Evolution now : a century after Darwin / edited by John Maynard Smith.

Date:
1982
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Evolution now : a century after Darwin / edited by John Maynard Smith. Source: Wellcome Collection.

    95/256 page 85
    THE EVOLUTION OF THE GENOME 85 X. laevis possesses a second a/3-globm gene cluster that codes for minor adult globins^®. This cluster appears to have arisen by tetraploidisation in an ancestor of X. laevis, and a contemporary equivalent of this ancestor, X. tropicalis, has a single aß cluster as expected. Thus chromosome duplication has generated globin diversity in X. laevis. The importance of chromosome duplication and polyploidisatíon in vertebrate evolution has been emphasised repeatedly'^'^''^. Human globin gene clusters still retain the capacity for expanding and contracting their numbers of genes by unequal crossing over. The fused öß-globin polypeptide in haemoglobin Lepore has been shown, by direct DNA analysis, to be the product of unequal crossing-over between 8- and /3-globin genes^^. Similarly, haemoglobin Kenya probably results from unequal crossing-over between ^y- and /3-globin genes (see Jones et alJ*). Chromosomes carrying three a-globin genes or a single a-globin gene have been detected in man^®'' and the chimpanzee^. These three- gene and one-gene chromosomes appear to be the reciprocal products of unequal crossing-over between duplicated a-loci. The point of crossover need not necessarily be within an a-globin gene, since each a-globin gene resides within an extensive (4 x 10^ base-pair) tandem repeat^® '®. Thus, the duplicated a-locus probably arose by tandem duplication of a large region containing an a-globin gene. The mechanism for this dupli¬ cation is unknown, although unequal crossing-over between sequences repeated throughout globin gene clusters®®-^^ could have been re¬ sponsible. By this argument, the initial spacing of gene duplicates would be dictated largely by the location of dispersed repetitive sequences relative to globin genes. PSEUDOGENES Recent analyses of globin gene families have revealed the existence of additional gene sequences that have accumulated mutations and no longer function as active globin genes. These additional pseudogenes have all been detected by cross-hybridisation with active globin gene sequences. The human ß-globin gene cluster contains two t/>/3 sequences, one at the 5' end of the cluster and one between the ^y- and ô-globin genes'*'®^ (see Fig. 1). The sequences of these i/ijS genes have not been published, and thus their true pseudogene status is not clear. These sequences are probably also present in apes and Old World monkeys^^. The rabbit yÖ-globin gene cluster contains a фß2 pseudogene between the /33 (y-like) and /31(/3-like) globin genes''^ (see Fig. 1). Pseuodogenes are also seen (at probably similar positions) in the yS-globin gene cluster of the mouse^*, lemur^^ and goat^'. An inactive a-globin pseudogene (фу!) has been found between the embryonic and adult a-globin genes in man^®. The extraordinary mouse a-globin pseudogene, which has lost both inter¬ vening sequences^® has already been described, although its linkage arrangement to functional a-globin genes has not been reported. Several globin pseudogenes have been sequenced completely. The
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