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GENETICS OF NATURAL POPULATIONS. XXVIII. SUPPLEMENTARY DATA ON THE CHROMOSOMAL POLYMORPHISM IN DROSOPHILA WILLISTONI IN ITS RELATION TO THE ENVIRONMENT
... Natural populations of Drosophila are composed primarily of heterozygotes. Consequently, selection must act on genes in heterozygous condition (Dobzhansky, 1952Dobzhansky, , 1955Dobzhansky, , 1959Mather, 1953Mather, , 1955Mayr, 1955Mayr, ,1959Wallace, 1956Wallace, , 1962; and others). Changes in the amount of concealed variability therefore reflect changes in the relative frequencies of different types of heterozygotes. ...
... Studies of salivary gland chromosomes have uncovered a large number of inversion polymorphisms (Dobzhansky, 1950;da Cunha et al., 1953;Kastritsis, 1969). The relationships between the chromosomal polymorphisms and certain ecological parameters have also been the subject of intensive studies (daCunha et al., 1959, and references therein). Drosophila willistoni and its relatives are excellent materials to study genetic differentiation in the process of speciation. ...
An important center in which genetic research started and was carried out in Brazil during the 20 th century was situated at the Faculty of Philosophy, Sciences and Linguistics of the University of São Paulo, led by André Dreyfus (1897–1952). Beginning in 1943, the Ukrainian geneticist Theodosius Dobzhansky (1900–1975) visited Dreyfus’s group four times. This paper evaluates the impact of Dobzhansky’s visits on the studies of genetics and evolution developed by the members of Dreyfus’s group during the 1940s and the 1950s. The study leads to the conclusion that Dobzhansky’s visits had an impact, not only in quantitative terms (the number of individual and joint publications), but also in qualitative terms. However, we also detect a decrease in the number of individual and joint publications related to the subject of the project during certain periods. The adoption of new experimental organisms by some members of the group; the involvement with subjects not related to the initial project, such as botany; Dobzhansky’s and his wife’s health problems during the third visit; and scientific disagreements between Dobzhansky and Brazilian researchers may have contributed to the decrease in publications.
On the one hand, much has been written on Theodosius Dobzhansky’s central role in the development of the field of population genetics and modern evolutionary theory, as well as on his sociopolitical worldview in the middle of the Twentieth Century. On the other hand, much has also been written on Dobzhansky’s role in the institutionalization of genetics in Brazil, where he spent a considerable amount of time. Unfortunately, these literatures developed without any points of intersection or cross-reference. This article places Dobzhansky’s work in Brazil in the broader contexts of the science and politics of its historical period.
We compare the dynamics of artificial populations of the annual Phlox drummondii introduced along a transect extending from within to beyond the species margin southwest of San Antonio, Texas. The margin is defined by a shift from sandy, nutrient poor soils to clay, nutrient-rich soils. Fourteen sites were chosen along the 6.5-mi transect. In each site seeds were planted into 1.5-m-square cleared plots and uncleared 10-m transects. Transplants also were introduced into cleared plots. Survival, flower production and seed set were monitored over the course of the growing season. Individual fitnesses and the reproductive rates of artificial populations generally declined as the distance from the margin increased. The results indicate soil type per se is not limiting Phlox and that plants can successfully grow and reproduce, albeit at a reduced level, in sites beyond their natural margin.
A model which visualizes a species as a set of biotype subsets is useful when considering the degree of fitness and homogeneity of populations in their environment. Under founder principle concepts we expect that the number of such subsets tends to decrease as local populations encounter new selective forces in environments and habitats other than those occupied by predecessor populations. Relations between biotypes, considered from the point of view of “variation associated with character correlation” or correlated response and polygene systems, imply that the magnitude of product-moment correlations between characters can be used as a general measure of selective stress impinging upon populations. Magnitude is defined as the per cent of the total possible pairs of characters whose correlation coefficients are significantly different from zero. The direction of magnitude expected under the founder principle is from low in an undisturbed or ancestral situation to high in an alien one. This hypothesis is demonstrated in the genus Froelichia (AMARANTHACEAE).
