Tracking the origin of the American colonization by Drosophila subobscura: genetic comparison between Eastern and Western Mediterranean populations

Several pieces of evidence indicate a Mediterranean origin of the colonization of America by Drosophila subobscura . To ascertain whether the origin was from the Eastern or the Western Mediterranean region, samples from Barcelona (Spain) and Mt Parnes (Greece) were collected and O chromosomal inversion polymorphism and lethal genes were analysed. The frequencies of lethal chromosomes were 0.244 ± 0.039 in Barcelona and 0.336 ± 0.043 in Mt Parnes, consistent with the expectations for large populations located in the central area of the species distribution. Lethal genes seem to be distributed at random along the O chromosome in both populations. The intra-populational allelism frequencies of Barcelona and Mt Parnes were 0.016 ± 0.007 and 0.012 ± 0.005 respectively. Thus, the estimates of the effective population size were high in both populations (between 6964 and 13 004 in Barcelona and 11 874 to 26 828 in Mt Parnes). The cases of allelism in Mt Parnes were observed only between individual lethal genes, but in Barcelona some concatenated clusters of allelism were detected. This pattern of allelism can be explained by synthetic lethality, hybrid dysgenesis, the induction of recurrent lethal mutations by different factors or an effect of microdifferentiation in subpopulations. In both populations, a reduction in fitness in the heterozygotes for lethal genes has been detected. Furthermore, the estimates of the migration coefficient (between 0.0085 and 0.0120 in Barcelona, and 0.0057 and 0.0087 in Mt Parnes) confirm the existence of gene flow between Palearctic populations of D. subobscura . Our lethal genes and chromosomal inversion results are consistent with a Mediterranean origin of the colonization of America by D. subobscura , but are inconclusive with regard to the identification of the population from which these colonizers came.     

Colonization of America by Drosophila subobscura: spatial and temporal lethal-gene allelism

About twenty years ago Drosophila subobscura, a western Palearctic species, colonized both North and South America. Lethal genes in the O chromosome has been subject to much research. Lethal gene allelisms between American populations far away have been studied. These allelisms were not negligible, but all cases were due to the lethal gene completely associated to the O5 chromosomal inversion. Here we analyze the lethal genes in a new American population of D. subobscura (Centralia, Washington), located fairly close to a previously studied population (Bellingham, Washington) and separated in space and time with other American populations (Gilroy I and II in California and Santiago de Chile). The frequencies of lethal and semilethal genes of Centralia were 16.9+/-4.6 and 6.2+/-3.0, respectively. The intrapopulational allelism of Centralia was 0.122+/-0.036. Interpopulational allelisms were studied using the lethal genes from the populations separated in space and time from Centralia. The interpopulational allelisms between Centralia and Gilroy I (California) and between Centralia and Bellingham (Washington) were higher than the intrapopulational allelism (0.155+/-0.032 and 0.153+/-0.024, respectively). In all these cases allelism was due to a complete association between a lethal gene and the O5 chromosomal inversion. Accordingly, no other lethal genes are shared in these populations.     

Lethal genes in O5, chromosomes of Drosophila subobscura from Europe and America

In populations of D. subobscura , a species that is know for its high chromosomal polymorphism, the O₅ inversion has a rather erratic frequency distribution in the Palearctic region. An O₅ lethal chromosomal line obtained from a Balkan population near Zanjic (South Adriatic, Montenegro, Yugoslavia) was tested for lethal allelism with other O₅ lethal chromosomal lines derived from American (USA and Chile) colonizing populations, and from the French population of Taulé. No allelism was found between the Balkan lethal gene and those from America and France. Thus, the lethal genes of the O₅ inversions are not of the same origin and it is most probable that the American colonizations did not start from the Zanjic population. The general difference in the chromosomal inversion polymorphism corroborates this conclusion. The cytological analysis confirms the assumption that all O₅ chromosomes studied are identical with respect to breakage points.     

