Inversions fail to account for allozyme clines

Allozyme and inversion data from natural populations of Drosophila melanogaster from the eastern United States were analyzed to determine whether the clines at allozyme loci are due to nonrandom associations with common cosmopolitan inversions. All inversions show strong clines. Clines were large and significant for half of the eight allozyme loci. An analysis of the contribution of inversions to clines of allozyme genes revealed three outcomes: the inversion cline (1) enhanced the allozyme cline, but was only partly responsible, (2) reduced the allozyme cline, and (3) had no effect. The allozyme clines were mainly determined by the pattern of allele frequencies within the chromosomal arrangements. Consequently, it was concluded that allozyme clines would exist in the absence of inversion clines.     

Patterns and levels of genetic differentiation in North American populations of the Alaskan wheatgrass complex

Levels and distribution of genetic variation were assessed using six allozymes in 27 populations of Alaskan wheatgrass (Elymus alaskanus) from different locations in Canada, USA, Greenland and Russia to obtain information on the genetic structure of these populations. The enzyme systems were ACO, DIA, GPI, MDH, PGM and SKD. Allozyme variation at the species level was high, with 64.3% (Ps) of the loci being polymorphic, an average number of alleles per locus of 1.9 (As), and an average genetic diversity of 0.17 (Hes). Differentiation was found in the populations studied, with the following findings: (1) statistically significant differences were found in allele frequencies among populations for every polymorphic locus (P < 0.001); (2) 63% of the total allozyme variation at polymorphic loci was partitioned among populations (GST = 0.63); (3) relatively low mean genetic distances between the populations were obtained (mean D = 0.029); (4) the genetic structure of Russian populations are clearly distinct from the other populations, the cluster and principal component analyses revealed the same genetic patterns of relationships among populations. This study also indicates that E. alaskanus contains different levels of allozyme variation in its populations. Furthermore, some banding patterns at the loci Aco-1, Aco-2, Gpi-2, Mdh-1, Skd-1, Skd-2 can be used as markers to identify individual populations.     

Genetic variability in Irish populations of the invasive zebra mussel, Dreissena polymorpha: discordant estimates of population differentiation from allozymes and microsatellites

1. The recent arrival and explosive spread of the zebra mussel, Dreissena polymorpha (Pallas), in Ireland provided a rare opportunity to study the population genetics of an invasive species.
2. Eight polymorphic allozyme loci ( ACO-1, ACO-2 , EST-D, GPI, IDH-2, MDH, OPDH and PGM ) were used to investigate genetic diversity and population structure in five Irish populations, and the results were compared with those from a previous microsatellite study on the same samples.
3. The mean number of alleles per locus (2.7 ± 0.1) was similar to the mean for the same loci in European populations, suggesting that Irish founder populations were large and/or multiple colonization events took place after foundation. A deficiency of heterozygotes was observed in all populations, but was uneven across loci.
4. Pairwise comparisons, using Fisher's exact tests and F _ST values, revealed significant genetic differentiation among populations. The overall multilocus F _ST estimate was 0.118 ± 0.045, which contrasted with an estimate of 0.015 ± 0.007 from five microsatellite loci on the same samples in a previous study.
5. Assuming that microsatellites can be used as a neutral baseline, the discordant results from allozymes and microsatellites suggest that selection may be acting on some allozyme loci, specifically ACO-1, ACO-2 , IDH-2 and MDH, which contributed most to the significant differentiation between samples.     