Six ecologically central (old field) and marginal (strip mine) populations of the hexaploid Rumex acetosella were collected, grown under uniform conditions, and examined for genetic and morphological variation. Extensive electrophoretic variation was found in alcohol dehydrogenase and phosphoglucose isomerase, while other enzymes surveyed showed little or no variation. Hedrick's genotypic measure of identity revealed mean values of 0.506 for central populations, 0.836 for marginal populations, and 0.633 for comparisons of central with marginal populations. Alcohol dehydrogenase phenotypes had significantly fewer electromorphs per individual in marginal populations. Clones of individuals from both environments were subjected to different watering regimes. No significant differences in root/shoot ratio, leaf number, total leaf area, or relative growth rate were found between strip mine and old-field individuals within each watering treatment, although significant differences were found between watering treatments. There are small, but significant amounts of isozyme differentiation between central and marginal populations, while there was no such differentation for morphological characters.
Theodosius Grigorievich Dobzhansky1 (Феодосий Григорьевич Добржанский) was born on January 25, 1900, in the town of Nemirov, which is located about 130 miles (200 km) southeast of Kiev in the Ukraine, and which at that time had a population of about 5,000. He was the only child of the former Sophia Voinarsky and Grigory Dobrzhansky, a teacher of mathematics. After his graduation from secondary school in Kiev, Dobzhansky majored in biology at the University of Kiev, from which he was graduated in 1921. He remained in Kiev at the Institute of Agriculture until 1924. He then moved to Leningrad, where he was a Lecturer in Genetics at the University from 1924 to 1927. While at this post, in 1926 he led an expedition to Central Asia to study domesticated animals.
The six sibling species of the Neotropical Drosophila willistoni group have a long history in studies of evolutionary biology, yet to date only one molecular study, which used allozymes, has been published on the phylogeny of the group. Here we present a phylogeny of the siblings based on the sequences of two nuclear genes, period (per) and Alcohol dehydrogenase (Adh), as well as the mitochondrial gene Cytochrome oxidase I (COI). Taken individually, only per has a strong phylogenetic signal supporting a well-resolved phylogeny of the group, and this phylogeny is different from that obtained using allozymes. The COI dataset by itself produces trees that disagree with per, and neither that data nor the Adh data have a strong phylogenetic signal, as indicated by low bootstrap values for all analyses. Combining the Adh and COI datasets results in the same tree as per alone. Combining all three genes results in the same topology, which is strongly supported. Two problematic taxa, D. pavlovskiana and a "Carmody strain," which were identified as potentially separate species based on reproductive isolation, clearly cluster in the phylogenetic analyses within D. paulistorum and D. equinoxialis, respectively. Thus, there appears to be a conflict between the biological species concept and the phylogenetic species concept.
Identifying cryptic species is a challenging but critical task for conservation biology because such identifications improve the resolution of taxonomic inventories and increase our ability to precisely assess the biodiversity of environments as well as the conservation status of rare species. To understand the temporal and spatial distribution of the cryptic species contained in the Drosophila willistoni subgroup, we re-examined all of the individuals of this subgroup identified in a previous work that analyzed drosophilids associated with a forest patch in the Brazilian Cerrado from December 2007 to November 2008. Each collection consisted of banana-baited traps distributed in three vertical strata of the forest (0-, 4-, and 8-m heights) at three horizontal positions (edge, transition between edge and river, and river area). Our results revealed that the specimens originally identified as D. willistoni consisted of three cryptic species: D. willistoni (n=1,344), D. paulistorum (n=1,001), and D. tropicalis (one male). Most D. willistoni and D. paulistorum specimens were collected during the rainy season on the ground of the edge forest patch. However, D. paulistorum was more temporally and spatially restricted than D. willistoni. Our findings confirm that these three cryptic species of the subgroup D. willistoni differ not only in abundance but also in ecological tolerance. This study contributes to the improved understanding of the Brazilian savanna communities, whose high levels of species richness and endemism are well documented for plants but poorly known for insects.
In the present study, we investigated chromosomal polymorphism for paracentric inversions found in Drosophila polymorpha populations common in five fly communities (one continental and four insular) collected in Atlantic Forest remnants in Santa Catarina, Brazil, during a one-year sampling. Our analyses showed that the more polymorphic the populations, the greater the abundance of flies. The insular populations were more polymorphic than the continental one, probably because the greater environmental heterogeneity found in the islands provided more niches for fly colonisation and was more suitable for supporting a community. Stochastic effects, however, need to be taken into account to understand the low polymorphism found at the continental site, since the D. polymorpha sample collected there was small. We also present here the occurrence of six previously undescribed paracentric chromosomal inversions in heterozygosis, increasing to 7 the number of variants identified in the D. polymorpha polytene chromosomes.
