Search PubMed⌕ Search

Biomedical subjects

K S Rai

Publications and source records attributed to K S Rai.

At least 37 records · Page 2Linked to original sources
PubMed1989

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.

Aedes↗
PubMed1989

Laboratory study of competition between United States strains of Aedes albopictus and Aedes aegypti (Diptera: Culicidae).

Mosquito abatement workers in Houston, Tex., and New Orleans, La., have observed that the recent introduction of Aedes albopictus (Skuse) has been accompanied by a decline and virtual disappearance of Ae. aegypti (L.). This suggests competitive displacement but contradicts the direction of displacement observed in native habitats of Ae. albopictus. The paper reports a series of laboratory studies on competition between U.S. strains of the two species. Larval survivorship, size, and development time were monitored in increasing densities of pure and mixed cultures of the two species under limited and optimal diets. Oviposition preferences were examined by offering gravid females cups containing first or fourth instars of either species. Ethological isolation was studied by simultaneously offering virgin females of one species to males of both species and monitoring courtship behavior as well as rates of oviposition, percentage of hatch, and embryonation. The results of the larval competition studies suggest that interspecific competition increased development time only slightly and had no effect on survival of Ae. albopictus. Competition increased survival in Ae. aegypti. Size of both species was influenced by interspecific competition, but the magnitude and direction of the effect was inconsistent between strains and diets such that neither species emerged consistently larger. The results of the oviposition study indicated a higher fecundity in Ae. aegypti but otherwise demonstrated that both species oviposit without regard to the presence, age, or species of larvae in the oviposition container. The ethological isolation tests demonstrated that Ae. aegypti males were more sexually aggressive than males or Ae. albopictus. However, Ae. albopitus females placed with increasing densities of Ae. aegypti males demonstrated no decrease in rates of oviposition, and eggs laid showed equal rates of hatch and embryonation. In general, the results do not suggest that the U.S Ae. albopictus population is inherently more competitive in the laboratory than Ae. aegypti. Other reasons for the observed decline in Ae. aegypti in the United States are discussed.

Aedes↗
PubMed1988

Breeding structure of a colonizing species: Aedes albopictus (Skuse) in peninsular Malaysia and Borneo.

The mosquito, Aedes albopictus, has recently become established in a number of cities throughout the United States. An initial survey of allozyme and genotypic frequencies in U.S. populations (Black et al., 1988) revealed an extensive amount of local differentiation of populations and suggested that much genetic drift may have accompanied colonization. A study of gene flow was initiated in native habitats of Ae. albopictus in Malaysia to determine if the result observed in the U.S. was a consequence of colonization or simply followed the natural breeding structure of the species. Allelic and genotypic frequencies were monitored at ten enzymatic loci in 11 populations from peninsular Malaysia and Borneo. Multiple populations were sampled within the districts of Kuala Lumpur and Kuala Trengganu. Peninsular Malaysian and Borneo populations were strongly genetically differentiated. Allele frequencies were significantly different among and within districts in both regions. Variance in allele frequencies among all collections was partitioned into the variance among regions, districts within regions and collections within districts. Almost all of the variance within regions was attributable to local differentiation suggesting that genetic drift is an important component of the natural breeding structure of this species. This indicates that the large amounts of local differentiation found in U.S. populations was not a consequence of recent colonization.

Aedes↗
PubMed1988

Breeding structure of a colonising species: Aedes albopictus (Skuse) in the United States.

Allelic and genotypic frequencies were monitored at seven enzymatic loci in newly established Aedes albopictus (Skuse) populations in the United States. Populations were sampled within the cities of New Orleans, Louisiana, Houston, Texas, three counties surrounding Houston, Memphis, Tennessee, Jacksonville, Florida, and Evansville and Indianapolis, Indiana. Unique alleles and relatively high levels of heterozygosity were detected in New Orleans, Houston, surrounding counties and Indianapolis suggesting relatively large and independent introductions in these cities. No unique alleles and low heterozygosities were detected in Memphis, Evansville and Jacksonville suggesting that a population bottleneck may have accompanied the founding of these populations. The bottleneck may have resulted from a small number of founding individuals or may have been generated through repeated control efforts. Genetic distance estimates indicated that Houston, New Orleans and Indianapolis were genetically similar. Evansville and Memphis were also found to be similar. Significant differentiation of allele frequencies existed among and within cities. Variance in allele frequencies among all samples was partitioned into the variance among cities and among locations within cities. Most of the variance was attributable to local differentiation. The most parsimonious explanation of this result is that much genetic drift accompanied the establishment of local populations in cities and that there has been little subsequent gene flow. Analysis of genotypic frequencies detected a slight but consistent excess of homozygotes suggesting inbreeding.

