In this article we will discuss about the inversions in natural populations which is caused when the genes in a section of a chromosome is reversed.
Inversions occur spontaneously in a large number of plant and animal species. But the most extensive work has been done in Drosophila in which they have been detected in a rather high frequency. In certain species, such as, D. willistoni, they occur more frequently than in others. Bridges and Brehme in 1944 listed a total of 126 inversions in D. melanogaster.
Of these, 112 were paracentric, while 14 were pericentric inversions. The occurrence of pericentric inversion was more frequent in chromosome 2, whereas paracentric inversions were preponderant in chromosomes 1 and 3. Paracentric inversions are restricted to the chromosome 3 of D. pseudoobscura and D. persimilis.
They occur on the X chromosome also. There are certain species where paracentric inversions are practically absent, e.g., in D. virilis and D. hydei.
Complex inversions have been observed in the wild populations of Drosophila, especially in D. pseudoobscura. Extensive studies of inversion in this species were made by Dobzhanzky. The frequencies of inversions in natural populations vary according to the season.
Chromosome pairing in complex inversions shows specific configurations for the different types, such as, independent, overlapping and included inversions (Fig. 15.14).
The karyotype is also changed due to inversions as a result of which they give rise to chromosomal polymorphism. Such polymorphism has been reported in about 30 species of Drosophila. Different Drosophila lines have evolved through gene rearrangements due to inversion.
The species D. pseudoobscura and D. persimilis have several gene arrangements for chromosome 3. The gene arrangement common to both species is called STANDARD. Almost all of these inversions were found to be overlapping inversions and were related to each other.
Using data from such studies, it was possible to construct a phylogenetic relationship between the various gene arrangements on chromosome 3 found in the natural populations of D. pseudoobscura and D. persimilis as shown by Dobzhansky in 1951.
Each arrangement was designated by the name of the geographical locality in which it was first found. It is assumed that either “standard” or “hypothetical” or “Santa Cruz” was the ancestral arrangement and from one of these, evolution of the gene arrangement on chromosome 3 proceeded.
One of the arrangements remains hypothetical as far as D. pseudoobscura and D. persimilis are concerned. But a related species D. miranda has shown the gene arrangement possessing the properties of this hypothetical arrangement.
Chromosome polymorphism due to inversion occurs in several animal species. Several grasshoppers, such as Trimerotropisgracilis, Keyacrisscurra, Aiolopusstrepens show stable polymorphisms due to pericentric inversions in chromosome. Stable polymorphism is maintained by heterosis and by the lower fitness values of the inversion homozygotes than that of the standard homozygotes.