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Cytology and Phylogeny of Drosophila

The rediscovery of the polytene chromosomes in the salivary glands of the Diptera has allowed for a high order of accuracy in the identification of chromosomal rearrangements. The various types of chromosomal mutations are described and their relative importance in the evolution of the genus Drosoph...

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Bibliographic Details
Published in:The American naturalist 1963-09, Vol.97 (896), p.333-352
Main Author: Wasserman, Marvin
Format: Article
Language:English
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Summary:The rediscovery of the polytene chromosomes in the salivary glands of the Diptera has allowed for a high order of accuracy in the identification of chromosomal rearrangements. The various types of chromosomal mutations are described and their relative importance in the evolution of the genus Drosophila is briefly discussed. Paracentric inversions are the most common changes observed; and these have been used in the construction of phylogenetic trees; the repleta group where the relationships among 46 species have been determined, is used as an example of such a study. Errors in the construction of the phyletic lines resulting from observational mistakes, cytological parallelism, cytological convergence and the segregation of independent inversions are discussed. Experimental work on naturally occurring paracentric inversions indicates that the selective values of these rearrangements appear not to be correlated with their actual breakage points, but are determined by the genetic material tied together by the action of the inversion as a suppressor of recombination. The genetic content of a rearrangement varies from locality to locality: each inversion, per se, therefore, has no definable adaptive value. Survival of a newly arisen inversion is dependent upon its fortuitous occurrence in a chromosomal region which is preadapted for a recombinant suppressor, that is, the region contains cooperating loci, among which recombination would be disadvantageous. Moreover, given the conditions for preadaptation, any one of a large number of distinguishable rearrangements could equally serve the purpose. In the repleta group, only about 144 inversions have survived during the history of the 46 species studied, indicating a very low incidence of such preadaptation in this species group. In view of the above, it is felt that the occurrence and survival of a pair of indistinguishable inversions in two phyletic lines, although possible, is highly unlikely. The probability of cytological parallel or convergent evolution involving two or more indistinguishable pairs of inversions in two phyletic lines can be considered zero. Phylogenies, therefore, based on inversional changes have an exceedingly high degree of accuracy. Phylogenetic relationships among the repleta group species are shown and their evolution is briefly discussed.
ISSN:0003-0147
1537-5323
DOI:10.1086/282284