Wilke van Delden

Latitudinal variation for two enzyme loci and an inversion polymorphism in Drosophila melanogaster from Central and South America

Abstract.— Many organisms show latitudinal variation for various genetically determined traits. Such clines may involve neutral variation and originate from historical events or their maintenance may be explained by selection. For Drosophila melanogaster, latitudinal variation for allozymes, inversions, and quantitative traits has been found on several continents. We sampled D. melanogaster populations in Panama and along a transect of 40 latitudinal degrees on the west coast of South America. Negative correlations with latitude were found for Adhs and αGpdhF allele frequencies and for the frequency of the cosmopolitan inversion In(2L)t in AdhsαpdhF chromosomes. A positive correlation existed between wing length and latitude. Significant correlations were found between these traits and climatic variables like temperature and rainfall. The observed clines show considerable resemblance to those found on other continents. Gametic disequilibrium between Adhs and αGpdhF occurred predominantly at higher latitudes and was caused by the presence of In(2L)t. The reasons for the clinal distributions are discussed and it is argued that selection is the most likely explanation. However, the exact nature of the selective force and the interactions of allozymes with each other and with In(2L)t are complex and not fully understood. In tropical regions In(2L)t‐containing genotypes have higher fitness than ST/ST and Adh and αGpdh hitchhike with the inversion, but there is also evidence for balancing selection at the Adh locus.

Genetic analysis of ecological relevant morphological variability in Plantago lanceolata L. I: Population characteristics

Morphological variation in Plantago lanceolata L. was studied in a greenhouse and in an experimental garden, using members of full-sib families from four populations. The study included two hayfield populations and two pasture populations. The morphological differences found in the natural populations were also observed in both experiments, it was thus concluded that for most characters the variation was partly genetic. In addition to significant interpopulational genetic variation also appreciable intrapopulational genetic variation (i.e., between and within full-sib families) was observed. The population varied in amount of variation, heritability estimates, genetic correlations and in environmental sensitivity. Each population seems well adapted to its habitat. The results suggest that microevolution is retarded in some populations by unfavourable genetic correlation structures among characters under simultaneous selection, by inhomogeneous habitat characteristics or by relatively high levels of phenotypic plasticity. Selection forces responsible for the population characteristics are discussed.

Evidence for autotetraploidy in Plantago media and comparisons between natural and artificial cytotypes concerning cell size and fertility

The hypothesis of autotetraploidy in Plantago media (2n = 12 and 2n = 24) was tested. The two cytotypes are morphologically very similar. Rare 4X and 3X hybrids were obtained in inter-ploidy crosses. The reproductive isolation between 2X and 4X is post-zygotic and can be broken down and introduced by colchicine treatment. The studied diploid and tetraploid populations shared the same alleles for nine allozyme loci. Together this provides strong evidence for autotetraploid origin of tetraploid P. media. Fertility of colchicine tetraploids was comparable to natural tetraploids and slightly lower than natural diploids. In contrast to colchicine tetraploids, natural tetraploids did not show gigas-characteristics. Tetraploid hybrids between diploids and natural tetraploids showed partial gigas-characteristics. The possible reasons for the differences in cell size between cytotypes are discussed. Differentiating characters between the natural cytotypes were not affected by colchicine treatment and are therefore not a direct consequence of polyploidization.The hypothesis of autotetraploidy in Plantago media (2n = 12 and 2n = 24) was tested. The two cytotypes are morphologically very similar. Rare 4X and 3X hybrids were obtained in inter-ploidy crosses. The reproductive isolation between 2X and 4X is post-zygotic and can be broken down and introduced by colchicine treatment. The studied diploid and tetraploid populations shared the same alleles for nine allozyme loci. Together this provides strong evidence for autotetraploid origin of tetraploid P. media. Fertility of colchicine tetraploids was comparable to natural tetraploids and slightly lower than natural diploids. In contrast to colchicine tetraploids, natural tetraploids did not show gigas-characteristics. Tetraploid hybrids between diploids and natural tetraploids showed partial gigas-characteristics. The possible reasons for the differences in cell size between cytotypes are discussed. Differentiating characters between the natural cytotypes were not affected by colchicine treatment and are therefore not a direct consequence of polyploidization.

