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PubMed2000

A biogeographic genetic approach for testing the role of reinforcement: the case of Drosophila pseudoobscura and D. persimilis.

The role of reinforcement in speciation can be explained by two distinct models. In model I, two diverged populations hybridize and produce fertile hybrids that successfully backcross (hybridization with gene flow). In model II, two populations hybridize but succeeding backcrosses are unproductive (hybridization without gene flow). Using Drosophila persimilis and D. pseudoobscura, we have tested model I by comparing the extent of heterospecific introgression in sympatric versus allopatric populations. We show that certain expectations of this particular model of reinforcement, which is based on hybridization and gene flow between divergent populations after secondary contact, are not realized in these two species. The evidence consists of the similarity of genetic distances as well as proportions of unique/rare alleles between sympatric and allopatric heterospecific populations and a negative correlation between genetic distance and geographical distance between heterospecific populations, which suggests ecological differentiation. This approach in quantifying differential gene flow has important consequences to studies that compare sympatric and allopatric isolation using genetic distance. Following model I, one would expect a pattern of higher prezygotic isolation in sympatric species compared to allopatric species of the same genetic distance simply as a result of an underestimation of genetic distance due to introgression between sympatric populations. We suggest more parsimonious explanations such as reinforcement without genetic exchange (model II) and ecological differentiation, which require high levels of preexisting reproductive isolation between populations.

Animals↗
PubMed1993

Differentiation of U.S. and European isolates of porcine reproductive and respiratory syndrome virus by monoclonal antibodies.

Monoclonal antibodies (MAbs) to two U.S. isolates of porcine reproductive and respiratory syndrome (PRRS) virus were prepared. Two MAbs specifically recognized a conserved epitope on the putative 15-kDa nucleocapsid protein of U.S. and European isolates of PRRS virus. Four other MAbs recognized epitopes on the 15-kDa protein of U.S. but not European isolates of PRRS virus. Collectively, this indicates that PRRS viruses contain both conserved and divergent epitopes on the 15-kDa viral protein.

Animals↗
PubMed2000

Trichinella murrelli n. sp: etiological agent of sylvatic trichinellosis in temperate areas of North America.

Trichinella T5, collected from sylvatic carnivores in North America, was identified previously as a different phenotype of Trichinella, with an uncertain taxonomic level due to the availability of only 2 isolates. Cross-breeding experiments carried out with single female and male larvae of 2 strains of Trichinella T5, with single female and male larvae of 2 strains of Trichinella spiralis, Trichinella nativa, Trichinella britovi, Trichinella pseudospiralis, Trichinella nelsoni, and Trichinella T6, showed a reproductive isolation of Trichinella T5. Viable offspring were obtained only when a female of Trichinella T5 was crossed with a male of T. britovi, but not vice versa. Furthermore, the analysis of biological, biochemical, and molecular data of 32 isolates collected from sylvatic animals in the Nearctic region and identified as Trichinella T5 permitted its reassessment at the species level. Trichinella murrelli n. sp. is characterized by the following: distribution in temperate areas of the Nearctic region; newborn larvae production in vitro of 29-36/72 hr; nurse cell development time between 24 and 70 days postinfection; reproductive capacity index in Swiss mice 1.2-9.5, in wild mice 29.5-159.8, in rats 0.7-2.4, and in pigs 0.03-0.0004; no resistance to freezing; ribosomal DNA fragments of 7.2 kb and/or 11.4 kb, plus 2.2 kb and 1.8 kb present after Dra I digested DNA when probed with total T. spiralis RNA; a specific amplicon of 179 bp after polymerase chain reaction (PCR) amplification with the primer set SB147G; a specific fragment of 1,600 bp after PCR amplification with the primer set Ts43CA and Hhb I digestion; long incubation period; and moderate to severe pathogenicity for humans. The new species is most similar to T. britovi, though it differs from T. britovi in the pattern of 2 allozymes, in the patterns of major ribosomal DNA and PCR-restriction fragment length polymorphism fragments, and in geographical distribution.

Animals↗
PubMed1981

Patterns of mortality in the the Old Order Amish. I. Background and major causes of death.