Aedes↗
PubMed1987

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.

Aedes↗
PubMed1987

Variation in the vector competence of geographic strains of Aedes albopictus for dengue 1 virus.

Eight geographic strains of Aedes albopictus from Asia and North America and one North American strain of Aedes aegypti were tested for their vector competence with dengue 1 virus. Three groups of Ae. albopictus were established based on their vector competence: a) the OAHU laboratory strain, b) the three Malaysian strains, and c) the TOKYO and three North American strains. The three North American strains were similar to the strain of Ae. aegypti from Houston, Texas in their ability to transmit dengue 1 virus. A comparison of barriers to infection and transmission suggests that Ae. albopictus HOUSTON represents an introduced strain distinct from the more similar MEMPHIS and NEW ORLEANS strains. Based on these studies the North American strains were seen as more similar to a northern Asian strain (TOKYO) than to the three Malaysian (southern Asia) strains, supporting the current hypothesis that the indigenous strains of Ae. albopictus recently introduced into the United States had a northern Asian origin.

Aedes↗
PubMed1986

Chromosome banding homologies in three species of Aedes (Stegomyia).

The chromosome complements of the mosquitoes Aedes aegypti, Aedes mascarensis, and Aedes albopictus, belonging to the subgenus Stegomyia, gave a uniform response to the Q-, H-, and R-banding techniques. Of the three homomorphic chromosome pairs, only the shortest or sex pair (I) showed a consistent banding pattern. In the three species, a bright yellow intercalary band was present on one arm of both chromosomes of the sex pair after heat treatment and staining with acridine orange. The rest of the chromosome and the other two pairs fluoresced orange-red. The same intercalary region appeared completely dark with the fluorochromes quinacrine and Hoechst 33258, while the rest of the chromosomes fluoresced dull. The same banding pattern was present in males and females. Size variations of the Q- and H-negative and R-positive intercalary bands were observed within each species. The results are interpreted in terms of structural homology of the sex-determining chromosomes, which is retained within the subgenus.

Aedes↗
PubMed1986

Genetics of Aedes albopictus.

Aedes albopictus is an important vector of dengue fever and dengue hemorrhagic fever in Southeast Asia. Its distribution extends from Madagascar to Hawaii and is currently expanding. From its proposed origin in Southeast Asia, Ae. albopictus has migrated as far as Mauritius and Madagascar to the west and Korea, Japan, Guam, Hawaii and other Pacific Islands to the east. In the continental United States, it was originally reported in the Texas area in August 1985 and is now well established in several states. This paper reviews information on distribution, cytology and genetics of Aedes albopictus. In addition, it includes comments on its competitive interaction with several other species. Relevant information on evolutionary genetics of certain other related sibling species is also included for comparative purposes.

Aedes↗
PubMed1985

Genetic relationships among certain species of the Aedes (Stegomyia) scutellaris group (Diptera: Culicidae).

Genetic affinities between species of the Aedes (Stegomyia) scutellaris group were determined through experimental hybridization. In this investigation Aedes hebrideus and Ae. scutellaris katherinensis were reciprocally crossed with Ae. polynesiensis, Ae. pseudoscutellaris, Ae. kesseli, Ae. malayensis, Ae. alcasidi and Ae. albopictus. Data on insemination and egg hatch rates were recorded in the reciprocal crosses. Of these, Ae. hebrideus and Ae alcasidi, Ae. hebrideus and Ae. s. katherinensis, Ae. malayensis and Ae. hebrideus, and Ae. malayensis and Ae. s. katherinensis, were unidirectionally compatible using female parent of former species of the two. The reciprocal crosses of these laid infertile eggs. Aedes s. katherinensis and Ae. alcasidi were bidirectionally compatible. All other interspecific crosses were incompatible producing infertile eggs. In the compatible crosses, i.e. where the viable hybrids were obtained, F2 and hybrid fertilities were determined. Species hybrids obtained by crossing Ae. malayensis females and Ae. s. katherinensis males yielded lower fertility in F2 and differential fertility when backcrossed. Species hybrids obtained by crossing Ae. s. katherinensis females and Ae. alcasidi males resulted in lower fertility in F2 and also when backcrossed to Ae. s. katherinensis females. All other species hybrids were fertile. Based on the data obtained, Ae. hebrideus, Ae. s. katherinensis, Ae. malayensis and Ae. alcasidi formed a sub-grouping which showed remote genetic affinities individually or as a group with rest of the species investigated.