COLONIZATION OF AMERICA BY DROSOPHILA SUBOBSCURA: LETHAL GENES AND THE PROBLEM OF THE O5 INVERSION

In this work, the process of colonization of North and South America by the species Drosophila subobscura has been studied by analyzing the variability of lethal genes. The genetic structures of a Palearctic natural central population (Bordils, Spain) and a colonizer population from America (Gilroy, California) have been compared. The frequencies of lethal chromosomes and their allelism are 29.007% and 0.0069 in the first population and 14.414% and 0.0526 in the American population. A founder effect is detected after the computation of some population parameters (N_e, h, he and the lethal load). Furthermore, the allelism of lethal chromosomes has revealed a strong association between a lethal gene and the O₅ inversion both in Gilroy and in the population of Puerto Montt (Chile). The interpopulation allelism shows that the O₅ arrangement from the USA and Chile is the same, confirming that the colonizing processes of North and South America are correlated. The O₅ arrangement can also be useful as a genetic marker to trace the origin of the colonization. The frequency of the O₅ arrangement in the original population of the colonization could be used to estimate the number of colonizers. This population is still unknown, but taking the extreme values of the frequency of the O₅ inversion in natural Palearctic populations (1–15%), the number of colonizers could vary between 9 and 149 individuals.     

The genetic structure of Balkan populations of Drosophila subobscura

Although Drosophila subobscura has been a model organism for European and American population geneticists, little information is available on the genetic structure of its natural populations. In this paper we report the estimates of some population parameters. We have used data from lethal allelism tests in four Balkan populations (Kamariste, Djerdap and Petnica in Serbia and Zanjic in Montenegro). In all populations, lethal genes were found to have a deleterious effect on heterozygotes. The N(e) values varied greatly from 370 (Petnica) to 19413 (Kamariste), depending on the habitat conditions and some environmental factors. Finally, gene flow between the Balkan populations was detected by the estimates of N(m) (from 4.68 in Petnica to 106.2 in Kamariste) and m (from 0.0041 in Djerdap to 0.0126 in Petnica). These results agree with those obtained in a previous study where the frequencies of allelism between populations were greater than predicted by independently arising lethal genes.     

Genetic load and coadaptation of chromosomal inversions. II. O-chromosomes in Drosophila subobscura populations

We have analysed the inversion polymorphism and genetic load of O-chromosomes in three populations of D. subobscura from southeastern Europe. As expected for a central populations the inversion polymorphism was extensive. In a like fashion, the genetic load, in particular the frequency of lethals, was heavy in all three populations. There were significant differences in the frequency of moderately deleterious genes. These differences in viability can be attributed to balancing selection. A comparison of these two kinds of genetic polymorphism indicates that there are differences in mean viability among different gene arrangements of O-chromosomes in the three populations. The differences observed are due to an unequal distribution of various viability classes among O-chromosome gene arrangements. We here show for the first time a specific distribution of lethal genes among these arrangements within the Palearctic distribution area of D. subobscura. The lethal allelism test showed lethals are non-randomly associated with the Ost gene arrangement. The amount of genetic load is heavy in gene arrangements with a high frequency, in comparison with the ones with a low frequency. Lethal genes may be protected in combinations of low and moderate frequency gene arrangements that harbor more lethal genes, as the Ost in the one population. Some arrangements that are less protected against recombination have a higher load than ones that are more protected against recombination. This can be taken as evidence for coadaptation.     

Genetic load and viability variation in korean natural populations ofDrosophila melanogaster

Summary Natural populations of Drosophila melanogaster from Anyang and Susac (suburbs of Seoul) have been analyzed with respect to viability variation on the second chromosome. Homozygotes as well as random heterozygotes for wild chromosomes were studied. The frequency of lethal factors was about 16 per cent, that of drastics 26 per cent. The average viability of homozygotes was 0.650 including lethal lines and 0.858 for quasinormals; that for random heterozygotes was 1.125. Allelism tests have been performed for the lethals. The allelism rate turned out to be as high as 0.036 and 0.0214, respectively. Using a formula by Nei, the effective population size can be estimated from these data. Korean D. melanogaster populations proved as small as 2000 to 3000 individuals. No correlation between homozygous and heterozygous viabilities could be found. According to these observations, along with the fact that partly big clusters of identic lethals could be found in the allelism tests, it is concluded that in Korean populations quite a large part of the hard genetic load is balanced. The connection between population size, population structure and associative or genuine overdominance is discussed.     

Tracking the origin of an invasive species: Drosophila subobscura in Argentina

Biological invasions are excellent opportunities to study the evolutionary forces leading to the adaptation of a species to a new habitat. Knowledge of the introduction history of colonizing species helps tracking colonizing routes and assists in defining management strategies for invasive species. The Palearctic species Drosophila subobscura is a good model organism for tracking colonizations since it was detected in Chile and western North America three decades ago and later on in the Atlantic coast of Argentina. To unravel the origin of the Argentinean colonizers two populations have been analysed with several genetic markers. Chromosomal arrangements and microsatellite alleles found in Argentina are almost similar to those observed in Chile and USA. The lethal allelism test demonstrates that the lethal gene associated with the O₅ inversions in Argentina is identical to that found in Chile and USA, strongly supporting the hypothesis that all the American colonizing populations originated from the same colonization event. A secondary bottleneck is detected in the Argentinean populations and the genetic markers suggest that these populations originated from the invasion of 80–150 founding individuals from Chile.     