The major causes of death were studied in the Old Order Amish people in three settlements in Indiana, Ohio and Pennsylvania to determine if lifestyle and genetic isolation altered their mortality risk compared to neighboring non-Amish. The Amish are a conservative religious group who live in farm settlements, use horses for work and travel, exercise vigorously, and avoid cigarettes and alcohol. They are reproductively isolated and highly inbred. Death certificates and Amish censuses were used to determine mortality risks, which were summarized using age-adjusted mortality ratio (MRs). Amish mortality patterns were not systematically higher or lower than those of the non-Amish, but differed by age, sex, and cause. Amish males had slightly higher all-cause MRs as children and significantly lower MRs over the age of 40, due primarily to lower rates of cancer (MR = 0.44, age 40-69), and cardiovascular diseases (MR = 0.65, age 40-69). Amish females MRs for all causes of death were lower from age 10-39, not different from 40-69, and higher over age 69. MRs were not significantly different for all cancer sites combined in Amish women and they had higher cardiovascular mortality ratio aged 70 and over (MR =1.34). Other major causes of death were also examined. Because the Amish and other farming groups have similar mortality patterns, it is suggested that lifestyle may be the primary determinant of the overall mortality patterns in the Amish.

Adolescent↗
PubMed1995

Antigenic comparison of Canadian and US isolates of porcine reproductive and respiratory syndrome virus using monoclonal antibodies to the nucleocapsid protein.

Fifteen Canadian field isolates of porcine reproductive and respiratory syndrome (PRRS) virus from Quebec and Ontario were compared with 5 US PRRS virus (PRRSV) isolates and with the European Lelystad isolate using monoclonal antibodies (MAbs) SDOW17, EP147, and VO17 directed to the 15-kDa nucleocapsid protein of PRRSV. All Canadian and US isolates tested by indirect immunofluorescence were recognized by the 3 MAbs, and individual titers of MAbs towards Canadian and US PRRSV isolates were similar as well. In contrast, the Lelystad virus isolate reacted only with the SDOW17 MAb and showed no reactivity with either EP147 or VO17. The reactivity pattern with these MAbs suggests that the Canadian isolates of PRRSV tested are antigenically similar to US isolates of PRRSV, and that these North American isolates share highly conserved epitopes on the 15-kDa nucleocapsid protein that clearly differentiate them from the European Lelystad virus isolate.

Animals↗
PubMed1997

Genetics of a pheromonal difference affecting sexual isolation between Drosophila mauritiana and D. sechellia.

Females of the sibling species Drosophila sechellia and D. mauritiana differ in their cuticular hydrocarbons: the predominant compound in D. sechellia is 7,11-heptacosadiene (7,11-HD), while that in D. mauritiana is 7-tricosene (7-T). We investigate the genetic basis of this difference and its involvement in reproductive isolation between the species. Behavioral studies involving hydrocarbon transfer suggest that these compounds play a large role in the sexual isolation between D. mauritiana males and D. sechellia females, while sexual isolation in the reciprocal hybridization results more from differences in female behavior than hydrocarbons. This interspecific difference in hydrocarbon profile is due to evolutionary change at a minimum of six loci, all on the third chromosome. The localization of evolutionary change to the third chromosome has been seen in very other genetic analysis of female hydrocarbon differences in the D. melanogaster group. We suggest that the high 7,11-HD phenotype seen in two species evolved twice independently from ancestors having the high 7-T phenotype and the recurrent third-chromosome effects are evolutionary convergences that may be due to a concentration of "hydrocarbon genes" on that chromosome.

Alkadienes↗
PubMed2000

Genetic differentiation in Tetranychus urticae (Acari: Tetranychidae): polymorphism, host races or sibling species?

Based on allozyme electrophoresis at the Pgm locus and nuclear ribosomal DNA (ITS2) sequences, we studied the genetic variation of the two-spotted spider mite Tetranychus urticae Koch collected on rose bay, Nerium oleander L. (Apocynaceae), from several localities around the Mediterranean basin. In addition, we compared these results with those of Navajas et al. (1998) and Tsagkarakou (1997) who collected from several other host plants from the Mediterranean. In the western part of this area (Spain, France, Tunisia), we found the individuals collected from rose bay to be clearly genetically differentiated from other samples. No evidence of such host-associated differentiation was detected in the eastern Mediterranean (Italy and Greece). The genetic differentiation of mites collected on rose bay was investigated further by studying the reproductive incompatibilities between populations in Greece and in France and a laboratory strain reared on bean, Phaseolus vulgaris, in France. Reciprocal crosses performed between these strains revealed variable levels of incompatibility, spanning from partial to complete reproductive isolation. In all cases incompatibility was asymmetric. We designed a test based on double-mating to establish the fertilization status of females in fully incompatible crosses. These crosses showed that the females had been inseminated, which suggests that the barrier to reproduction is not of a prezygotic behavioral nature. The data raises the question of the relative role of ecological factors (host plant) and geographical distance, in the ongoing differentiation process potentially leading to speciation.