Evidence for a link between local and seasonal cycles in gene frequencies and latitudinal gene clines in a cyclic parthenogen

In an earlier study (Rhombergh et al., Can. J. Genet. Cytol. 27: 224-232, 1985) of natural populations of the cyclic parthenogenetic Rose aphids, Macrosiphum rosae, 6 out of 31 loci were found to be polymorphic and one locus (Esterase-4) showed cyclic seasonal changes in gene and genotypic frequencies. Assuming that the Est-4 polymorphism was balanced and due to some climatic factor that varies seasonally, and realizing that most environmental factors that vary seasonally also vary latitudinally, we predicted existence of a latitudinal gene cline at this locus. In the present study we surveyed four polymorphic loci (chosen to be used as markers) in six geographic populations spanning over 1200 km between the United States and Canada and found all four loci to have latitudinal clines. We think that the gene clines are due to a latitudinal cline in the degree of advancement of local populations through the seasonal cycle, and have called such a pattern a 'seasonal phase cline'. The results are discussed in relation to the temporal instability of local patterns and persistence of genetic variability on the large scale in aphids. It is argued that population structure of aphids makes retention of selectively neutral or weakly selected polymorphisms difficult.     

Immunochemical differentiation of glucose phosphate isomerase (GPI) allozymes of the mouse

Polyclonal xenoantisera against mouse GPI-1B and GPI-1C were produced in rabbits and analyzed for their ability to recognize allozyme-specific determinants. These studies showed a high degree of serological similarity among the three allozymes of mouse glucose phosphate isomerase (GPI). However, GPI-1B and GPI-1C could be differentiated from GPI-1A as well as GPI-1A and GPI-1B from GPI-1C using quantitative solid-phase immunobinding assays. In addition, polyclonal and monoclonal alloantibodies specific for GPI-1C were produced in BALB/c (Gpi-1a/Gpi-1a) mice. As indicated by immunoblotting data, the allozyme specificity of rabbit antisera and monoclonal alloantibodies against GPI-1C is dependent on the native structure of that allozyme.     

Allozyme Variation in a Threatened Freshwater Fish, Spotted Murrel (Channa punctatus) in a South Indian River System

Samples of the spotted murrel (Channa punctatus) were collected from three rivers of Tamil Nadu and Kerala. The allozyme variation of C. punctatus was investigated by polyacrylamide gel electrophoresis. Eighteen enzymes were detected, but only 10 (EST, PGM, G3PDH, G6PDH, SOD, GPI, ODH, GDH, XDH, and CK) showed consistent phenotypic variations. Allele frequencies were estimated at the 18 polymorphic loci representing 10 enzymes. Two rare alleles, EST-4*C and G6PDH-2*C, were noted in the Tamirabarani and Kallada populations but were absent in the Siruvani population. The allele frequencies of the Tamirabarani and Kallada populations were similar, except for a few loci. Among the three populations, the maximum genetic distance (0.026) and FST (0.203) were found between the geographically distant Siruvani and Kallada populations. Overall the study showed that among the three populations, the Tamirabarani and Kallada have similar genetic structures.     

Latitudinal Variation of Allozyme Frequencies in Populations of Two Freshwater Isopods in Western Europe

Populations of the freshwater isopods, Proasellus meridianus Racovitza and P. coxalis Dollfus were found to be polymorphic at loci coding for the enzymes glucose phosphate isomerase (GPI) and phosphoglucomutase (PGM). PGM allele frequencies in populations of both species were significantly associated with latitude as has been previously reported for the closely related Asellus aquaticus (L.). Co-variation of PGM allele frequencies was also found in sympatric populations of P. meridianus and A. aquaticus. Significant interpopulation variation in GPI allele frequencies occurred in P. coxalis but not P. meridianus, however, this variation was not associated with latitude.     

The effects of genetic drift during range expansion on geographical patterns of variation: a computer simulation of the colonization of Australia by Bufo marinus

A computer simulation is performed of allele frequency changes resulting from genetic drift at eleven loci during the probable course of colonization of Australia by populations of the Giant Toad, Bufo marinus. The history of twelve populations for which allele frequency data are available is modelled. Account is taken of the likely pattern of relationship among the populations, the effective size of the populations (as indicated by the observed variance of allele frequencies) and the probable isolation of the populations from each other following their separation. In all simulations, allele frequencies at some loci show a significant association with latitude while others do not. In six of ten simulations, there is a significant association between degree of variation at a locus and the presence of a latitudinal cline of allele frequencies. There are also indications of this kind of association in simulations of a uni-directional range expansion. These results demonstrate that such associations, which were also observed in the data from the actual populations, can result from genetic drift during a range expansion, and therefore cannot be taken as evidence of the action of natural selection.     