Clan Lethals and Inter-Year Allelism in Death Valley DROSOPHILA PSEUDOOBSCURA

A chromosome 2 lethal allelism rate of about 3% was found in the 1974 population of D. pseudoobscura in Death Valley, California. This rate was significantly higher than allelism rates in other Southern California populations. The Death Valley population was sampled again in 1975 and 1977, with allelism rates of 1% and 0.5%, respectively. In 1974, several lethals were in high frequencies (about 1%), a pattern that reappeared in 1975 and 1977. However, none of the lethals in high frequency one year were in high frequency another year; the particular lethal alleles present in this ephemeral population appear to be due to their random presence in the flies which refound the population every winter. The results for the Death Valley population are compared with a Japanese population of D. melanogaster in which lethals in high frequency one year are also in high frequency in succeeding years and with earlier work on chromosome 3 of D. pseudoobscura, which showed a lower lethal frequency and higher allelism rate.     

Gene Flow in Natural Populations of DROSOPHILA MELANOGASTER with Special Reference to Lethal Allelism Rates and Protein Variation

A simultaneous survey of 14 protein loci, together with frequencies and within- and between-population allelism rates of lethal chromosomes, was carried out in five (four Japanese and one Korean) natural populations and one cage population of Drosophila melanogaster. It was found that lethal allelism rates decrease rapidly as geographic distance between two populations increases, while variation at protein loci shows a remarkable similarity over all populations examined. These findings suggest that there are very high levels of gene flow in these natural populations and that selection at protein loci which can maintain substantial geographic variation, if present, is overshadowed by gene flow. There is no indication that invasion of D. melanogaster to the Far East occurred so recently that the frequencies of lethal chromosomes are still in nonequilibrium.     

The distribution of lethal allelism in finite populations

It is shown that, under certain conditions, the distribution of lethal allelism in equilibrium populations is independent of the fitness of heterozygotes. For these conditions, an expression for the over-generation variance of allelism is given.     

Low Diversity of T Haplotypes in the Eastern Form of the House Mouse, Mus Musculus L

In previous studies, 13 different recessive embryonic lethal genes have been associated with t haplotypes in the wild mice of the species Mus domesticus. In this communication we have analyzed five populations of Mus musculus for the presence and identity of t haplotypes. The populations occupy geographically distant regions in the Soviet Union: Altai Mountains, western and eastern Siberia, Azerbaijan and Turkmenistan. No t haplotypes were found in mice from eastern Siberia. In the remaining four populations, t haplotypes occurred with frequencies ranging from 0.07 to 0.21. All the t haplotypes extracted from these populations and analyzed by the genetic complementation test were shown to carry the same lethal gene tcl-w73. In one population (that of western Siberia), another lethal gene (tcl-w5) was found to be present on the same chromosome as tcl-w73. This situation is in striking contrast to that found in the populations of the western form of the house mouse, M. domesticus. In the latter species, tcl-w73 has not been found at all and the different populations are characterized by the presence of several different lethal genes. The low diversity of t haplotypes in M. musculus is consistent with lower genetic variability of other traits and indicates a different origin and speciation mode compared to M. domesticus. Serological typing for H-2 antigenic determinants suggests that most, if not all, of the newly described t haplotypes might have arisen by recombination of tw73 from M. musculus with t haplotypes from M. domesticus either in the hybrid zone between the two species or in regions where the two species mixed accidentally.     

Simultaneous Spawning of Six Species of Echinoderms in Barkley Sound,British Columbia

Summary

Six species of echinoderms were observed spawning (1 holothuroid, 1 echinoid, 4 asteroids) during a single 40 min dive to 10 to 18 m depth in Barkley Sound, Vancouver Island, on the afternoon of 9 August 1987, the day after the full moon. Most of the spawning animals were not close to conspecific individuals. None were observed spawning the following day. Published reports indicate that it is not uncommon for widely scattered individuals of echinoderms to broadcast spawn in the field. Fertilization success often may be low. Moreover, simultaneous spawning by different species, as we and others have observed, indicates that the incidence of hybridization (either lethal or viable) could be high and have important consequences to the biology of the species involved.     

