UBC Theses and Dissertations
Concerted evolution of a cluster of X-linked tRNA4 7 genes from Drosophila melanogaster Leung, Jeffrey
Multigene families have posed an acute problem for evolutionary biologists ever since the revelation that many families exhibit unexpected sequence homogeneity within and between individuals of a species. A family that is shared between several species, in contrast, often reveals substantial heterogeneity between them. This cohesive and species-specific pattern of variation, which disengages from the classical mode of random genetic drift and selection, has been formally described as Molecular Drive (Dover, 1982). Based on initial observations (Cribbs 1982), the tRNA₄Ser and tRUA₇Ser genes on the X-chromosome of Drosophila melaaogaster also showed intriguing characteristics reminiscent of Molecular Drive. However, in this unusual case, the coevolution process would not only encompass the individuals within a family, but would also ensnare members from a different family. This thesis is an in depth study on the concerted evolution of both gene families and provides evidence consistent with the view that they are undergoing Molecular Drive. Eight tRNA₄,₇Ser genes have been cloned from bands 12DE on the X-chromosome of D metanogaster by molecular walking. There are two tRNA₄Ser and two tRNA₇Ser genes that contain sequences expected from their known tRNAs (Cribbs et. al., 1987a). Of the 86 nucleotides, they only differ from each other at positions 16, 34 and 77 (non-standard numbering, see Sprinzl et al., 1987). The difference at position 34 corresponds to the anticodon and accounts for their difference in codon recognition. These genes have been designated as either 444 or 777 genes, based solely on the three diagnostic differences. However, there is also a single 474 and two 774 genes, which are recombinant structures of the bona fide genes. The remaining gene, 444*, has the three nucleotides diagnostic of tRNA₄Ser but contains a mutation at the tip of the extra arm. Thus collectively, the entire caste of tRNA₄,₇Ser genes at 12DE forms a graded series of transitional states, bridging the narrow sequence variability between true tRNA4Ser and tRNA7Ser. Flanking sequences of these hybrid and the 444* genes show segmental homologies related to both the 444 and 777 genes within the cluster, again a strong indication that both gene types are undergoing concerted evolution. Examination of selected genes from two distantly related sibling species, D, erecta and D. yakuba, shows their equivalent flanking sequences have diverged from those of melanogaster. As expected, the base changes in these species, often occurring as clusters, are also non-random and appear to have been propagated to certain respective members to maintain a species-specific and cohesive pattern of variation consistent with Molecular Drive. One possible mode of spreading sequence variation and creating the hybrid genes in the process could involve an initial stage of asymmetric pairing between 444 and 777 DNA. To examine this possibility, a tRNAArg gene cluster also from 12DE was conveniently exploited as independent "monitors". This family shows fluctuations in the number of genes among the different species and strains (Newton, unpublished), which could also be explained by asymmetric pairing of DNA followed by unequal exchange. Thus, even though the tRNAArg and tRNA₄,₇Ser genes have embarked on different evolutionary pathways, both phenomena may be explained by their common susceptibility to local asymmetric pairing of DNA.
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