Patterns of variation in the rDNA cistron within and among world populations of a mosquito, Aedes albopictus (Skuse).

A restriction map was constructed of the ribosomal cistron in a mosquito, Aedes albopictus (Skuse). The 18s, 28s and nontranscribed spacer (NTS) regions were subcloned and used to probe for intraspecific variation. Seventeen populations were examined throughout the world range of the species. No variation was detected in the coding regions but extensive and continuous variation existed in the NTS. The NTS consisted of two nonhomologous regions. The first region contained multiple 190-bp AluI repeats nested within larger XhoI repeats of various sizes. There was a large number of length variants in the AluI repeat region of the NTS. No repeats were found in the second region and it gave rise to relatively fewer variants. An analysis of NTS diversity in individual mosquitoes indicated that most of the diversity arose at the population level. Discriminant analysis was performed on spacer types in individual mosquitoes and demonstrated that individuals within a population carried a unique set of spacers. In contrast with studies of the NTS in Drosophila populations, there seems to be little conservation of spacers in a population. The importance of molecular drive relative to drift and selection in the generation of local population differentiation is discussed.

Resource partitioning by three species of hemipteran herbivores on the basis of host plant density.

Three hemipteran herbivores partition a common resource plant, Senecio smallii, on the basis of bud density and plant patch size. The monophagous herbivore, Neacoryphus bicrucis, is most abundant in larger patches where bud density is greatest. The oligophagous herbivore, Harmostes reflexulus, is most abundant in small patches of high bud density. The polyphagous species, Lygus pratensis, is most abundant in small patches where abundance is independent of bud density. The aggressive nature of the monophagous herbivore appears to mediate the responses of the other species.

Variation in ribosomal DNA internal transcribed spacers 1 among eastern populations of Ixodes scapularis (Acari: Ixodidae).

The base sequence of the internal transcribed spacer 1 (ITS 1) of ribosomal DNA of the tick Ixodes scapularis Say (= I. dammini Spielman, Clifford, Piesman & Corwin) was determined to assess genetic divergence between populations along the eastern (Atlantic) seaboard of the United States. Twenty sequences were obtained from localities down the eastern margin of the species's range: 10 from the southeast (Georgia and Florida), seven from the middle east (North Carolina, Maryland), and three from the northeast (Massachusetts, New Jersey, New York). Both the neighbor-joining and parsimony methods cluster most of the southeastern sequences together and most of the middle eastern sequences together but fail to cluster those from the northeast. In addition, an F ratio test revealed significant between-region sequence variation. Thus, there appears to be genetic structuring on at least a macrogeographic scale. Only 23% (SEM = 6.4%) of the sequence variation occurs between regions, with the vast majority of variation, 77% (SEM = 6.4%), being within region. These data, plus other published data, indicate that I. scapularis constitutes a single species. However, the pattern of variation is consistent with restricted gene flow between regions or, alternatively, with recent introgression between northern and southern types in the middle-eastern part of the species's range.

Conspecificity of the ticks Ixodes scapularis and I. dammini (Acari: Ixodidae).

Reciprocal crosses between Ixodes dammini Spielman, Clifford, Piesman & Corwin from Massachusetts and Ixodes scapularis Say from Georgia produced offspring through the F3 generation when the experiment was discontinued. Reciprocal I. dammini x Ixodes pacificus Cooley & Kohls (California) and I. scapularis x I. pacificus crosses produced F1 progeny; however, all progeny were sterile. Assortative mating experiments between I. dammini and I. scapularis indicated that males and females of both species mated with the opposite sex of heterospecific or conspecific ticks when there was a choice. Conventional discriminant analysis of morphometric measurements of ticks from Georgia, North Carolina, Maryland, Massachusetts, and two populations of F1 hybrids indicated that there were recognizable differences. However, size-free (sheared) discriminant analysis indicated that these differences were largely size-dependent, with much overlap of the four eastern and two hybrid populations but no overlap with I. pacificus from California. Analysis of chromosomes (morphology and C band) indicated no differences between the Georgia and Massachusetts populations but showed a difference between them and the California population of I. pacificus. Analysis of isozymes showed that the genetic identity value for the Georgia and Massachusetts populations was within the normal range for conspecific populations, whereas the California population indicated congeneric but not conspecific relatedness to the Georgia and Massachusetts populations. Life cycle data collected under similar laboratory conditions showed no differences in length of feeding and molting periods among Georgia, Massachusetts, and California populations. These data and results of the work of other authors on tick host preferences and vector competence indicate that I. dammini is not a valid species separate from I. scapularis. Because the name Ixodes scapularis Say, 1821, has priority over the name Ixodes dammini Spielman, Clifford, Piesman & Corwin, 1979, I. dammini is relegated to a junior subjective synonym of I. scapularis (based on Article 23 of the International Code of Zoological Nomenclature).