The Frequency and Allelism of Lethal Chromosomes in Isolated Desert Populations of DROSOPHILA PSEUDOOBSCURA

Second-chromosome lethals were extracted from four populations of Drosophila pseudoobscura in Southern California. Two of the populations were from desert oases and two from the classic habitat on Mt. San Jacinto, previously studied by Dobzhansky. Allelism tests were made on the lethals within and between all locations. The frequency of lethal second-chromosomes in each location was 0.18, and this was not different from the results of other workers for samples throughout the species range. Interpopulational allelism rates were about 0.005, and not different from earlier results of Dobzhansky. Intrapopulational rates in this study were, with one exception, the same as the interpopulational rates, and significantly lower than Dobzhansky found using the third chromosome. This may be due to lethals being linked with heterotic third-chromosome inversions. The allelism rate of the exceptional population (about 0.03 and equal to Dobzhansky''s intrapopulational results) may be due to heterotic lethals, or a founder effect. Two lethals were found in three populations each, possibly due to migration among these populations, which are up to 334 km apart.     

Genetic Evidence of the Contribution of Ethnic Migrations to the Propagation and Persistence of the Rare and Declining Scrambling Shrub Caesalpinia bonduc L

This paper examines the contribution of human migrations to the propagation and maintenance of Caesalpina bonduc by means of an analysis of its population genetics and distribution patterns. One hundred and forty seven sites were surveyed in the three climatic zones of Benin and all individuals of the species were recorded. A set of individuals was randomly selected and sampled from seven populations and morphological variation and genetic diversity were assessed. The study confirmed the presence of the species in all climatic zones but its abundance varied greatly. Morphological variability between populations and zones was low in comparison with the high amount of variation within populations. AFLP and cpDNA fingerprinting revealed an extremely low genetic diversity within populations and a low genetic differentiation, suggesting parental links between populations. The results support the hypothesis of human involvement in Caesalpinia dispersal and persistence in Benin. However, the low genetic diversity may imply high risks for future extinction. We recommend that gene flow among the remaining populations be supported in order to conserve the species.     

Failure to detect simian immunodeficiency virus infection in a large Cameroonian cohort with high non-human primate exposure

Hunting and butchering of wildlife in Central Africa are known risk factors for a variety of human diseases, including HIV/AIDS. Due to the high incidence of human exposure to body fluids of non-human primates, the significant prevalence of simian immunodeficiency virus (SIV) in non-human primates, and hunting/butchering associated cross-species transmission of other retroviruses in Central Africa, it is possible that SIV is actively transmitted to humans from primate species other than mangabeys, chimpanzees, and/or gorillas. We evaluated SIV transmission to humans by screening 2,436 individuals that hunt and butcher non-human primates, a population in which simian foamy virus and simian T-lymphotropic virus were previously detected. We identified 23 individuals with high seroreactivity to SIV. Nucleic acid sequences of SIV genes could not be detected, suggesting that SIV infection in humans could occur at a lower frequency than infections with other retroviruses, including simian foamy virus and simian T-lymphotropic virus. Additional studies on human populations at risk for non-human primate zoonosis are necessary to determine whether these results are due to viral/host characteristics or are indicative of low SIV prevalence in primate species consumed as bushmeat as compared to other retroviruses in Cameroon.     

Viabilities of originally natural O-chromosomal inversion homo- and heterokaryotypes in Drosophila subobscura

The influence of epistatic interactions of lethal and non-lethal genes upon viability of Drosophila inversion karyotypes is poorly known. In this paper we present comparative results for viabilities of 21 originally natural O-inversion homo- and 38 heterokaryotypes in a D. subobscura population. We observed strong heterotic effect in viability of O-lethal heterozygotes irrespective of different inversion backgrounds, which indicates a mechanism for protection of a great number of lethal genes. In O-non-lethal heterozygotes the heterotic effect in viability was absent irrespective of different inversion backgrounds. Our results showed a great extent of genetic load and high abundance of O-chromosomal arrangements in the population analyzed. It belongs to the set of central European populations. An epistasis of lethal genes present in O-inversion hetero- and not present in O-inversion homokaryotypes of moderate or low frequencies could be good example for co-adaptation of chromosomal inversions with regard to the genetic load. This represents a more efficient mechanism for limitation of genetic load than alternative mechanisms for protection of lethals. Except for lethal genes, possible epistatic interactions of mildly deleterious (subvital) genes, could also be responsible for limiting the extent of genetic load in natural D. subobscura populations. We demonstrated a non-random distribution of several combinations of viability classes among different O-inversion homo- and heterokaryotypes. As a consequence of that, the viabilities of the O-inversion homokaryotypes compared to heterokaryotypes were significantly higher at low frequencies than in moderate or high frequencies. This shows frequency-dependence as a mechanism of balancing selection for protection of O-chromosomal inversions in natural D. subobscura populations. In addition, the viabilities of the O-inversion homokaryotypes of lower frequency, compared to homokaryotypes of moderate or high frequency, were significantly higher. This again indicates the existence of supergene selection as another mechanism for protection of chromosomal inversions, as co-adapted complexes in natural D. subobscura populations.     