Phenotypic variation is as poorly understood as it is ubiquitous. Many biologists hold simplistic and even contradictory views of the relation between genic diversity and phenotypic variation. The correlation of electrophoretically detectable enzyme polymorphisms and morphological variation in island populations of lizards is described. A model explaining the different levels of genetic-phenetic variation among the populations is suggested by a multiple regression analysis. The principal conclusions are that gene flow between habitats significantly elevates variation, and isolation by distance results in a variation asymptote.
The male meiotic chromosomes of Drosophila willistoni are described and compared to Giemsa stained mitotic chromosomes. Meiosis was examined in males from two wild type stocks: WIP-4, an almost monomorphic stock, and 17A2, a high-inversion polymorphic stock. Hybrids between these stocks were also examined. Stocks and hybrids were cultured at 25ºC and at 29ºC, a temperature condition thought to induce male recombination associated with transposable element(s) mobilisation. Chromatin in primary spermatocytes is seen as pachytene and early diplotene bivalents, representing the two autosomal bivalents and a steady association between X and Y. Bivalents remain relatively extended from diplotene through anaphase I, which is followed by the conventional stages of meiosis II. Neither chiasmata nor chromatid bridges and fragments were detected in both control and experimental males. Thread-like bubble structures on the distal region of the X chromosome were observed in early diplotene cells from F1 male offspring between WIP-4 and 17A2 reared at 29ºC; their functional meaning is unknown.
A survey of a natural population of Anopheles cruzii from Aldeia dos Indios, São Paulo, Brazil, has disclosed the existence of a highly polymorphic genetic system characterized by the presence of 14 independent paracentric inversions and at least other five inversions belonging to two complexes. Most of the inversions have heterozygous frequencies higher than 25%. The mean number of inversions per individual was estimated as 5.9. Besides the two complexes, where the inversions were completely linked, four intrachromosomal and six interchromosomal nonrandom inversion associations were observed. In six of these nonrandom associations the 3Rd inversion was present. The presence of a highly polymorphic inversion system in this population is discussed, taking into account the variety of habitats explored by this species.
A first case of polymorphism of a pericentric inversion on X-chromosome was detected in six wild populations of Drosophila willistoni from islands and the mainland of Santa Catarina State, southern Brazil. The high representativeness of this inversion, with frequencies between 10 to 42 % in samples of wild populations from Santa Catarina State, suggests that it could be adaptive and that the establishment of such polymorphism was probably due to crossing over restriction in the chromosomal region involved in the rearrangement.
1.
Data are given on the quality and quantity of the chromosomal polymorphism in a population ofDrosophila subobscura from Dröbak (Norway).
2.
The Norwegian population shows the characteristics of a marginal population. Compared with the Middle- and South-European populations the chromosomal variability is considerably reduced.
3.
An apparently high number of females and males of the Norwegian wild population did not produce offspring in the laboratory.
4.
Intercrosses of different strains ofDrosophila subobscura from Norway and from various other localities were made. All different groups of intercrosses produced fertile F1 progenies. However, the frequency of successful single pair crosses was significantly lower if males from the Norwegian strain were used. It may be assumed to be due to the low developmental homeostasis of the Norwegian strain not adapted to the laboratory environments.
We record the occurrence of male recombination, detected in cytological preparations of testes in 2 Drosophila willistoni population samples. Comparative analysis of meiotic figures in imaginal discs of testes and the salivary gland polytene chromosomes of male third instar larvae were employed. We observed anaphase bridges, suggesting the occurrence of chiasmata and fragmented chromatids involving the second chromosomal pair and the heterozygous inversions IILF and IILD+E segregating in the same individuals. This was observed in non-stressed larvae maintained at physiological temperature, and opens a wide field to study the factors that regulate crossing-over in natural populations of the highly polymorphic D. willistoni. We also observed what appears to be crossing-over in the tip of chromosomal arm IIL, in a sole male heterozygote for the inversion IILH of the G3 natural population.