Alleles↗
PubMed2001

DNA analysis of transferred sperm reveals significant levels of gene flow between molecular forms of Anopheles gambiae.

Anopheles gambiae populations in west Africa are complex, being composed of multiple, sympatric subpopulations. Recent studies have failed to reveal significant genetic differences among subpopulations, stimulating a debate regarding the levels of gene flow among them. The observed homogeneity may be the consequence of substantial contemporary gene flow or it may be that reproductive isolation is complete, but too recent for the accumulation of significant levels of genic divergence. Here, we report the results of a study estimating contemporary levels of gene flow between An. gambiae subpopulations by analysing females and transferred sperm removed from their reproductive systems. A total of 251 female and associated sperm extracts was analysed from a single site in Mali. Two molecular forms of An. gambiae, the M- and S-forms, occurred in sympatry at this site. Overall, we found very strong positive assortative mating within forms, however, we did observe significant hybridization between forms. In the M subpopulation 2/195 females (1.03%) contained sperm from S-form males and in 55 S-form females we found one female containing M-form sperm (1.82%). We also identified a mated M xS hybrid adult female. From mating frequencies, we estimate the Nem between the M- and S-form at 16.8, and from the adult hybrid frequency at 5.6. These values are consistent with our earlier estimate, based on FST for 21 microsatellite loci in which Nem = 5.8. We conclude that the general lack of genetic divergence between the M and S subpopulations of An. gambiae can be explained entirely by contemporary gene flow.

Animals↗
PubMedDate not supplied

Counter current transfer and back transport of 3H-PGF2 alpha in the cow's broad ligament vasculature ipsilateral and contralateral to the corpus luteum.

Eight cows of similar age (5-7 years) were chosen for the experiment. Isolated reproductive tract was supplied with autologous oxygenated and heated (40 degrees C) blood through the uterine artery and ovarian artery. 3H-PGF2 alpha in total dose of 2 MBq (10(7) cpm) was injected into each of the uterine lumen of isolated organ. Blood samples were collected at 5 min intervals during 120 min of experiment using cannulae inserted into the branches of uterine arteries about 1 cm below the horns and from ovarian arteries inserted 0.5 cm below the ovaries. The concentration of 3H-PGF2 alpha found in blood plasma taken from uterine artery or from ovarian artery on the side with active corpus luteum (CL) was significantly lower (p less than 0.001) compare with contralateral side to active CL. Radioactive PGF2 alpha found in branches of uterine arteries on both ipsilateral and contralateral side to CL was significantly higher (p less than 0.001) compare to ovarian artery of the same side. It is concluded that absorption of 3H-PGF2 alpha from uterine lumen into venous blood as well as its counter current transfer in area of broad ligament vasculature were reduced on the side of uterine horn with active CL probably as an effect of estrogen:progesterone ratio on vascular constriction in area of uterine vasculature.

Adnexa Uteri↗
PubMed1994

Cytoplasmic incompatibility. Giant steps sideways.

The horizontal transfer of a bacterial endosymbiont that is intimately associated with reproductive isolation in insects is now feasible and may, in principle, lead to new strategies for biological pest control.

Animals↗
PubMed1996

Natural selection and random genetic drift as causes of evolution on islands.

The evolutionary processes responsible for adaptation and speciation on islands differ in several ways from those on the mainland. Most attention has been given to the random genetic drift that arises when a population is founded from just a few colonizing genomes. Theoretical obstacles to "founder effect speciation' are discussed, together with recent proposals for avoiding them. It is argued that although certain kinds of epistasis can facilitate the evolution of strong reproductive isolation, this favours divergence by selection as much as by random drift.

Alleles↗
PubMed1994

Postcopulatory, prezygotic isolation: intraspecific and interspecific sperm precedence in Tribolium spp., flour beetles.

