Background: Repetitive elements have contributed to the formation of chromosomal rearrangements in a variety of taxa. In the course of investigating the breakpoints of the XR-chromosome inversion distinguishing Drosophila pseudoobscura and D. persimilis, we identified a novel repetitive element that we name "Leviathan" that may have contributed to its formation.
Results: Leviathan occurs in arrays of a highly palindromic 319-bp motif, and its sequence is not found in any databases or within other Drosophila species for which genome sequences are available. We found evidence for active transcription of the Leviathan sequence, and we note that it is spread broadly across the genomes of these two species. To better understand the evolutionary dynamics of Leviathan, we apply several computational tests to its sequence and distribution. We conclude that the appearance of Leviathan likely predates the divergence of these species and may have contributed to their divergence through the generation of a chromosomal rearrangement. We also fail to find support for the master gene hypothesis regarding the spread of this sequence across the genome.
Conclusions: We have characterized a novel repetitive element using computational tests and laboratory techniques, thus further demonstrating the value of adding computational genomic characterization of repetitive elements to traditional laboratory studies to better understand both the molecular and evolutionary dynamics of repetitive sequence.
The last few decades have seen a surge of interest in speciation, the genetic changes underlying it, and the evolutionary forces driving it. As with most disciplines, however, the nature of the questions addressed has changed with time. Many studies from the 1980's and 1990's often asked questions about whether certain processes ever occur in nature (e.g., speciation with gene flow). Since that time, case studies have provided evidence that nearly all evolutionary processes thought to be involved in speciation have occurred at least once. As a result, we are now in a new era where the "big questions" must go beyond demonstrations that a phenomenon has happened at least once. Here we discuss a few open questions in speciation—questions that we feel are not only exciting but tractable. We focus our discussion most sharply on recent studies in Drosophila and related species, the area of our expertise. However, we also emphasize the importance of broad taxonomic meta-analyses testing the importance or frequency of various processes thought to cause speciation.
Resource use in phytophagous insects can influence survivorship as well as the ability to find mates. Therefore, the ability to find a resource and the potential to survive in its presence can have important effects on micro-evolutionary processes in insect populations. Here, we study tolerance of and attraction to varying concentrations of ethanol in Drosophila pseudoobscura adult flies. We document substantial heritable variation for ethanol attraction and tolerance among outbred strains and observe a borderline significant correlation between attraction and tolerance. We also note significantly greater survivorship of females over males when exposed to low concentrations of ethanol.
To assess the frequency and importance of reinforcement in nature we must begin by looking for its signature in the most likely places. Theoretical studies can pinpoint conditions that favor and inhibit reinforcement, and empirical studies can identify both how often these conditions occur and whether reinforcement results. We examine how well these tools have addressed these questions by searching for gaps and mismatches in theoretical and empirical studies of reinforcement. We concentrate on five areas: (a) a broad assessment of selection against interspecific mating, (b) the mode and genetic basis of nonrandom mating, (c) the geography of speciation, (d) divergent selection on mating cues, (e) and the genetics of reproductive isolation. We conclude that reinforcement has probably not been looked for where it is most likely to occur. We pinpoint however, many further areas of study that may ultimately provide a strong assessment of the importance of reinforcement in speciation.
We used an enrichment technique to isolate 60 microsatellite loci in Ardea herodias. We developed primers for 17 loci, screened for variation in A. herodias and attempted to amplify these loci in three closely related species (A. alba, A. cinerea and A. cocoi). Fifteen loci were polymorphic in A. herodias. Observed heterozygosities ranged from 0.10 to 0.81. Two loci appeared to be monomorphic in A. herodias, but exhibited variation in product size among species within the genus. Our ability to amplify polymorphic products in closely related species suggests these markers may be useful in other herons.
Alvarez, D., M. A. F. Noor, and M. Ruiz-Garcia. 2001. Comparative genetic structure between tropical Colombian and North American Drosophila pseudoobscura populations. Biotropica, 34: 81-92.
Since the discovery of Drosophila pseudoobscura in the tropical highlands of the Colombian Andes in the 1960´s, this population has been subject of study for many evolutionary biologists due to its isolation from the main geographical range of this species in North America. Here, we used five highly variable microsatellite loci (DPSX001, DPS2001, DPS3001, DPS3002 and DPS4001) to analyze the genetic structure of three Colombian populations and the genetic relationships with four North American populations. We found that the average heterozygosity was consistent among the three tropical Colombian populations (H= 0.665-0.675), but they had less variability than their North American counterparts. Nonetheless, the genic diversity found in the Colombian populations was higher than that found previously using other genetic markers. The average genic heterogeneity estimate among the Colombian populations (RST = 0.042), although statistically significant, was substantially lower than that found among the North-American populations (RST = 0.088). We identified alleles in the Colombian populations not reported in North American populations, suggesting further divergence between the populations on these continents. We estimated that the populations on the two continents diverged approximately 80,000 years ago, consistent with independent sequence analyses of these populations but contrary to some suggestions in the literature. Finally, we estimated an average effective population size of the Colombian populations to be on the order of 100,000.
Noor, M. A. F. 2000. Effects of dominance on runaway sexual selection. Adaptive Behavior, 8: 255-266.