Natural populations of the blackfly Simulium vittatum (Zett.) show a high degree of inversion polymorphism. Except for seven different two-step overlapping sequences in the short arm of chromosome I, all inversions are simple paracentrics. In all, 134 different arrangements have been found so far. A small inversion (IIIL-1) that is restricted to the Y chromosome is present in some populations from Ontario, New York, and Wisconsin.In the populations studied, there is no sibling speciation. Hardy-Weinberg tests of 226 inversions showed seven inversions that are not in equilibrium. Tests for intra-arm associations revealed that a few inversions in some chromosome arms are carried in the repulsion or coupled position to excess. It was demonstrated that non-random associations exist between certain inversions on different chromosomes. The inversion limits appear in clusters along chromosome arms IS, IL, IIL, and IIIL. The frequencies and distribution of all the inversions are presented in an appendix. The discussion compares these observations with the results from other dipteran inversion studies.
An official journal of the Genetics Society, Heredity publishes high-quality articles describing original research and theoretical insights in all areas of genetics. Research papers are complimented by News & Commentary articles and reviews, keeping researchers and students abreast of hot topics in the field.
Natural populations of Drosophila willistoni collected every three months over banana baits between September, 1978 and May, 1982 at Parque de Itapuã and Parque do Turvo, Rio Grande do Sul, Brazil, localised in different regions in terms of climate, flora and fauna, were studied with respect to the variation of their chromosomal polymorphism associated with meteorological variables and population fluctuations.
Inhalt: Einleiung S. 542. — 2. Methode S. 543. — Erklrung von Ausdrücken S. 543. — 3. Qualitative geographische Variation: a) Die Elemente e und E S. 545. — b) Die Elemente D, F, A und B S. 546. — c) Das Element C und die Tonhöhenfolge der Elemente a', a, A, B und C S. 546. — 4. Quantitative Variation: a) Lnge der Strophen S. 548. — b) Zahl der Elemente S. 550. — c) Zahl der Zacken der Elemente E und F S. 550. — d) Verschiedene Hufigkeit eines Elementtyps S. 550. — 5. Vergleich mit der geographischen Variation morphologischer Merkmale des Gartenbaumlufers S. 550. — 6. Zusammenfassung der Befunde S. 551. — 7. Summary of findings S. 552. — 8. Erörterung: a) die mosaikartige Verbreitung S. 553. — b) die gröz̀ere Variation im Süden S. 556. — c) die gröz̀ere Variation im Norden S. 556. — d) Die Unterschiede der Strophenlnge usw. S. 558. — e) Die verschiedene Hufigkeit eines Elementtyps S. 558. — f) Verschiedene Ausbildungen in groz̀en Rumen mit schmalen Mischzonen S. 559. — g) Die Rolle von Angeborenem und Erlerntem für das Zustandekommen der Verbreitungsmuster S. 560. — h) Die Entstehung von Isolationsmechanismen S. 560. — 9. Zusammenfassung der Erörterung S. 563. — 10. Summary of the discussion S. 564. — 11. Literatur S. 565.
The chromosomal polymorphism in three Danish lake populations of the midge Chironomus plumosus L. was followed during a period of nearly three years. The populations exhibited differences in the composition of the chromosomal polymorphism and in the level of heterozygosity (= number of inversion heterozygotes per individual). The frequencies of larvae with a certain paracentric inversion located in the largest of the chromosomes showed remarkable differences between the three populations. In one of the lakes, in addition to a large frequency of larvae with this inversion, specimens heterozygous for the inversion were in great excess compared to the Hardy-Weinberg distribution. The frequency of the heterozygote varied during the year. The adjustments of the chromosomal polymorphism to the environments are discussed.
Genetic distances (D's) within and between four Daphnia species were estimated and the genetic structures of the species were compared from frequencies of protein electromorphs at 7 loci. The average withinspecies distances varied from 0.067 in D. pulex to 0.282 in D. magna. The D¯ values for the between-species comparisons were about 0.3 (0.271-0.373) except for the species pair D. longispinal D. pulex, which had a D¯ value of only 0.101. The most striking aspect of the data was the high degree of genetic differentiation within Daphnia species. The very highest value was found in D. magna, which mainly inhabits small temporary ponds. The level of heterozygosity proved lowest in populations inhabiting northern marginal habitats. The effects of cyclical parthenogenesis on the genetic structure are discussed.
