We describe allelic variation at 28 gene loci in natural populations of D. willistoni. Seventy samples were studied from localities extending from Mexico and Florida, through Central America, the West Indies, and tropical South America, down to South Brazil. At least several hundred, and often several thousand, genomes were sampled for each locus. We have discovered a great deal of genetic variation. On the average, 58% loci are polymorphic in a given population. (A locus is considered polymorphic when the frequency of the most common allele is no greater than 0.95). An individual fly is heterozygous, on the average, at 18.4% loci.—Concerning the pattern of the variation, the most remarkable finding is the similarity of the configuration of allelic frequencies from locality to locality throughout the distribution of the species. Our observations support the conclusion that balancing natural selection is the major factor responsible for the considerable genetic variation observed in D. willistoni.

/pdf/enzyme-variability-in-the-drosophila-willistoni-group-iv-1lt5ngx0z9.pdf

Enzyme variability in the drosophila willistoni group. iv. genic variation in natural populations of drosophila willistoni

In less than half a century, molecular markers have totally changed our view of nature, and in the process they have evolved themselves. However, all of the molecular methods developed over the years to detect variation do so in one of only three conceptually different classes of marker: protein variants (allozymes), DNA sequence polymorphism and DNA repeat variation. The latest techniques promise to provide cheap, high-throughput methods for genotyping existing markers, but might other traditional approaches offer better value for some applications?

The evolution of molecular markers — just a matter of fashion?

Avise, John C. (Department of Genetics, University of California, Davis, California 95616) 1975. Systematic value of electrophoretic data. Syst. Zool. 23:465-481.-Two consistent observations from recent multi-locus electrophoretic studies are: (1) levels of genic similarity between conspecific populations appear very high (populations nearly identical in allelic content at 85 percent or more of their loci) and (2) genic similarities between different, even very closely related species, are generally much lower and more widely dispersed (congeneric species pairs often completely distinct at one-fifth to four-fifths of their loci). These observations have valuable implications regarding the practical utility of electrophoresis: (1) one or a few samples often yield adequate data for the description of an entire species for systematic purposes and (2) closely related species may be arranged according to percentages of shared alleles or genotypes. A survey of the literature indicates that when such arrangements are made, they usually correspond very closely to previously recognized relationships of various species groups based on classical systematic criteria. This observation, coupled with several theoretical advantages of the study of allozymes, makes it clear that electrophoretic techniques will provide an extremely valuable tool for systematists. [Electrophoresis; genetics; systematics.] Electrophoretic techniques were first used by Tiselius (1937; cited by Brewer, 1970) to distinguish multiple fractions of serum proteins migrating through solution under the influence of an electric current. During the next 25 years, advances in electrophoretic methodology and knowledge centered on three fronts: (1) improvements in types of supporting media including the development of starch gels (Smithies, 1955) which are widely used today; (2) the application of histochemical staining methods (Hunter and Markert, 1957), which allowed analysis of electrophoretic variation in enzymatic proteins; and (3) the demonstration that much of the variation was inherited in simple Mendelian fashion. Prior to 1963, most studies described variation in single proteins, but by the mid 1960's, electrophoretic techniques were sufficiently refined to permit examinations of large numbers of different proteins in the same organisms (Hubby, 1963; Hubby and Throckmorton, 1965; Hubby and Lewontin, 1966; Johnson et al., 1966; Lewontin and Hubby, 1966; Harris, 1966). These multi-loci studies were the prototypes for a profitable new method of analysis of levels of genic variability and population structure (review by Gottlieb, 1971). Ironically, some of their findings may also have had an initially retarding influence on the evaluation of electrophoretic data in systematics because, although it was immediately recognized that a quantification of allozyme differences between populations (based on allele or genotype frequencies) might offer potentially valuable information for systematics (Hubby and Throckmorton, 1965), other allozymic results stimulated far greater interest. The disclosure of very high levels of genic variability in natural populations was in apparent conflict with classical population genetic models of balanced load, and hence generated great controversy. Researchers turned their attention to the question of whether most allozymes were maintained by natural selection or were selectively neutral (Proc. VI Berkeley Symp., 1972). Thus, even by 1970, a major review of electrophoretic literature included almost no discussion of uses of electrophoretic data in systematics, other than for description and identification of species (Manwell and Baker, 1970). Today, the selectionist versus