Description and genetics of glucose phosphate isomerase (GPI) and phosphoglucomutase (PGM) polymorphisms in Asellus aquaticus (L.)

Analysis of Western European populations of Asellus aquaticus uncovered 10 electrophoretic phenotypes of glucose phosphate isomerase (GPI) and 7 of phosphoglucomutase (PGM). Breeding studies indicate that the variation is controlled by codominant alleles at two autosomal loci. Genotype frequencies in the two sexes do not differ significantly, mating between genotypes is random, and no structural linkage is detectable between the two loci. PGM shows nongenetic, "secondary" banding, particularly in animals stored at -20 degrees C prior to electrophoresis. This secondary banding confounds the identification of the genetic variation but can be controlled by the reducing agent 2-mercaptoethanol.     

Genetic variation underlying local adaptation of diapause induction along a cline in a butterfly

Diapause is a life history strategy allowing individuals to arrest development until favourable conditions return, and it is commonly induced by shortened day length that is latitude specific for local populations. Although understanding the evolutionary dynamics of a threshold trait like diapause induction provides insights into the adaptive process and adaptive potential of populations, the genetic mechanism of variation in photoperiodic induction of diapause is not well understood. Here, we investigate genetic variation underlying latitudinal variation in diapause induction and the selection dynamics acting upon it. Using a genomewide scan for divergent regions between two populations of the butterfly Pararge aegeria that differ strongly in their induction thresholds, we identified and investigated the patterns of variation in those regions. We then tested the association of these regions with diapause induction using between‐population crosses, finding significant SNP associations in four genes present in two chromosomal regions, one with the gene period, and the other with the genes kinesin, carnitine O‐acetyltransferase and timeless. Patterns of allele frequencies in these two regions in population samples along a latitudinal cline suggest strong selection against heterozygotes at two genes within these loci (period, timeless). Evidence for additional loci modifying the diapause decision was found in patterns of allelic change in relation to induction thresholds over the cline, as well as in backcross analyses. Taken together, population‐specific adaptations of diapause induction appear to be due to a combination of alleles of larger and smaller effect size, consistent with an exponential distribution of effect sizes involved in local adaption.     

Clinal variation for amino acid polymorphisms at the Pgm locus in Drosophila melanogaster

Clinal variation is common for enzymes in the glycolytic pathway for Drosophila melanogaster and is generally accepted as an adaptive response to different climates. Although the enzyme phosphoglucomutase (PGM) possesses several allozyme polymorphisms, it is unique in that it had been reported to show no clinal variation. Our recent DNA sequence investigation of Pgm found extensive cryptic amino acid polymorphism segregating with the allozyme alleles. In this study, we characterize the geographic variation of Pgm amino acid polymorphisms at the nucleotide level along a latitudinal cline in the eastern United States. A survey of 15 SNPs across the Pgm gene finds significant clinal differentiation for the allozyme polymorphisms as well as for many of the cryptic amino acid polymorphisms. A test of independence shows that pervasive linkage disequilibrium across this gene region can explain many of the amino acid clines. A single Pgm haplotype defined by two amino acid polymorphisms shows the strongest correlation with latitude and the steepest change in allele frequency across the cline. We propose that clinal selection at Pgm may in part explain the extensive amino acid polymorphism at this locus and is consistent with a multilocus response to selection in the glycolytic pathway.     