Genetic variation in Phoca vitulina (the harbour seal) revealed by DNA fingerprinting and RAPDs

Genetic variation in two harbour seal (Phoca vitulina) populations from the Dutch Wadden Sea and Scotland was examined by RAPD analysis and DNA fingerprinting. For comparison a population of grey seals (Halichoerus grypus) was studied. The RAPD method revealed a very low number of polymorphic bands. The multilocus DNA fingerprinting technique showed very low levels of variation both within and between populations of the harbour seals. For one probe a significant difference in variation between populations was demonstrated. The estimated average heterozygosity, however, diverged more strongly between the two harbour seal populations because of more frequent occurrence of rare alleles in the Scottish population. Low levels of genetic variation seem to be characteristic for the subspecies Phoca vitulina vitulina, but not for all marine mammals in the North Sea, as the grey seal population showed significantly higher variation.

Worldwide latitudinal clines for the alcohol dehydrogenase polymorphism in Drosophila melanogaster: What is the unit of selection?

Geographical clines may reflect the action of natural selection on genetic polymorphisms. In Drosophila melanogaster several latitudinal clines occur for many characters like allozymes, inversions and quantitative traits. The identical nature of these clines on the various continents, both on the Northern and Southern Hemispheres strongly suggests adaptation to specific stress factors. The alcohol dehydrogenase (Adh) polymorphism shows high frequencies of the S allele in tropical regions and declines with latitude. The reasons for this cline are difficult to determine because of the entanglement with other polymorphisms varying with latitude. In this paper the tentative connections with other polymorphisms like alpha-Gpdh, In(2L)t, body size and development time are reviewed with respect to the possible environmental stress factors involved. It is concluded, also from recent experiments, that the (2L)t inversion plays a dominant role in resistance to high temperature and is partly responsible for the Adh cline. Further research is aimed at the specific selective forces acting on Adh, focussing on the physiological and life history aspects.

Genetic analysis of ecological relevant morphological variability in Plantago lanceolata L. IV response and correlated response to bidirectional selection for leaf angle

Bidirectional selection for leaf angle was imposed for four generations in a population of Plantago lanceolata. A fifth generation was made by intra- and interline crosses. The population originated from an extremely wet hayfield with a high and dense vegetation. A high level of additive genetic variability was indicated by a realized heritability of 0·42. From the correlated responses realized coheritabilities and genetic correlations were calculated. Several genetic correlations, positive and negative, between leaf angle and other characters were found. The negative correlation of flowering date and leaf angle may retard further evolution of these correlated characters as this is the opposite of what the conditions in this habitat require, namely early flowering plants with a small leaf angle. The fifth generation crosses revealed a substantial directional dominance in the direction of smaller leaf angle.

Genetic variation for oviposition behavior inDrosophila melanogaster. I. Quantitative genetic analysis of insertion behavior

The genetic basis of egg insertion behavior was analyzed in twoDrosophila melanogaster strains (N and E strain), which were homozygous for theSlow allele of the alcohol dehydrogenase locus. The E strain had been selected for increased tolerance to ethanol. This study originated from the observation that nearly all of the eggs laid by females of the E strain were inserted, while in the control strain (N strain) which was kept on regular food, considerable variation for egg insertion occurred. Crosses between the E and the N strains were made, and quantitative genetic tests were performed. It was shown that the inheritance of egg insertion is polygenic, some of the genes are probably sex linked, and in-laying is partly dominant over on-laying. The heritability of the trait, derived from mother-daughter regression analysis, was .59±.18. The repeatability was high. Bidirectional selection in the E strain was unsuccessful, but an asymmetrical response to selection was obtained in the N strain and in lines derived from crosses between the N and the E strains. Selection for in-laying was more successful than for on-laying. The realized heritability in the former lines was .35±.10.