Heterozygosity for chromosomal arrangements was investigated in four species of the nasuta complex. D. sulfurigaster albostrigata, D. albomicans and D. kohkoa are highly polymorphic whereas D. pulaua is monomorphic. Inversions were identified with the aid of photographic chromosome maps. The geographic distribution and frequencies of inversions detected and their possible phylogenies were discussed. Most inversions in D.s. albostrigata and D. albomicans occur on chromosome IIL. In D. kohkoa there is marked preponderance of inversions on chromosome III. Non-random association of certain inversions was noted. Variation in frequencies of inversions is interpreted as a result of adaptation to local ecogeographic conditions. Shared extant polymorphisms indicate phylogenetic relationships between species.
By means of the male sample method samples of D. subobscura from the north and south coast of Asia Minor and from the south coast of the Caspian Sea were examined with regard to the gene arrangements contained in them. The samples were judged by the following three criteria: (1) the number of different gene arrangements present, (2) the frequencies in which these gene arrangements occur, and (3) the rate of zygotic combinations.
The samples from Asia Minor were found to be highly polymorphic. In this respect, they are comparable with other samples from the northern border of the Mediterranean region.
The Persian populations are poor as regards number of gene arrangements. Commonly, some gene arrangements are highly frequent and, accordingly, high rates of homozygous combinations dominate. Presumably, these are marginal populations.
Comparisons between samples from the north and south coast of Asia Minor confirm the findings from other parts of the species' range. As a rule, the frequencies of various gene arrangements vary according to the latitude in which they occur. Since climate and vegetation are also largely correlated with latitude, it is assumed that this is due to adaptive reaction to ecological factors.
Comparing populations from the Western and Eastern Mediterranean region, similar differences were recorded. Some gene arrangements seem to be confined to either the western or eastern part of the area, and other gene arrangements differ in frequency. Since the ecological conditions in these areas appear to be comparable, such differences might be due to the history of the formation and dispersal of gene arrangements.
In general, the frequencies of the gene arrangements in the chromosomes A, U, E and O are most equal along the northern margin of the Mediterranean area and, correspondingly, the frequencies of the heterokaryotypes are highest. It may be assumed that many heterokaryotypes reach their highest degree of heterotic supremacy in this area, and/or that the different homokaryotypes are uniform with respect to adaptive values. The reasoning rests on the additional assumption that in this area the biological conditions are the least extreme, or, in other words, that D. subobscura is best adapted to this environment. As it is, the region appears to lie geographically near the center of the species' range. In the I-chromosome the most equal frequencies of gene arrangements along with the highest frequency of the heterokaryotypes are to be found in Central Europe.
On the other hand, in various populations lying supposedly at the borders of the species' range, the frequencies of gene arrangements of at least some chromosomes are strongly unequal and, accordingly, the frequencies of the homokaryotypes are high.
In populations where the polymorphism is high, the character of the polymorphism varies slightly from chromosome to chromosome. In the O-chromosome usually one gene arrangement is abundant, while a large number of others are rare; two gene arrangements tend to dominate in the I- and U-chromosome, whereas in the E-chromosome several gene arrangements are apt to share similar frequencies.
1.
Eine Population vonDrosophila subobscura der nordwestlich von Sizilien gelegenen Insel Ustica wurde hinsichtlich ihres chromosomalen Strukturpolymorphismus cytologisch analysiert und die Hufigkeit der verschiedenen Strukturtypen ermittelt.
2.
Es wurden die fr die Populationsstruktur charakteristischen Gren Heterozygotiegrad, Inversionsdichte und Index der freien Rekombination errechnet.
3.
Bei einem Vergleich der Insel Ustica mit den Inseln Ponza, Ventotene und Lipari und dem Festlandstandort Formia ergeben sich gesicherte Unterschiede in bezug auf Heterozygotiegrad und Inversionsdichte. Der Index der freien Rekombination hingegen ist auf allen Inseln annhernd gleich gro und nur dem Festland gegenber gesichert verschieden.
4.
In bezug auf die Anzahl der koexistierenden Strukturtypen konnten keine Unterschiede zwischen den untersuchten Insel-und Festlandpopulationen festgestellt werden. Es wurden daher die Unterschiede im Heterozygotiegrad und der Inversionsdichte auf eine unterschiedliche Hufigkeitsverteilung der Strukturtypen an den verschiedenen Standorten zurckgefhrt und die sich daraus ergebenden Probleme diskutiert.