DNA sequence variation and latitudinal associations in hsp23, hsp26 and hsp27 from natural populations of Drosophila melanogaster

Heat shock genes are considered to be likely candidate genes for environmental stress resistance. Nucleotide variation in the coding sequence of the small heat shock genes (hsps) hsp26 and hsp27 from Drosophila melanogaster was studied in flies originating from the Netherlands and eastern Australia. The hsp26 gene was polymorphic for an insertion/deletion of three extra amino acids and two nonsynonymous changes in all populations. The hsp27 gene exhibited two nonsynonymous changes and three synonymous mutations. The hsp26 polymorphism showed a latitudinal cline along the east coast of Australia. This pattern was not confounded by the fact that the shsps are located in the inversion In(3 L)P which also shows a latitudinal cline in eastern Australia. A similar latitudinal cline was found for the previously described variation in hsp23, while frequencies of hsp27 alleles did not change with latitude. These findings suggest that variation at two of the shsps or closely linked loci are under selection in natural populations of D. melanogaster.     

Extensive amino acid polymorphism at the pgm locus is consistent with adaptive protein evolution in Drosophila melanogaster

PGM plays a central role in the glycolytic pathway at the branch point leading to glycogen metabolism and is highly polymorphic in allozyme studies of many species. We have characterized the nucleotide diversity across the Pgm gene in Drosophila melanogaster and D. simulans to investigate the role that protein polymorphism plays at this crucial metabolic branch point shared with several other enzymes. Although D. melanogaster and D. simulans share common allozyme mobility alleles, we find these allozymes are the result of many different amino acid changes at the nucleotide level. In addition, specific allozyme classes within species contain several amino acid changes, which may explain the absence of latitudinal clines for PGM allozyme alleles, the lack of association of PGM allozymes with the cosmopolitan In(3L)P inversion, and the failure to detect differences between PGM allozymes in functional studies. We find a significant excess of amino acid polymorphisms within D. melanogaster when compared to the complete absence of fixed replacements with D. simulans. There is also strong linkage disequilibrium across the 2354 bp of the Pgm locus, which may be explained by a specific amino acid haplotype that is high in frequency yet contains an excess of singleton polymorphisms. Like G6pd, Pgm shows strong evidence for a branch point enzyme that exhibits adaptive protein evolution.     

Genetic Differentiation between Geographically Distant Populations of DROSOPHILA MELANOGASTER

We have studied allozyme variation at 26 gene loci in nine populations of Drosophila melanogaster originating on five different continents. The distant populations show significant genetic differentiation. However, only half of the loci studied have contributed to this differentiation; the other half show identical patterns in all populations. The genetic differentiation in North American, European and African populations is correlated with the major climatic differences between north and south. These differences arise mainly from seven loci that show gene-frequency patterns suggestive of latitudinal clines in allele frequencies. The clinal variation is such that subtropical populations are more heterozygous than temperate populations. These results are discussed in relation to the selectionist and neutralist hypotheses of genetic variation in natural populations.     

Chromosome inversion polymorphisms in Drosophila melanogaster III. Gametic disequilibria and the contributions of inversion clines to the Adh and Gpdh clines in Australasia

Gametes from each of 19 Australasian collections of Drosophila melanogaster were characterised for polymorphic Adh and Gpdh allozyme loci and the two chromosome-2 common cosmopolitan inversions In(2L)t and In(2R)NS.-No consistent gametic disequilibria for the loosely linked combinations of Adh-Gpdh and Adh-In(2R)NS were found over the 19 collections. However, strong and consistent gametic disequilibria were found for the tightly linked combinations of Adh-In(2L)t and Gpdh-In(2L)t. The results for each of these four combinations agree with those previously reported for Northern-Hemisphere collections. -Notwithstanding its parallel latitudinal cline and consistent gametic disequilibrium with Adh, In(2L)t was found to account for only a small fraction of the Adh latitudinal cline in Australasia. This is consistent with the previous finding that the North American Adh latitudinal cline also exists largely independently of In(2L)t. However, In(2L)t accounts for all of the Australasian Gpdh latitudinal cline and so contrasts the finding from North American collections where the corresponding Gpdh cline exists largely independently of In(2L)t.     