Distinguishing among theories of sexual selection requires that one develop diagnostic predictions that can be tested in living systems. Recently, genetic studies of female species preferences in Drosophila supported the predictions of a model of sexual selection through pleiotropy with adaptive traits: preferences generally behaved as recessive characters. However, the dominance predictions of female preferences resulting from runaway sexual selection have not been investigated. Here, I present an extension of previous simulation models of runaway sexual selection by varying the dominance of the female preference and incorporating genetic drift. I show that runaway sexual selection is generally more likely to favor the evolution of dominant female preferences than recessive ones. Also, in contrast to the results of a previous study, dominant preferred male characters spread more quickly by runaway sexual selection than recessive ones under some conditions. Overall, the predictions derived from this model of runaway sexual selection are not supported by empirical data on species preferences, suggesting that runaway sexual selection may not be a major force in the evolution of such preferences. More empirical studies will be necessary to further evaluate both the predictions and the assumptions of this model, however.
Noor, M. A. F. 2000. On the evolution of female mating preferences as pleiotropic byproducts of adaptive evolution. Adaptive Behavior, 8: 3-12.
Although evidence is mounting that female mating preferences evolve at least in part as incidental (pleiotropic) consequences of alleles favored by natural selection, it is less clear how preferences can evolve when they are initially maladaptive, as by delaying reproduction. I extend a previous model by Tomlinson and O'Donald (1996, Heredity 77:303-312) to investigate how dominance, sex-linkage, and sex-limitation affect the evolution of a costly new female preference. I find that recessivity of the new female preference can allow it to spread as a pleiotropic byproduct of adaptive evolution even when the novel preference is initially extremely detrimental. Further, three predictions of this model are satisfied by empirical data on the genetics of female mating preferences in Drosophila. Taken together, these findings suggest that incidental association of novel female preferences with alleles under selection could be a potent force in the origin and evolution of novel female mating preferences.
Noor, M. A. F. 1998. Diurnal activity patterns of Drosophila subobscura and D. pseudoobscura in sympatric populations. American Midland Naturalist, 140: 34-41.
The Old World species, Drosophila subobscura, has recently invaded North America and become sympatric with the native obscura-group species. This study investigates the summer diurnal activity patterns of two northwestern North American populations of D. subobscura and compares them to the documented Old World trend as well as that of its native North American congener, D. pseudoobscura. Both species peak in activity at temperatures between 15 and 20°C, and activity in both species increases with decreasing sun angles, thus causing high levels of activity near sunrise and sunset. However, D. subobscura becomes more abundant than D. pseudoobscura at lower temperatures and sun angles. No differences in diurnal activity were noted between Old World and New World D. subobscura. D. subobscura might have contributed to the reduced abundance of a native species, D. persimilis, in areas of recent sympatry, though this causal connection is not certain.
Noor, M. A. F. 1997. Genetics of sexual isolation and courtship dysfunction in male hybrids of Drosophila pseudoobscura and D. persimilis. Evolution, 51: 809-815.
Despite the importance of sexual isolation to speciation, few studies have analyzed the genetic basis of interspecific mating discrimination, particularly using hybrid males. In this study, I investigated the genetic basis of sexual isolation using male hybrids of Drosophila pseudoobscura and D. persimilis. Hybrid male mating success was caused by interactions between the X-chromosome and autosomes (or Y-chromosome), and different arms of the X-chromosome contributed to mating success with females of each species. Further, although there was an X-chromosome component to mating success, its magnitude was not disproportionately large when compared with the proportion of the genome contained on this chromosome. Some hybrid males courted with an anomalously low intensity, so I simultaneously mapped the genetic basis of this "courtship dysfunction." The courtship dysfunction was caused by an interaction between the left arm of the X-chromosome in D. persimilis with the autosomes or Y-chromosome from D. pseudoobscura. Anomalous courtship behavior in interspecific hybrids can obscure the conclusions of studies of the genetics of sexual isolation, so courtship intensity should be evaluated in all such investigations.
Noor, M. A. F. 1996. Absence of species discrimination in Drosophila pseudoobscura and D. persimilis males. Animal Behaviour, 52:1205-1210.
Despite frequent assumptions in both the theoretical and empirical literature that males are indiscriminate in their courtship, species mating discrimination by males and females are approximately equally frequent in Drosophila. This study looked for evidence of male species discrimination in D. pseudoobscura and D. persimilis through both choice and no-choice experiments. The results indicate that males of these species court females indiscriminately. Thus, although male mate choice sexually isolates many other Drosophila species, it does not appear to serve as an isolating mechanism in these sibling species. The implications of the lack of species discrimination by males to speciation and the respective roles of choice and no-choice experiments are discussed.
Noor, M. A. 1995. Incipient sexual isolation in Drosophila pseudoobscura bogotana Ayala & Dobzhansky (Diptera: Drosophilidae). Pan-Pacific Entomologist, 71: 125-129.
Incipient sexual isolation was found between the subspecies Drosophila pseudoobscura bogotana (Ayala and Dobzhansky) and D. p. pseudoobscura (Frolova). Males took longer to initiate courtship with heterosubspecifics than consubspecifics. This divergence could represent an early step in the speciation of these taxa. The subspecies' pheromones do not appear to have diverged, so the difference is attributed to female behavior.
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