Systematic Value of Electrophoretic Data

The rapid development of molecular techniques offers a palette of technical approaches for population biologists interested in a wide range of questions. For example, these tools can be used to determine individual reproductive success or to measure rates of genetic divergence among populations. Which technique is most appropriate for a par- ticular question depends upon (1) the extent of genetic polymorphism required to best answer the question, (2) the analytical or statistical approaches available for the technique's application, and (3) the pragmatics of time and costs of materials. Here we evaluate the application of several major techniques (protein electrophoresis, nuclear and mitochondrial RFLPs (restriction fragment length polymorphisms), minisatellite and microsatellite VNTRs (variable number tandem repeats), RAPDs (random amplified polymorphic DNA), and DNA sequencing) to an array of questions regarding individual identification, exclusion and assignment of parentage, and various levels of population structure. In our evaluation, we briefly explain the technical components of each molecular approach and assess whether the typical outcomes expected from each approach will provide useful information as applied to each level of inquiry. For studies of population genetic structure, protein electrophoresis remains a powerful tool for most taxa, although techniques based on nucleic acids (par- ticularly DNA sequencing and mitochondrial DNA RFLPs) are useful here as well. Recently developed nucleic acid techniques (e.g., VNTRs) can often identify enough genetic vari- ability to address questions of self-identification or parentage. Some of the newest tech- niques (RAPDs and microsatellites) are potentially useful across a number of levels of inquiry, although procedures for adopting them are still developing.

/pdf/what-molecules-can-tell-us-about-populations-choosing-33rzbo2u7s.pdf

What molecules can tell us about populations: choosing andusing a molecular marker

Heritable variation in regulatory or coding regions is the raw material for evolutionary processes. The advent of microarrays has recently promoted examination of the extent of variation in gene expression within and among taxa and examination of the evolutionary processes affecting variation. This review examines these issues. We find: (i) microarray-based measures of gene expression are precise given appropriate experimental design; (ii) there is large inter-individual variation, which is composed of a minor nongenetic component and a large heritable component; (iii) variation among populations and species appears to be affected primarily by neutral drift and stabilizing selection, and to a lesser degree by directional selection; and (iv) neutral evolutionary divergence in gene expression becomes nonlinear with greater divergence times due to functional constraint. Evolutionary analyses of gene expression reviewed here provide unique insights into partitioning of regulatory variation in nature. However, common limitations of these studies include the tendency to assume a linear relationship between expression divergence and species divergence, and failure to test explicit hypotheses that involve the ecological context of evolutionary divergence.

/pdf/variation-within-and-among-species-in-gene-expression-raw-4zwlqeoum5.pdf

Variation within and among species in gene expression: raw material for evolution.

