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Modern Genetic Analysis Chapter 8. Chromosome Mutations
Evolution of the Genome

Different phylogenetic groups differ considerably in chromosome number and structure. In some cases, there are differences between closely related groups, and there can even be normal variation within or between populations of one species. In this section, we consider some of the ways that chromosome mutations contribute to the evolution of genomes. We are now in a position where we can begin to explore and describe the comparative molecular anatomy of genomes.

Chromosomal Polymorphism

Some natural populations show two or more chromosomal forms, a situation called chromosomal polymorphism. Drosophila, because of its polytene chromosomes, has been a favorite organism for the study of natural chromosomal variation. In particular, Drosophila shows abundant polymorphism for chromosomal inversionsspecifically, paracentric inversions. Inversion polymorphism is so common that determining the "wild-type gene order" in a species of Drosophila is quite arbitrary. There are many different gene orders, depending on the inversions present in a given individual fly. Inversion polymorphism might be prevalent because it allows genes in an inverted region to coevolve, because inversion heterozygosity blocks the production of viable crossover gametes. Other kinds of chromosomal rearrangement polymorphism seem to be rarer, because they tend to be more deleterious than inversions.

Chromosomal Changes and Speciation

How do chromosomal mutations contribute to speciation? No general answer can be given to this question at present. However, we can describe the kinds of differences that we see between species, and these differences provide clues about the types of events that might underlie speciation.

First, consider gene families. Some types of genes are found in several copies per hapoid genome; such groups of clearly related genes are called gene families (see Chapter 12). In some phylogenetic comparisons the number of copies of certain gene families varies in interesting ways. For example, there are frequently four times as many members of a given gene family in mammals as there are in primitive chordates (a group considered to be mammalian ancestors). This finding has led to the suggestion that the primitive chordate genome underwent two successive genome doublings in the evolutionary lineage that eventually formed the mammals. This is another example of how polyploidization might have contributed to speciation.

Second, many chromosomal regions appear to have arisen through tandem duplication. In any case of duplication (and indeed wholesale polyploidization), one of the duplicates is free to undergo gene mutation because the necessary basic gene functions will be provided by the other copy. This provides an opportunity for divergence in the function of the duplicated genes, which could be advantageous in genome evolution. Indeed, in groups of adjacent genes with closely related functions, such as the human globin genes (Figure 8-29a on the following page), it seems certain that these genes arose as tandem duplicates (Figure 8-29b).The linked members of the gene family are similar enough that crossing-over can occur between them, resulting in higher-order duplication and deletion (Figure 8-29c).

MESSAGE
Duplication and polyploidization supply additional genetic material capable of evolving new functions.

Chromosomal Synteny

Inversions and translocations also are notable in comparisons of different species, but the evolutionary roles fulfilled by these kinds of chromosomal rearrangements are not well understood. Perhaps gene fusions that take place at the end points of such rearrangements are occasionally of selective advantage to an organism, and so the rearrangement producing such a gene fusion would then be maintained. Regardless of the particular properties that might be under evolutionary selection, it is a fact that we are now encountering many examples in which related species differ by some number of rearrangements. Consider humans and mice. The human karyotype includes 23 pairs of chromosomes, the mouse only 20 pairs. Sufficient mapping of the same genes from mouse and humans has been done to see that many blocks of closely linked genes in humans are in closely linked blocks in the mouse as well. Such blocks of genes are said to be syntenic. A synteny map shows how the chromosomal locations of the blocks in one species relate to the comparable blocks in another (Figure 8-30 on page 261). Between 50 and 100 different inversion and translocation events have mixed and swapped these blocks, to enable them to reside in their present arrangements in humans and mice.

MESSAGE
The study of synteny shows that chromosomal rearrangements have been instrumental in evolution at the chromosome level.

© 1999 by W. H. Freeman and Company. All rights reserved.