Rita Alicchio

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.

Phenotypic variability and divergence in disruptive selection.

SummaryTwo types of disruptive selection (HL and LH) were performed on three lines (M, F and K) which had reached different plateaus, following a previous directional selection applied independently on the two sexes. Increase in phenotypic variability and the appearance of a bimodal frequency distribution of wing length measurements were considered as responses to disruptive selection.These responses are shown not to be linearly related; moreover both responses seem not to be related to the genetic and phenotypic variability present in the population when disruptive selection was started.These results and the persistent differences between LH and HL selections are interpreted as to suggest that the effects of disruptive selection are mainly dependent on changes in the developmental patterns of the genes involved and of their connections.The suggestion is advanced that the results presented are inconsistent with the assumption that the effects of disruptive selection be explained by genetic or developmental switch mechanisms or by chromosomal polymorphism.

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.

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.