The level of chromosomal inversion polymorphism is much higher in three populations from South Morocco than in the nearby Canary and Madeira Islands. The composition of the chromosomal arrangements is also strikingly different in both areas. The similarity among the populations from the islands excludes drift or the founder principle as an explanation of these differences. A variety of reasons support the conclusion that the polymorphism in the islands is a relict polymorphism, corresponding to a less evolved stage than the one present in the mainland. The populations from South Morocco have a relatively low index of free recombination, like others from the southern range of the distribution ofD. subobscura. This contrasts with the high values found in the northern range. The populations from the North are relatively recently established ones, whereas those from the South are longer established and have become marginal only in relatively recent times. This difference in evolutionary history is sufficient to explain the differences in the index of free recombination.
In natrlichen Populationen vonDrosophila subobscura
Coll. aus der Schweiz, Tanger, Trkei und Persien wurden 13 neue Strukturtypen gefunden. Davon gehen 7 auf neu beschriebene Inversionen zurck, 2 auf Duplikationen und 4 auf Rekombination bereits bekannter Inversionen.Es wird angenommen, da die aufgefundenen Duplikationen auf austauschbedingte Abspaltung aus Inversionskomplexen zurckgehen.An zwei frher beschriebenen Inversionen wurden Korrekturen in der Bruchstellenlokalisation angebracht.Der Bestand an bekannten natrlich vorkommenden Chromosomenstrukturen umfat zur Zeit 49 Inversionen, 2 Duplikationen und 61 Strukturtypen.Die bisherige Auffassung ber die nicht zufallsmige Verteilung der Bruchstellen wird durch das neue Material besttigt.
Rica, Panama, Colombia, Trinidad and Brazil. In summary, the results show that prosaltans is a polymorphic species due to the presence of 13 paracentric inversions, most of them occurring in chromosome arm XL. The evolutionary relationships ofprosaltans with the other 6 species in the same subgroup, established on cytological basis by Bicudo (1973b), suggest that this species is ancestral in the evolution of the saltans subgroup. In the present study additional data on chromosomal polymorphism of 15 strains and a survey of the data on chromosomal variability of prosaltans accumulated to now are presented.
Inversion polymorphism has been studied in three natural populations of Drosophila ananassae. The three cosmopolitan inversions have been detected in all the three populations analysed. Some quantitative variations in the inversion frequencies seem to exist and the level of inversion heterozygosity varies between the different populations.
Drosophila paulistorum
Dobzhansky
et
Pavan is a complex of six races or incipient species. The races are mostly allopatric, but they are reproductively isolated sufficiently to permit them to exist also sympatrically in some places. The gene arrangements in the chromosomes of the races have been compared by means of examination of the giant chromosomes in the larval salivary glands; 28 strains of all races, and about an equal number of interracial hybrids have been studied.Chromosomal inversion polymorphism has been discovered in all races, even in the Guianan race of which only a single strain is available. Inversion heterozygotes are found in every one of the five chromosomal strands which the species has. Interracial hybrids tend to be heterozygous for more inversions than are present in the strains of the parental races. The Transitional race has however much the same gene arrangements as the widespread Andean — South Brazilian race.With the exception of the Transitional race, and of three other possible exceptions, each race has a collection of its own race-specific inversion polymorphs, not found in the other races. This very striking finding is discussed in connection with the hypothesis which envisages the origin of new species from marginal colonies at the periphery of the geographic distribution area of the ancestral species.
Relative frequencies for heterozygous inversions in nine populations of D. nebulosa and six of D. willistoni were analyzed. The analysis corresponds to a grand total of 1828 arm chromosomes in which their genotype were determined, of them 404 correspond for each one of the two polymorphic chromosomes, X and III, of D. nebulosa and 204 per chromosome arm in D. willistoni. The more abundant inversions, according to their relative frequencies in D. nebulosa were the XL inversion with 7.9 % and inversion “A” in the third chromosome with 15.6 %, the remaining inversion found did not reach the ten percent. In the case of D. willistoni the more frequent inversions were for the XL arm inversions “A” and “D” with 13.7 and 12.2 percent respectively the remaining five did not reach a ten percent; in the XR arm the higher frequencies were for inversions “D” and “E” with 9.3 % each been the other four inversions with values less than five percent; in the IIL arm none of the seven inversions registered had values higher than six percent; inversion “E” in the IIR arm showed a frequency of 24.5 % and the five remaining barely reached a frequency of 5 %; among the 12 different inversions recorded in the third chromosome, only inversion “B” surpassed the ten percent in this case with 16.1 %. We must mention that the Standar sequence in all cases, for both species, always had high frequencies as shown in Tables II and III. The corresponding variability for this characteristic is for D. nebulosa 61.4 %.and 72.2% for D.willitoni. Concerning other parameter observed we have the average number of inversions per female in our case for D. nebulosa was 1.4 and for D. willistoni 3.6 which indicate the both species are marginal with respect to the distribution of this character in both species since in the central areas of distribution reach values up to nine inversions per female.