There is mounting evidence that Drosophila populations are mlore polymorphic at the level of genetic loci than was formerly thought by many geneticists. There have been many experimental demonstrations of the extensive chromosomal polymorphis^ms, of the presence of considerable quantities of hidden deleterious and lethal alleles in the populations, and of the extensive complexity of the coadapted genetic systems integrated through natural selection.' Nevertheless, the proportion of polymorphisms among the majority of all loci is largely unknown because of inadequate screening techniques. Furthermore, the mechanisms for the maintenance of polymorphisms -remain elusive to experimental verification. The ideal situation of screening populations for genetic comparisons at and between individual loci has been brought a step closer to reality by electrophoretic separation of enzyme molecules. Though enzyme differences are not necessarily related with genetic differences on a one-to-one basis, most may be. At any rate, zymogram analysis of individual flies for electrophoretic mobility or qualitative activity differences gives a good first approximation of the underlying genetic situation for a different class of variation than has previously been studied directly. However, as Lewontin and Hubby3 have pointed out, the randomness of the samples of loci studied by this technique is critical for broader generalizations which might be made from the findings in this class of loci. Sample characteristics are yet to be determined. Lewontin and Hubby3 have found about 40 per cent of all electrophoretically separable enzyme systems and protein bands of no known enzymatic activity of D. pseudoobscura to be polymorphic, having two or more electrophoretic forms of the same protein. One expects the difference in molecular forms to represent at least as much genetic difference, so that the variation of molecular types reflects similar variation of genotypes. If only a moderate bias is involved in estimating this proportion for the genome, then there exists a major discrepancy between the data and the expectation based upon the theory of segregational load. According to the theory, a population may bear, without being jeopardized by reduced average fitness, only a relatively small portion of loci in heterotic polymorphic conditions. In order to establish the meaning of the apparent inconsistency between the theory and observations, it must be established whether or not these isozyme polymorphisms are maintained more or less independently by some form of selection. It is very important to know if these variations are random representatives of a large number of such loci. It has been demonstrated in a laboratory population of D. busckii4 that the polymorphism at the "leucine aminopeptidase" locus is maintained by heterotic selection, though this was not the case for an esterase locus in D. inelanogaster and D. simulans.5

An analysis of polymorphisms among isozyme loci in dark and light Drosophila ananassae strains from American and Western Samoa.

Tissue specificity of the esterases of two closely related species, Drosophila aldrichi and D. mulleri, is determined by starch gel electrophoresis of dissected tissues and organs and an esterase staining technique. The determination of esterase homology between the two species is supplemented by tissue specificity as an added criterion of catalytic properties. General biological functions of some of the esterases are suggested by their presence in certain tissues. The potential usefulness of knowledge of tissue specificities in evolutionary and population genetic studies is discussed.

Isozyme variability in species of the genus Drosophila. IV. Distribution of the esterases in the body tissues of D. aldrichi and D. mulleri adults.

Genetic variation in natural island populations of members of the Drosophila nasuta and Drosophila ananassae subgroups.

Two closely related species, Drosophila aldrichi and D. mulleri, are compared on the basis of their esterase isozyme patterns after starch gel electrophoresis. Comparable esterases between the two species are identified by substrate specificity, inhibition, and enhancement of esterase activity by various agents. The extensive electrophoretic variability of most of the esterases in nature is described and data relating to the inheritance of the various enzyme forms are presented.

Isozyme variability in species of the genus Drosophila. III. Qualitative comparison of the esterases of D. aldrichi and D. mulleri

The presence or absence of certain esterase types in ejaculatory bulbs of 93 Drosophilaspecies was determined by appropriate staining after starch gel electrophoresis. One entire subgenus (Sophophora)and several primitive species were found lacking in one type of esterase. This esterase was present in numerous species of the large Drosophilasubgenus, but presence and absence did not follow closely the lines of taxonomic subdivisions in the subgenus. Correlation of presence-absence of the bulb esterase and previous results in interspecific hybridization tests indicated a greater chance for a successful interspecific cross when males possessed the bulb esterase.

F. M. Johnson

Papers

An analysis of polymorphisms among isozyme loci in dark and light Drosophila ananassae strains from American and Western Samoa.

Isozyme variability in species of the genus Drosophila. IV. Distribution of the esterases in the body tissues of D. aldrichi and D. mulleri adults.

Genetic variation in natural island populations of members of the Drosophila nasuta and Drosophila ananassae subgroups.

Isozyme variability in species of the genus Drosophila. III. Qualitative comparison of the esterases of D. aldrichi and D. mulleri

Isozyme variability in species of the genus Drosophila. V. Ejaculatory bulb esterases in Drosophila phylogeny.