documented, but there have been few systematic studies of the genetic consequences of domestication in tephritids. Here, we used nine DNA microsatellite markers to monitor changes in genetic diversity during the early generations of domestication in replicated lines of the fruit fly Bactrocera tryoni (Froggatt) (Diptera: Tephritidae). The observed changes in heterozygosity and allelic richness were compared with the expected changes in heterozygosity generated by a stochastic simulation including genetic drift but not selection. The results showed that repeatable genetic bottlenecks occur in the early generations and that selection occurs in the later generations. Furthermore, using the same simulation, we show that there is inadvertent selection for increased productivity for the entire life on a mass-rearing colony, in addition to intentional selection for increased productivity. That additional selection results from the common practice of establishing the next generation of the breeding colony from a small proportion of one days pupae collection (the pupal raffle). That selection occurs during all generations and acts only on fecundity variation. Practical methods to counter that unavoidable loss of genetic diversity during the domestication process in B. tryoni are discussed.

High levels of recombination induced by homologous P elements in Drosophila melanogaster

SummaryP element transposons in Drosophila melanogaster are capable of mobilizing incomplete P elements elsewhere in the genome, and of inducing recombination. This recombination is usually only of the order of 1% or less. We show that two P elements, located at exactly homologous sites, induce levels of recombination of 20% or higher. The recombination appears to be exact, as determined by the lack of phenotypic effects in recombinant products and the lack of size changes detectable by Southern hybridization. Female recombination is increased, but to a lesser extent than male recombination. Somatic recombination levels are also elevated. Alternative explanations for the high recombination levels are given in terms of the consequences of repair of an excision site and in terms of recombination as part of the replicative transposition process.

Divergence between Human Populations Estimated from Linkage Disequilibrium

Observed linkage disequilibrium (LD) between genetic markers in different populations descended independently from a common ancestral population can be used to estimate their absolute time of divergence, because the correlation of LD between populations will be reduced each generation by an amount that, approximately, depends only on the recombination rate between markers. Although drift leads to divergence in allele frequencies, it has less effect on divergence in LD values. We derived the relationship between LD and time of divergence and verified it with coalescent simulations. We then used HapMap Phase II data to estimate time of divergence between human populations. Summed over large numbers of pairs of loci, we find a positive correlation of LD between African and non-African populations at levels of up to approximately 0.3 cM. We estimate that the observed correlation of LD is consistent with an effective separation time of approximately 1,000 generations or approximately 25,000 years before present. The most likely explanation for such relatively low separation times is the existence of substantial levels of migration between populations after the initial separation. Theory and results from coalescent simulations confirm that low levels of migration can lead to a downward bias in the estimate of separation time.

Variation in population structure across the ecological range of the Queensland fruit fly, Bactrocera tryoni

We sampled a pest fruit fly species, the Queensland fruit fly, Bactrocera tryoni, across its entire ecological range in eastern Australia, from ancestral high-density populations in tropical regions through to isolated outbreak populations in marginal arid areas. Using DNA microsatellite markers, we found that in ancestral areas, population differentiation was low and populations were genetically homogeneous over large distances. In more temperate areas, populations were far more genetically differentiated but there was no pattern of isolation-by-distance (no drift/migration equilibrium). Genetic drift appeared to be the major influence on population differentiation. The transition between these extremes was abrupt and unexpectedly far from the species border. Limited geographic structuring among the non-equilibrium populations was apparent from patterns of genetic differentiation, patterns of allelic richness and an ordination analysis. Our results also suggested that there might be recurring migration of flies into a neighbouring quarantine area.

X-chromosomal heterosis in Drosophila melanogaster.

Population cages were set up containing an X -chromosome balancer, and either a single wild-type chromosome(homozygous cages) or a mixture of wild-type chromosomes(heterozygous cages). The balancer chromosome was eliminated more rapidly from the heterozygous cages, indicating that chromosome heterozygotes are at an advantage over chromosome homozygotes. The disadvantage of X -chromosome homozygosity in the female is estimated to be about 40%. From earlier studies it is known that the average disadvantage of homozygosity for either of the two major autosomes of D. melanogaster is approximately 80%. Since these autosomes are both about twice as long as the X chromosome, the disadvantage per unit length is similar for both chromosomal types. Both X -chromosomal and autosomal heterosis can be explained by either dominance or overdominance at individual loci. However, a dominance model can only explain the similarity if many of the X -linked loci (about 50%) are limited in expression to the female.