We investigated intraspecific and interspecific patterns of paternity in Tribolium castaneum, the flour beetle, by mating females either to pairs of conspecific males or to one conspecific and one heterospecific male of the closely related species, T. freemani. Females of both species store sperm in the spermatheca after copulation and postcopulatory, prezygotic reproductive isolation has been reported between this pair of species. When conspecific males of contrasting genotype are mated simultaneously to T. castaneum females, we observed extremely high levels of variation among females in the pattern of sperm precedence as shown by the offspring genotypes. In contrast, T. castaneum females mated simultaneously to a conspecific and a heterospecific male produced over 99 per cent conspecific progeny. When conspecific males were mated sequentially to T. castaneum females, within 3-7 days, all offspring were sired by the second male. In contrast, when a conspecific male was replaced with a T. freemani male, most females continued to produce only first-male, conspecific offspring for the next 10 days. When a heterospecific male was replaced with a conspecific male, T. castaneum females changed from producing hybrid to conspecific offspring within 3 days. We discuss the evolutionary implications of these findings.

Animals↗
PubMed2001

Incipient sexual isolation in the nasuta-albomicans complex of Drosophila: no-choice experiments.

Drosophila nasuta nasuta and Drosophila nasuta albomicans are cross-fertile races of Drosophila. Hybridization between these races in the laboratory has given rise to new races (Cytoraces), among which karyotypic composition differs from one another and also from those of the parental races. In this study, we search for the evidence of incipient reproductive isolation among the parental races and four Cytoraces by assessing the fraction of no-matings, mating latency and copulation duration in all possible types of homo- and heterogamic crosses (N = 4184). In no-choice conditions, the latency time (time to initiation of copulation) is lower in homogamic crosses than in heterogamic crosses for both parental races and Cytoraces. Latency time and copulation duration are negatively correlated, whereas fraction of no matings is positively correlated with latency time. Thus these six closely related races of the nasuta-albomicans complex show the initiation of the earliest stages of pre-zygotic isolation, manifested as a tendency for matings to be initiated earlier and more often, and for a longer duration, among homogamic rather than heterogamic individuals

Animals↗
PubMed1995

Wolbachia and cytoplasmic incompatibility in mycophagous Drosophila and their relatives.

Bacterial symbionts belonging to the genus Wolbachia are associated with postzygotic reproductive incompatibility in a number of insect species. Using polymerase chain reaction (PCR) amplification of bacterial gene sequences, strains from 10 species belonging to the closely related quinaria, testacea and tripunctata groups of Drosophila were screened for the presence of Wolbachia in their reproductive tissues. Those screened included the mycophagous species D. falleni, D. recens, D. phalerata, D. testacea, D. neotestacea, D. orientacea, D. putrida and D. tripunctata, and the nonmycophagous species D. palustris and D. quinaria. Two species, D. recens and D. orientacea, were found to be infected with Wolbachia. Subsequent tests of four additional strains of D. recens found all to be infected with the bacteria. It was established that these bacteria cause partial cytoplasmic incompatibility in D. recens by antibiotic curing followed by crosses between cured and uncured strains. Curing was confirmed by a PCR assay. Although most species of insects shown to be infected with Wolbachia are cosmopolitan and/or have undergone recent range expansion in association with human activity, D. recens and D. orientacea are endemic species with specialized ecological habits. Preliminary molecular phylogenetic analysis indicates that, among the species we examined, D. quinaria is most closely related to D. recens. To determine whether the bacteria are involved in reproductive isolation between these two species, reciprocal crosses were carried out between D. quinaria and both infected and uninfected (cured) strains of D. recens. Although these species did mate with each other, all interspecific crosses failed to yield hybrid progeny, indicating that the bacteria are not responsible for reproductive incompatibility between these species.

Animals↗
PubMed1986

The heterochromatin of grasshoppers from the Caledia captiva species complex. I. Sequence evolution and conservation in a highly repeated DNA family.