Rate of recombination is a powerful variable affecting several aspects of molecular variation and evolution. A nonrecombining portion of the genome of most Drosophila species, the "dot" chromosome or F element, exhibits very low levels of variation and unusual codon usage. One lineage of Drosophila, the willistoni/saltans groups, has the F element fused to a normally recombining E element. Here, we present polymorphism data for genes on the F element in two Drosophila willistoni and one D. insularis populations, genes previously studied in D. melanogaster. The D. willistoni populations were known to be very low in inversion polymorphism, thus minimizing the recombination suppression effect of inversions. We first confirmed, by in situ hybridization, that D. insularis has the same E + F fusion as D. willistoni, implying this was a monophyletic event. A clear gradient in codon usage exists along the willistoni F element, from the centromere distally to the fusion with E; estimates of recombination rates parallel this gradient and also indicate D. insularis has greater recombination than D. willistoni. In contrast to D. melanogaster, genes on the F element exhibit moderate levels of nucleotide polymorphism not distinguishable from two genes elsewhere in the genome. Although some linkage disequilibrium (LD) was detected between polymorphic sites within genes (generally <500 bp apart), no long-range LD between F element loci exists in the two willistoni group species. In general, the distribution of allele frequencies of F element genes display the typical pattern of expectations of neutral variation at equilibrium. These results are consistent with the hypothesis that recombination allows the accumulation of nucleotide variation as well as allows selection to act on synonymous codon usage. It is estimated that the fusion occurred ∼20 Mya and while the F element in the willistoni lineage has evolved "normal" levels and patterns of nucleotide variation, equilibrium may not have been reached for codon usage.
Drosophila paulistorum populations colonizing the urban area of Porto Alegre, southern Brazil, were studied with the objective of characterizing their chromosomal polymorphism in this new environment. Despite being geographically and ecologically marginal and the fact that the colonization of the urban area seems to be a recent event, the populations showed a large number of inversions on all chromosome arms. Differences regarding inversion frequencies and percentage of heterozygosis were found when we compared the samples with respect to geographical, microenvironmental and temporal aspects. Such differences, however, could be attributed to both selective and stochastic factors
Two species of the Drosophila nasuta subgroup of the Drosophila immigrans group, D. sulphurigaster albostrigata and D. nasuta albomicans were investigated in this study. Collections of both species were made from Phuket, Thailand. Both species have similar salivary chromosomes, with four autosomal arms and one sex chromosome arm, and both are highly polymorphic for paracentric inversions. D. s. albostrigata accounted for the majority of the isolines collected and exhibited the greater number of inversions. One inversion, C1, was common to both species, indicating common ancestry.
A non-random distribution of inversions was observed on the proximal end of chromosome II in both D. s. albostrigata and D. n. albomicans. An inter-collection comparison revealed that both rigid and flexible chromosomal polymorphism were operating in the two species, with a seasonal variation noted for one inversion in D. s. albostrigata. A non-random association of two inversions was observed in D. n. albomicans.
Based on a comparison of the indices of crossing over, both D. s. albostrigata and D. n. albomicans were found to be more heterozygous than in previous studies, with D. n. albomicans appearing to have evolved further than D. s. albostrigata.
Phylogenetic relationships within the Drosophila sulfurigaster species-complex, which belongs to the D. nasuta subgroup, were investigated on the basis of chromosomal constitution and morphology. D. pulaua is thought to be the most ancestral species, from which D. s. sulfurigaster and D. s. bilimbata derived in one branch and D. s. albostrigata and D. s. neonasuta in another branch.