Evolution of transcript structure and base composition of rDNA expansion segment D3 in ticks.

Four to thirty-two copies of the rDNA 28S gene expansion segment D3 and flanking H14 stem were sequenced in six species of ticks (Ixodes: Ixodidae: Acari). Sequence match among species varied from 66% to 97%. Sequence length averaged 130 bases in I. persulcatus across eight Eurasian sites and averaged 186 bases in five other species across 19 Eurasian and North American sites. The difference in length represents one or more deletions totalling about 60 bases that correspond to stems S3 or S4 of the folded transcript. The typical transcript conformation was observed as one possible low energy structure in the five species of longer D3. The structure entails a basal loop with four stem/loop structures, S1-S4 (moving 5' to 3') atop stem H14. A secondary structure lacking S4 but possessing all other putative standard features of the D3 transcript is possible with the shorter I. persulcatus sequences. Interspecific sequence differences occur at higher frequency in loops and bulges vs. complementary pairing regions of stems. Insertion/deletion events (indels) and base substitutions accounted equally for sequence differences. Indels are flanked by similar sequences, suggesting that they occur by slippage during replication. The D3 of Ixodes species is composed of a degenerate set of subrepeats. Thus, unequal exchange among subrepeats may have caused the reduction in length of the I. persulcatus D3. Compensatory base substitution and compensatory insertion/deletion events are indicated by the failure of mutations to affect secondary structure. Transversions accounted for 64% of sequence differences and were biased toward the gain of G and U and the loss of A and C. This bias could re-establish intramolecular base pairing when disrupted by insertions or deletions that shift one side of a stem relative to the other. The distribution of sequence differences, biased substitution, and conservation of transcript conformation in D3 suggest selective constraint.

Structure of rDNA in the mosquito Anopheles gambiae and rDNA sequence variation within and between species of the A. gambiae complex.

The structure of the rDNA repeating unit of Anopheles gambiae (Diptera: Culicidae) was determined by restriction endonuclease mapping and hybridization analyses on four independent clones obtained from a genomic library of a colony (G3) from the Gambia (West Africa). rDNA gene coding sequences are conserved, but much intragenomic and intraspecific (geographic) variation occurs in the intergenic spacer. Hybridization of subclones from spacer and coding sequences to genomic DNA that was isolated from single mosquitoes from laboratory colonies of four other A. gambiae complex species reveals conservation of coding sequences but concerted evolution in the intergenic spacers.

Interspecific and geographical variation in the sequence of rDNA expansion segment D3 of Ixodes ticks (Acari: Ixodidae).

The base sequence of the rDNA D3 expansion segment and flanking H14 stem varies between six species of Ixodes ticks (Acari: Ixodidae) where only 33 invariant sites occur among sequences of 123-203 bases in length. Multiple copies of D3 were sequenced from localities across the geographical ranges of four species to investigate deep population genetic structure. Two species, I. pacificus, from western North America, and I. ricinus, from Europe, have no sequence variation indicating a lack of deep genetic structure. One species, I. scapularis, from eastern North America has two forms of the D3 sequence that are distributed differently among northern vs. southern populations, suggesting recent divergence and hybridization. I. persulcatus, from Eurasia, has sequence variation between localities of the order of that observed between other species, suggesting a long history of population isolation and deep genetic structure. With the exception of I. scapularis, sequence variation was not observed within localities. This indicates that cellular processes underpinning concerted evolution have homogenized populations and species for particular rDNA sequence variants.

Intraspecific and interspecific variation in the sequence and abundance of highly repeated DNA among mosquitoes of the Aedes albopictus subgroup.