The restriction enzyme TaqI digests 0.2% of the genomic DNA from the grasshopper Caledia captiva to a family of sequences 168 bp in length (length of consensus sequence). The sequence variation of this "Taq family" of repeat units was examined among four races from C. captiva to assay the pattern of evolution within this highly repeated DNA. The Taq-family repeats are located in C-banded heterochromatin on at least one member of each homologous pair of chromosomes; the locations range from centromeric to telomeric. Thirty-nine cloned repeats isolated from two population 1A individuals along with 11 clones from seven populations taken from three of the races demonstrated sequence variation at 72 positions. Pairwise comparisons of the cloned repeats, both within an individual and between different races, indicate that levels of intraspecific divergence, as measured by reproductive incompatibility, do not correlate with sequence divergence among the 168-bp repeats. A number of subsequences within the repeat remain unchanged among all 50 clones; the longest of these is 18 bp. That the same 18-bp subsequence is present in all clones examined is a finding that departs significantly (P less than 0.01) from what would be expected to occur at random. Two other cloned repeats, from a reproductively isolated race of C. captiva, have sequences that show 56% identity with this 18-bp conserved region. An analysis showed that the frequency of occurrence of an RsaI recognition site within the 168-bp repeat in the entire Taq family agreed with that found in the cloned sequences. These data, along with a partial sequence for the entire Taq family obtained by sequencing uncloned repeats, suggest that the consensus sequence from the cloned copies is representative of this highly repeated family and is not a biased sample resulting from the cloning procedure. The 18-bp conserved sequence is part of a 42-bp sequence that possesses dyad symmetry typical of protein-binding sites. We speculate that this may be significant in the evolution of the Taq family of sequences.

Animals↗
PubMed2000

Crossing relationships among ancient and experimental sunflower hybrid lineages.

Reproductive barrier formation between newly derived hybrid taxa and their parental species represents a major evolutionary hurdle. Here, I examine the development of a sterility barrier during hybrid speciation by examining the fertility of progeny from all combinations of crosses involving three experimentally synthesized sunflower hybrid lineages, their natural hybrid counterpart, Helianthus anomalus, and their parents, H. annuus and H. petiolaris. Crosses between the parental species and H. anomalus generated almost completely sterile offspring (pollen viability < 5%; seed set < 1%). A fairly strong sterility barrier also has developed between three hybrid lineages and both parental species (pollen viability 11.1-41.6%; seed set 0.84-20.1%). In contrast, the three hybrid lineages are almost fully interfertile (pollen viabilities 83.1-88.6%; seed set 72.1-75.3%), as predicted by molecular mapping studies that indicate they have converged on a similar set of gene combinations and chromosomal rearrangements. A modest decline in compability is observed in crosses between the three hybrid lineages and H. anomalus (pollen viabilities 64.1-70.7%; seed set 37-43%), a result that agrees well with prior data demonstrating significant congruence between the genomes of the natural and experimental hybrid lineages. These observations not only indicate that reproductive isolation can arise as a by-product of fertility selection in hybrid populations, but also testify to the repeatability of this mode of speciation.

Breeding↗
PubMed2000

Population structure of Hydropsyche exocellata. Genetic homogeneity in a zone of fragmented distribution.

Using four allozymic loci, we analysed the genetic structure and differentiation of Hydropsyche exocellata (Trichoptera; Hydropsychidae) larvae in seven sites along the Upper Loire River (France). The genetic differentiation is low for this species despite distribution patchiness, and only due to one locus in one study site. No trend relating genetic differentiation to geographic distance (isolation by distance) was observed. Significant deviations from Hardy-Weinberg equilibrium were observed at most sites and loci. The population genetics analysis of an amphibiotic insect such as H. exocellata in light of current ecological and population genetics knowledge is discussed following two main factors: 1) sampling of partially reproductively isolated, genetically distinct subpopulations, and 2) form of selection. Despite knowledge on several aspects of the ecology of H. exocellata in the River Loire, numerous points limit genetic data interpretation. These points are underlined and discussed with reference to studies on other amphibiotic insects.

Animals↗
PubMed1976

Olfactory discrimination as an isolating mechanism in speciating mole rats.

Olfactory discrimination was tested in two chromosome forms of the speciating fossorial rodent, Spalax ehrenbergi, in Israel. Females of the chromosome forms 2n = 52 and 2n = 58 were tested for male odour discrimination, the source of odour being either case litter or urine. Estrous females of both forms preferred homochromosomal odurs, whereas diestrous females showed no discrimination. These results suggest that olfactory discrimination may serve as a reproductive isolating mechanism in the speciation of mole rats.

Animals↗