D. L. Palenzona

Karyotypic variability in plants of Solanum melongena regenerated from callus grown in presence of culture filtrate of Verticillium dahliae.

SummaryPlantlets were regenerated from calli derived from leaf expiants of three genotypes of Solanum melongena (two parental genotypes and their hybrid). The cytological analysis showed that a) plants regenerated were all mixoploid, b) toxic medium (basal medium added with filtrate culture of Verticillium dahliae) was able to evidence karyotypic differences between genotypes not displayed by plants regenerated from callus grown on control medium, c) chromosomal mosaicism persists up to plant maturity and also in the selfed progeny. The results are discussed in terms of a selective process involving genes controlling chromosome number and/or a direct effect of toxic medium on the activity of the same genes.

The relationship between fitness and response to selection in Drosophila melanogaster

SummaryThe relationship between the selection response exhibited by a morphological trait and the correlated response observed on fitness values has been investigated to improve our understanding of micro-and macro-evolutionary processes. The research was designed to test the validity of the “homeostatic” and “metric deviation” models (Robertson 1956). The results show that:1)correlated changes in fertility values are larger when selection for minus-variant wing length is applied to males than when it is performed on females,2)within each of the selected lines, M, F and MF, the reproductive fitness of the populations obtained from vg/vg x +/vg crosses differs from that of the populations derived from +/vg X vg/vg crosses,3)reproductive fitness values of populations derived from vg/vg X +/vg crosses are fairly constant in selected lines obtained by means of different selective procedures, while mean wing length changes; the corresponding populations from +/vg X vg/vg crosses show, on the contrary, a proportionality between fitness and selected trait values. The lack of consistency of these results with the currently accepted hypotheses on the relationship between selection response and correlated changes in fitness leads to the conclusion that the two models considered do not satisfactorily interpret our results; these are better explained by a developmental relationship between fitness and selected trait.

Differential mutagenic response in selected lines of Drosophila melanogaster

SummaryThe mutagenic efficiency of ionizing radiations has been tested on different lines of Drosophilamelanogaster. It has been shown that differential lethal effects are obtained when irradiated females from different lines are mated to flies carrying heterozygous lethal genes. The results seem not to be attributable to differential expression of the lethality in the various crosses performed with the irradiated flies. This might suggest that gene activity is involved in the expression of the mutagenic effects of radiations.

Selection and recombination in Drosophila melanogaster

SummaryArtificial selection for wing length in Drosophila melanogaster resulted in changed crossing-over frequencies between three marker genes on the 2nd chromosome, b, cn and vg.The results suggest that artificial selection is a causal agent in producing the observed changes; moreover it is suggested that the modifications in cross-over frequency are controlled by extra-nuclear factors.

FITNESS AND GENETIC HETEROGENEITY IN LABORATORY POPULATIONS WITH OVERLAPPING GENERATIONS

SUMMARY Three populations of Drosophila melanogaster Meig. (A, B, C) were reared in conditions of overlapping generations together with the three populations (A × B, A × C, B × C) derived from the crosses between them. The number of adult individuals present in each population was scored at 7 day intervals, as an estimate of fitness; wing length was measured at the end of the experiment. The comparison between A × B, A × C, B × C populations and the lines from which they were derived, suggest the existence of gene complexes controlling fitness values; these gene complexes seem to be built on specific gene interactions in the different populations considered. This is suggested by the lower than expected fitness values observed in populations A × B, A × C, B × C and by a large wing length variability. These results are discussed in connection with the mechanisms underlying population dynamycs.

RECOMBINATION RATE IN SELECTED LINES OF DROSOPHILA MELANOGASTER MEIG

SUMMARY The recombination frequency between three genes of the second chromosome of Drosophila melanogaster Meig. (black, cinnabar, vestigial) was estimated in two lines selected for short wing (C and CF) and in a cross between them (F1). The recombination rate in the selected lines was different from that in the Canton strain from which they were derived; moreover, the two selected lines differed between themselves, while the recombination rate in the F1 population was similar to that observed in the parental line C. These results suggest that the action of the factors which modify the recombination in the C line are «dominant» over those in the CF line. The results obtained have relevance in the study of population dynamics: it is argued that selection and recombination are part of an auto-regulating complex; it is also suggested that the «dominance» effect of the factors involved in recombination control may provide a «dynamic barrier» which causes the gene pools of the two populations to behave differ...

GENETIC ANALYSIS OF SEXUAL DIMORPHISM IN DROSOPHILA MELANOGASTER

SUMMARY Sexual dimorphism in Drosophila melanogaster is changed by selecting for wing length on females while a similar selection on males does not produce detectable variations. The genetical analysis performed by means of backcrosses to the parental strains suggests that changes occur in selection on males too, but they do not appear in the selected lines. The observed values of sexual dimorphism in the selected lines are transmitted to the B1 and B2 progeny only by the males. It is hypothesized that sexual dimorphism depends on the interaction of factors carried by the Y chromosome, the second chromosome (vg+ locus) and extra-nuclear. A model of the mechanism of interaction is proposed which may be of importance in understanding evolutionary processes.

Univariate and multivariate analysis on phenotypic divergence in Phleum

SummaryAn analysis of phenotypic and genetic variability was performed on Phleum ambiguum populations obtained by vegetative propagation and grown in different environments. The investigation on a single character and the canonical analysis on several plant traits indicate that, while genetic variability (h2) is essentially constant, phenotypic variance and covariance are affected by different environments and successive clonations. The observed changes seem not to be reversible when plants, transferred to a different environment, are returned to the original one. Within-clones variability seems to be affected by environmental conditions without being directly related to them. The results reported seem to be in good agreement with the phenotypic divergence hypothesis.

Interaction between artificial and natural selection.

SummaryIn order to understand how divergence may appear within a gene pool without limitations on gene flow, experiments were performed to investigate whether the genetic structure of a population may be effective in controlling the response to natural selection. Starting from plateaued populations of Drosophila melanogaster selected for wing length on the two sexes separately, the response to natural selection was studied after artificial selection had been discontinued. The results show that populations derived from artificial selection applied to the different sexes respond to subsequent natural selection in dissimilar ways, so as to suggest that the relationships between fitness and wing length have been changed. A second experiment comparing the response to natural and to artificial reversed selection suggests that different genes or gene complexes may be involved in wing length determination: these different genes show a dissimilar response in the different sexes and to natural and artificial selection. This variety of responses suggests that the interactions of different genes or gene complexes with artificial and natural selection could possibly lead to differentiation within the population.

A multivariate analysis of dominance inDrosophila

SummaryEleven wing measurements in twoDrosophila melanogaster lines (Canton and FLL) and their reciprocal crosses were analyzed by a multivariate technique in order to investigate changes in variability when dominance is manifested. FLL line was obtained by artificial selection for short wing applied on one measurement on the rightwing. Differences between left and right wing were observed in variabilities after selection. The comparison between F1 and parental populations shows that there is strong evidence for non-additivity which is manifested by a component of the variability due to multiple regression. This is interpreted as meaning that relationships between characters are affected in the heterozygote in a non-additive way, apparently due to interactions among the developmental patterns of the characters considered. It is suggested that a “character” should be defined in such a way as to include its developmental patterns, in order to achieve a better understanding of the dominance phenomenon.