Science in Christian Perspective

BIOLOGY
Irving W. Knobloch, Ph.D.

The Role of Chromosome Rearrangements In Evolution

In the last issue we assessed the part played by gene mutations in evolution. It was pointed out that the term "mutation" is used quite broadly in the literature and that many so-called "mutations" are not point or gene mutations but are due to other phenomena. The difficulty of pinning a point mutation down was also indicated. We might add here the definition of point mutations as given by Stadler "- (point mutations) are those alterations in the germ plasm for which a mechanical chromosome basis cannot be detected."

In this article we wish to cite some evidences of changes in phenotypes originating by chromosome rearrangements. Goldschmidt, by the way, regards this method as the fundamental agency of phenotypic differentiation.

There are four main types of chromosome rearrangements recognized. A deletion or deficiency involves the loss of a portion of a chromosome and, at synapsis, one of the homologous chromosomes has a loop in it where synapsis could not occur because of the deficiency. A duplication comes about when a certain chromosomal segment with its genes is repeated. An inversi on would refer to a case where a segment is found in an inverted order to that normally present. A translocation, as the name implies, is the transfer of a part of a chromosome to a non-homologous chromosome. Such cases are usually reciprocal. None of the above types are point mutations. What evidence is there that rearrangements are important in speciation?

Dubinin found that the "hairy gene" in Drosophila a normal chromosome but dominant

This sort of thing is acts deceptively like a is due, however, to a transthat the fish Misgurnus pairs of chromoso es and was recessive in in a translocated chromosome. called a "position effect" and point mutation. The effect location. Makino found anguillicaudatus has 26 Barbatula oreas has 24 pairs. All the chromosomes in both fish are rod-shaped except two pairs in B. areas which are V-shaped. He took this to mean that the phenotypic differences between the fish were due to translocations in what were originally distinct chromosomes.

Sturtevant and Morgan showed that in cross-over experiments between other genes near the "bar eye' gene, the so-called mutation from "bar-eye" to "double bar-eye" and to "normal-eye" were due to unequal crossin-over, i.e., a duplication. Individuals with one bar gene were "normal", those with two bar genes were "bar" and those with three such segments in succession were "double bar". In the fly Sciara, two of the species, ocellaris and reynoldsi are said to differ from each other because of both deficiencies and duplications. McClintock, in work on maize, reported that many types of variegated brown midrib plants were produced because of a duplication of factors. 

Some workers in Drosophila lean to the opinion that the mutant 'notch wing" is due to a deficiency in the X chromosome and that "curly" and "dichaete' wings are due to inversions of the second and third chromo somes respectively. Dr. F. A. Saez noted that "notched wing" is found in chromosome I and is produced by the loss of a segment of 1.5 units: it is lethal in the males and, when homozygous, also in the females. Thus the flies which continue living with this deficiency are heterozygous." He goes on to say that "the species of Drosophila differ among themselves according to the different arrangement of their genes which have re sulted from successive rearrangements of the chromo somes during the course of their phylogeny." Dr. G. L. Stebbins in his great work cited below says "It is pos sible that a considerable proportion of the genetic changes that have been regarded as point mutations are actually minute deficiencies."

In view of the above evidence about chromosome rearrangements it seems fair to state that they probably play a very important role in the formation of what we call species. Point mutations, assuming that they exist, are another method of speciation. Rearrange ments are being detected more frequently now than formerly and, in my opinion, their importance will in crease. It should be plain that it is not necessary to have new genes to have new species. In a subsequent article, we plan to discuss the role of polyploidy in evolution.