Aedes↗
PubMed1984

Genetics of speciation in the Aedes (Stegomyia) scutellaris subgroup (Diptera:Culicidae). 4. Chromosomal relationships of Aedes cooki with four sibling species.

Morphology and behavior of chromosomes and development of testes and sperm were examined in hybrids from interspecific crosses involving Aedes cooki and four sibling species of the Aedes (Stegomyia) scutellaris subgroup of mosquitoes. The degree of abnormality in hybrid spermatogenesis in interspecific crosses involving Aedes cooki males and females of four sibling species paralleled the geographic distributions of these species and the genetic divergence indicated by other genetic studies. Hybrids from crosses involving Aedes malayensis females and Aedes cooki males were characterized by atrophied testes and extensive chromosome breakage. Hybrids from crosses involving Aedes alcasidi females and Aedes cooki males suggested a possible pericentric inversion distinguishing the largest autosome of Aedes alcasidi from that of Aedes cooki. Hybrids from interspecific crosses involving females of Aedes polynesiensis and Aedes pseudoscutellaris and males of Aedes cooki showed high percentages of univalents of the smallest chromosome pair. Hybrid spermatogenesis in two interspecific crosses involving Aedes cooki females differed from results of reciprocal crosses. Data were scant, however, and interpretation was difficult in view of negligible hatch in all interspecific crosses involving Aedes cooki females.

Aedes↗
PubMed1983

Linkage relationships of eleven enzyme loci in the Aedes scutellaris group.

Linkage relationships of 11 enzyme loci were determined in backcrosses between Aedes polynesiensis and Aedes kesseli. Three linkage groups established were Aat2-Lap2-Me-Sex, Cat-Ao-Pgm-Idh2-Est6, and Gpi-Odh-Pgd. Lap2 and Cat have not been previously mapped in Aedes. Locus order and linkage groups were the same as those observed for seven loci mapped in Aedes aegypti. The significance of the observed similarities in chromosome organization and differences in crossover values among closely related Aedes are discussed.

Aedes↗
PubMed1978

Cell cycle and DNA synthesis in the mosquito Aedes aegypti.

Chronology of the cell cycle has been determined for brain cells in Aedes aegypti (L.) larvae by autoradiography. The time of the cell cycle is approximately 10 h and 30 min. Areas of late DNA synthesis in metaphase chromosomes were localized mainly on the female determining (= m) chromosome of the sex pair. Correlations between late replication patterns and C-banding are discussed.

Aedes↗
PubMed1975

Trisomy in Aedes aegypti.

A trisomic (2n=6+1) pupa of the yellow fever mosquito Aedes aegypti has been found. The trisomy involved chromosome 3 which is intermediate in size between 1 and 2. The extra chromosome formed a univalent or a trivalent during meiosis.

Aedes↗
PubMed1975

Chemosterilant (apholate)-induced ultrastructural changes during oogenesis in Aedes aegypti.

The effect of chemosterilant, apholate, on-oogenesis has been studied in Aedes aegypti. Treatment of larvae to 20 ppm of the chemical induced ultrastural changes in the presumptive and primary follicles of the adult ovary. These changes comprised condensation of chromatin, disruption of nuclear envelope and extensive degeneration as evidenced by numerous myelin figures and residual bodies. In some primary follicles, where cellular degeneration was restricted to epithelial cells, no arrest in development was observed. However, in comparison with controls, these follicles also were retarded. Larval treatment with 30 ppm apholate completely supressed ovariolar development. High incidence of autophagy was observed in tissues at both dose levels.

Aedes↗