A. Brito Da Cunha

Chromosomal polymorphism in the Diptera.

Publisher Summary In this chapter, chromosomal inversions present in natural populations of Dipters have been shown to be heterotic in heterozygous conditions in all cases in which tests have been made. Even in cases in which no tests have been carried out, there are good indications that the inversions are important in the adaptedness of their carriers. The adaptive significance of the chromosomal inversions is due to the complexes of genes that they hold together. The function of the inversions is to tie together adaptive gene combinations. The heterosis produced by chromosomal inversions develops through natural selection by means of formation of coadapted gene combinations. As the heterozygotes have adaptive values higher than the homozygotes, the populations become genetically variable and acquire a high degree of ecological plasticity.

Adaptive Polymorphism of Behavior Developed in Laboratory Populations of Drosophila willistoni

Laboratory populations of Drosophila willistoni kept in population cages developed an adaptive behavioral polymorphism. The larvae were able to survive only on the food when the populations were first established. Larvae able to survive outside the food cups appeared with time and became frequent in the populations. The genetic difference between larvae that prefer to pupate on the food and larvae that prefer the bottom of the cages is simple and due mainly to a single major gene difference. Outside and inside larvae differ in their preferences for a place to pupate, in their rates of development, in number of bristles, in weight, in degree of activity and phototaxis, and in eclosion rate. Polymorphic populations with both types of flies reach larger sizes and greater biomass than monomorphic populations. The genes of preference for pupation outside the food are rapidly eliminated in populations kept in 250-cm3 bottles.

A comparative study of chromosomal polymorphism in certain south american species of Drosophila

A comparative study of chromosomal polymorphism in certain south american species of Drosophila

Recombination in Drosophila willistoni

KNOWLEDGE of Drosophila genetics was acquired mainly through work on Nearctic species. Conclusions reached from studies of these species are frequently extended to those of other regions. Biological generalisations are hazardous especially when they involve adaptive characters of organisms living under different conditions. This is especially true for genetic systems which, owing to their crucial adaptational importance, must be highly sensitive to natural selection. The comparative analysis of the genetic systems of related species living in different environments is necessary before generalisations can be made and this is one of the ways of understanding their adaptive meaning and evolutionary origin. Recombinant chromatids produced by crossing-over within relatively inverted segments are eliminated. Data obtained in Nearctic species of Drosophila show that heterozygous inversions also suppress effective crossing over outside the inverted segments. This is well known for inversions like Payne and Curly of D. melanogaster and for inversions in D. pseudoobscura, .D. persimilis, D. virilis and particularly of D. robusta (review in Carson, 1953). No recombination was found by Carson (1953) within or between two heterozygous inversions in opposite arms of the second chromosomes of D. robusta and separated by a distance corresponding to 376 per cent, of the chromosome length. Similar results were obtained for inversions in the X chromosome. On the basis of results obtained with Nearctic species, it is generally believed that there is no crossing over at spermatogenesis in Drosophila. The absence of crossing over in D. melanogaster males has been well known since Morgans work (1914), which has been confirmed by many authors. Patterson and Suche (1934), for example, found only one recombinant among 8329 third chromosomes marked from tip to tip with eight recessive

On lethals and their suppressors in experimental populations of Drosophila Willistoni

Abstract Naturally occurring and radiation-induced lethals were studied in experimental popu populations maintained in population cages. The populations were started with four lethals, two wild and two induced, each of them having frequency of 0.25. The lethals still had frequencies from 0.03 to 0.18 when the populations were 348 days old. The analysis of the lethals showed that all of them were able to survive in homozygous condition. The survival of the flies homozygous for the lethals was due to the presence of recessive suppressors. The proportion of the lethals accompanied by theirs own suppressors was measured in a sample taken when the populations were 866 days old and varied from 0 to 58.3%. The implications of the presence of suppressors regarding the behaviour of lethals in populations are analysed and discussed.