Drosophila buzzatii feeds and breeds on the decaying cladodes and fruits of several species of Opuntia (prickly pear) which contain significant levels of ethanol and isopropanol. The potential influence of these two alcohols on the inversion polymorphism of the second and fourth chromosomes was investigated in ten experimental populations with different amounts of alcohol (either ethanol or isopropanol) added to the culture medium. All populations were started with the offspring of 29 wild females collected at Adeje (Tenerife, Canary Islands) and their genetic composition was monitored for about two years (more than 30 generations). Consistent changes in the frequency of most second- and fourth-chromosome arrangements occurred in all populations including those without alcohol (control). The comparison of inversion frequency through the various treatments revealed a significant influence of the alcohol on the frequency changes of the four second-chromosome arrangements. Moreover, this influence was of a different type for every one of them, sometimes with opposite effects between alcohols and/or concentrations. These results indicate genetic differentiation among second-chromosome arrangements with regard to alcohol and suggest that the alcohol heterogeneity found in the species' trophic niche may play an important role in the maintenance of this polymorphism and also in the recent historical changes in the frequency of some arrangements associated with colonization.
1. The structural heterozygosity in sixteen populations of D. subobscura from Central and Western Europe and from Israel is compared by the index of free recombination proposed by Carson (1955). This index is significantly lower in France and Portugal than in most Swiss populations. It is higher in Israel than in the populations of Europe. 2. The index of free recombination is compared with other measures of polymorphism and some of their biological implications are discussed.
The purpose of the present study has been to test further the working hypothesis that the genetic polymorphism in Mendelian populations enables them to exploit a greater variety of adaptive niches than could be within reach of genetically more uniform populations. The chromosomal variability has been used as an index of the degree of genetic polymorphism in populations of Drosophila willistoni. Samples of natural populations of D. willistoni have been collected in four regions in South America, for which expectations of the degrees of polymorphism to be found in the populations of the species in question were formulated on the basis of the above working hypothesis and of data secured earlier (by da Cunha, Burla, and Dobzhansky, 1950). The expectations were confirmed in three regions, and contradicted in the fourth. The diversity of adaptive niches occupied by D. willistoni in different regions has been evaluated in quasi-quantitative terms, following methods suggested by Dansereau (1952). The values obtained show a positive correlation with the degrees of polymorphism in the populations of the fly (figs. 2 and 3). This correlation would have been more pronounced than it is if it were not for the populations which inhabit the tropical coastal forests in the State of Bahia and adajacent territories. These exceptional populations are chromosomally more uniform than expected in the rich and diversified environments in which they live.
Natural populations of Drosophila willistoni contain many individuals heterozygous for inverted sections in their chromosomes. At least 40 different inversions have thus far been recorded; inversions occur in every chromosome and chromosome limb; one individual has been found heterozygous for 16 inversions. Most of the inversions are very widespread geographically. Nevertheless, the incidence of certain gene arrangements is quite different in different parts of the geographic range of the species. Every population can be described in terms of the relative frequencies of the gene arrangements it carries, and different populations are racially distinct. The relative frequencies of gene arrangements in populations of some localities vary from month to month. Whether these variations are cyclic and seasonal cannot be decided from the available data. The mean numbers of heterozygous inversions per individual vary in different bioclimatic regions from 0.8 (deserts of Bahia) to 9 (central Goyaz). These variations can be accounted for on the supposition that the amount of chromosomal polymorphism, and in general the amount of adaptive genetic variability, is proportional to the variety of habitats which the species has mastered in the environment of a given region.
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Studies of chromosomal polymorphism in natural populations of D. guaramunu and D. griseolineata in Rio Grande do Sul, Brazil, confirmed and extended previous findings in these species. The more common D. guaramunu is much more polymorphic than its closest but less common relative D. griseolineata. In the former species, the frequencies of four inversions showed differences from locality to locality, and five inversions of the same species showed temporal changes in the population of Bexiga. It was not possible to correlate the differences with any particular factor in the environment, though the temporal changes were perhaps due to seasonal climatic cycles.
Inversion heterosis was not evident in either species — instead, inversion Ee of D. guaramunu was found to be less frequent than expected on the basis of the Hardy-Weinberg equilibrium in the localities of Emboaba, Bexiga and Feliz. The implications of these findings are discussed.
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AND TH. DOBZHANSKY. 1954. A further study of chromosomal polymorphism, in Drosophila willistoni in its relation to the environment
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