Evidence for a link between local and seasonal cycles in gene frequencies and latitudinal gene clines in a cyclic parthenogen

In an earlier study (Rhombergh et al., Can. J. Genet. Cytol. 27: 224–232, 1985) of natural populations of the cyclic parthenogenetic Rose aphids, Macrosiphum rosae, 6 out of 31 loci were found to be polymorphic and one locus (Esterase-4) showed cyclic seasonal changes in gene and genotypic frequencies. Assuming that the Est-4 polymorphism was balanced and due to some climatic factor that varies seasonally, and realizing that most environmental factors that vary seasonally also vary latitudinally, we predicted existence of a latitudinal gene cline at this locus. In the present study we surveyed four polymorphic loci (chosen to be used as markers) in six geographic populations spanning over 1200 km between the United States and Canada and found all four loci to have latitudinal clines. We think that the gene clines are due to a latitudinal cline in the degree of advancement of local populations through the seasonal cycle, and have called such a pattern a seasonal phase cline. The results are discussed in relation to the temporal instability of local patterns and persistence of genetic variability on the large scale in aphids. It is argued that population structure of aphids makes retention of selectively neutral or weakly selected polymorphisms difficult.     

O chromosome inversion polymorphism in Northern and Atlantic Europe and its implications in the American colonization by Drosophila subobscura

The Western Mediterranean populations of D. subobscura have a very similar chromosomal inversion polymorphism to the American colonizing populations, with the exception of the O₅ inversion. This inversion, which is found in appreciable frequencies in colonizing populations and is distributed according to a significant latitudinal cline, both in North and South America, has never been reported in that Palearctic region. Therefore, an analysis of the distribution of this inversion, along a latitudinal cline, has been carried out in Europe. The O Chromosome inversion polymorphism has been studied in seven populations: Gävle and Lilla Edet (Sweden), Gesten (Denmark), Ter-Apel (Holland) and Crézy, Aizenay and Taulé (France). The O₅ inversion was only detected in three populations and in low frequencies: Gävle (3.7%), Lilla Edet (1.8 %) and Taulé (1.2 %). However, none of these three populations has all the inversions found in the American populations. The clinal distribution of some O chromosomal arrangements has also been studied according to latitude and the temporal changes of this polymorphism.     

Genetic differences in glucose phosphate isomerase activity among mouse embryos

We have compared mouse embryos of three heterozygous, congenic genotypes (with high, medium and low levels of oocyte-coded glucose phosphate isomerase (GPI-1) activity respectively) to test whether 1) the survival time of oocyte-coded GPI-1 activity in the early embryo is affected by its activity level in the oocyte and 2) whether embryo-coded GPI-1 is detected earlier in embryos that inherit low levels of oocyte-coded GPI-1. The oocyte-coded GPI-1 was entirely GPI-1A allozyme in the high and medium groups but was the less stable GPI-1C allozyme in the low group. We determined total GPI-1 activity and the ratio of different GPI-1 allozymes in early embryos and calculated the activity of oocyte-coded and embryo-coded GPI-1. In all three groups, the oocyte-coded enzyme activity remained at a more or less constant level for the first 21 1/2 days. Some oocyte-coded GPI-1 remained in 4 1/2 day embryos from the high and medium groups but was gone by 5 1/2 days. Very little remained in 4 1/2 day embryos that inherited low levels of a less stable form of the enzyme (GPI-1C allozyme). Despite a 4- to 5-fold difference in initial oocyte-coded GPI-1 activity, no differences were seen among the three genotypically distinct groups of embryos in the time of activation of the embryonic Gpi-1s genes. The embryo-coded GPI-1 was first detectable in 3 1/2 day compacted morulae in all three groups. The level of oocyte-coded GPI-1, in the high group, when embryo-coded GPI-1 was first detected was higher than the level in the low group at any stage prior to detection of embryo-coded GPI-1.(ABSTRACT TRUNCATED AT 250 WORDS)