Genetic Basis of Self-Incompatibility in the Lichen-Forming Fungus Lobaria pulmonaria and Skewed Frequency Distribution of Mating-Type Idiomorphs: Implications for Conservation

Fungal populations that reproduce sexually are likely to be genetically more diverse and have a higher adaptive potential than asexually reproducing populations. Mating systems of fungal species can be self-incompatible, requiring the presence of isolates of different mating-type genes for sexual reproduction to occur, or self-compatible, requiring only one. Understanding the distribution of mating-type genes in populations can help to assess the potential of self-incompatible species to reproduce sexually. In the locally threatened epiphytic lichen-forming fungus Lobaria pulmonaria (L.) Hoffm., low frequency of sexual reproduction is likely to limit the potential of populations to adapt to changing environmental conditions. Our study provides direct evidence of self-incompatibility (heterothallism) in L. pulmonaria. It can thus be hypothesized that sexual reproduction in small populations might be limited by an unbalanced distribution of mating-type genes. We therefore assessed neutral genetic diversity (using microsatellites) and mating-type ratio in 27 lichen populations (933 individuals). We found significant differences in the frequency of the two mating types in 13 populations, indicating a lower likelihood of sexual reproduction in these populations. This suggests that conservation translocation activities aiming at maximizing genetic heterogeneity in threatened and declining populations should take into account not only presence of fruiting bodies in transplanted individuals, but also the identity and balanced representation of mating-type genes.     

Incidence of diapause varies among populations of <Emphasis Type="Italic">Daphnia pulicaria</Emphasis>

Dormancy is a common way in which organisms survive environmental conditions that would be lethal to the active individual. However, while dormant, individuals forgo reproduction. Hence theory suggests an optimal time in which to enter dormancy, depending on risks associated with both remaining active and entering dormancy. When these relative risks differ among habitats, dormancy strategies are predicted to vary as well. For freshwater zooplankton, it has been suggested that sensitivity to the cues that initiate dormancy should be selected against when females have the opportunity to remain in the water column year round. We tested this prediction with 12 populations of lake-dwelling Daphnia pulicaria (Crustacea: Cladocera). Differences among lakes in basin morphometry, predators and resources create a gradient of risk for Daphnia in the water column. Some populations persist in high numbers year round while others are abundant only in spring. We used this difference in persistence ability as an estimate of risk in the water-column. For 3 years of field sampling we found consistent differences among the lake populations in the incidence of dormancy. In some populations, only a small fraction of females switched to producing dormant eggs each year whereas in others the majority of eggs produced in the late spring were dormant. In general, populations that experienced predictably low abundances in the active form exhibited higher incidence of dormancy than did populations that persisted in high abundance year round, but there were exceptions. Our results confirm that the incidence of dormancy varies considerably among populations in a fashion consistent with general theory, but suggest that persistence in the water column is not the sole predictor of the diapause strategy found in any particular lake.     

Modeling the eradication of invasive mammals using the sterile male technique

Large vertebrates, like the domestic goat (Capra hircus), have been transported all over the world and are an ecological disaster to numerous island and mainland ecosystems. Eradication measures for such species are generally centered on lethal methods of removing individuals, an increasingly difficult process as populations become smaller and individual animals become much more difficult to detect. In addition, methods of lethal removal are becoming less desirable in the public eye, prompting the necessity to explore alternatives. Here we investigate the use of the sterile males technique as an effective strategy in the eradication of large mammals. The results of our simulations suggest that the use of sterile males as a single strategy would only be an effective measure to eradicate relatively small (no more than 100 individuals) isolated feral vertebrate populations. However, our results indicate that the technique could be employed as a successful and potentially cost-effective end-point complement to lethal control and/or as a preventative measure against re-invasion.