Interspecific variation in abundance of highly repeated DNA sequences has been examined in three species of the Aedes scutellaris and three species of the A. albopictus subgroup of the A. scutellaris group. Sequences from a population of A. albopictus were hybridised with whole genomic content from other species and strains. Copy number estimates were determined by dot-blot hybridisation. Variation in sequence abundance between strains of A. albopictus was as great as between it and the other six species. Two clusters were formed by principal components analysis, each of which contains populations of A. albopictus. Copy numbers of highly repeated sequences do not correlate with genome size. The results indicate extensive sequence divergence and rapid evolution. These findings are discussed in relation to the importance of concerted evolution and natural selection in the evolution of the species group.

Investigation of the validity of species status of Ixodes dammini (Acari: Ixodidae) using rDNA.

The two internal transcribed spacers (ITS1 and ITS2) of rDNA of three members of the Ixodes ricinus "complex" (Acari: Ixodidae) were sequenced. Sequence variation was assessed for the North American species I. scapularis, I. dammini, and I. pacificus at three levels: within individual/population, between individuals of different geographic origin within a species, and between species. Both spacers are highly variable, particularly with regard to small deletions and additions which may arise via replication slippage. Homogenization of rDNA multigene arrays for particular sequence variants appears to occur at a relatively rapid rate, since I. pacificus sequences differ from the others at numerous invariant sites, facilitating the use of these sequences to assess sibling species relationships. Based on maximum parsimony and two distance methods (unweighted pair-group with arithmetic means and neighbor-joining), sequence variation in ITS1 and ITS2 suggests that I. scapularis and I. dammini are not distinct species and that even individuals from geographically isolated locations are very similar. Individuals from geographically separated populations of I. pacificus appear to be relatively less closely related to each other but distinct from those of I. scapularis/dammini. In I. scapularis/dammini, diversity within and between individuals from geographic populations contributed equally to total sequence diversity.

Evolution of the rDNA spacer, ITS 2, in the ticks Ixodes scapularis and I. pacificus (Acari: Ixodidae).

Evolution of the rDNA spacer, ITS 2, is examined by comparing 17 DNA sequences of the ticks, Ixodes scapularis and I. pacificus. The distribution of fixed interspecific differences and the relative frequency of base changes vs. insertions/deletions (indels) matches the distribution and relative frequency for intraspecifically variable sites. This suggests that most intraspecific variation is not effectively selected against. The base composition of the ITS 2 transcript is G- and U-biased. But, 5-base regions enriched (> 80 per cent) for A or U occur more frequently than expected while G- and C-enriched regions occur less frequently than expected. Enriched sequences may be prone to replication slippage, accounting for the A/T bias in insertions. Slippage-mediated gains and losses of A/T-rich tandem repeats apparently account for most indels. Minimum-energy conformations of the two species' folded transcripts share major structural features. Structural inertia arises from intramolecular base pairing within stems that allows most mutations to be absorbed as new bulges off stems. Yet, there is evidence of selection to maintain the conformation. First, intraspecifically variable sites are concentrated at the ends of stems in loops and intersections, structures that do not contribute to intramolecular base pairing. Moreover, some indels that have become fixed in one species compensate for the presence of conformation-destabilizing indels. However, high rates of sequence evolution within stems and absence of compensatory base evolution contraindicates selective constraint. Degenerate dispersed and tandem copies of two subrepeats, each approximately 20 bases long, may account for much of the ITS 2 sequence. These are approximately inverses of each other and are, consequently, capable of significant intramolecular hydrogen bonding to produce folded transcripts of low energy. Evolution of the ITS 2 sequence may largely entail replication slippage-mediated gains and losses of these repeats or their composite subrepeats.

Microgeographic variation in rDNA intergenic spacers of Anopheles gambiae in western Kenya.

The genetic population structure of Anopheles gambiae (Diptera: Culicidae) in western Kenya was investigated by hybridizing a rapidly evolving rDNA intergenic spacer sequence to restriction endonuclease digests of genomic DNA extracted from single mosquitoes from seven localities. Significantly different distributions of restriction fragment arrays were obtained from field sites less than 10 km apart, which suggests restricted gene flow and a subdivided population structure. Eight of twenty-one possible comparisons between pairs of populations yielded significant differences. An eastern Kenya coastal population did not share its restriction fragment arrays with any of the western populations, suggesting that isolation by distance can be complete on a relatively small geographic scale (700 km).