Evolving Sideways Horizontal Gene Transfer

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Standard Darwinian evolution involves alterations in genetic information passed on from one generation to its descendants. However, it is also possible for genetic information to be passed "sideways" from one organism to another that is not one of its descendents or even a near relative. The term vertical gene transfer refers to gene transmission from the parental generation to its direct descendants. Vertical transmission thus includes gene transmission by all forms of cell division and reproduction that create a new copy of the genome, whether sexual or not. This contrasts with "horizontal gene transfer" (also known as "lateral gene transfer") in which genetic information is passed sideways, from a donor organism to another that is not its direct descendent.

For example, when antibiotic resistance genes are carried on plasmids they can be passed between unrelated types of bacteria (see Ch. 16). Since genes carried on plas-mids are sometimes incorporated into the chromosome, a gene can move from the genome of one organism to an unrelated one in a couple of steps. The complete genomes of many bacteria have now been fully sequenced. Estimates using this data suggest that about 5-6% of the genes in an average prokaryotic genome have been acquired by horizontal transfer. The effects of horizontal transfer are especially noticeable in a clinical context. Both virulence factors and antibiotic resistance are commonly carried on transmissible bacterial plasmids.

horizontal gene transfer Transfer of genetic information "sideways" from one organism to another that is not directly related lateral gene transfer Movement of genes sideways between unrelated organisms. Same as horizontal gene transfer vertical gene transfer Transfer of genetic information from an organism to its descendents

Problems in Estimating Horizontal Gene Transfer 565

FIGURE 20.28 Horizontal Transfer of Type-C Virogene in Mammals

The type-C virogene was present during evolution of old world monkeys from their common ancestor. Surprisingly, a version of this gene closely related to the one in baboons was identified in North African and European cats. Since baboons and cats are not closely related, the gene must have moved from one group to another via horizontal transfer. Further supporting the idea of horizontal transfer, the gene is not found in cats like the lion or cheetah, which developed before the North African and European cats branched off.

Several species of baboons


European wildcat cat

African wildcat



/African ^^ wildcat

Retroviral transfer ^^

Sand cat

Blackfooted cat




Horizontal gene transfer usually involves viruses, plasmids or transposons.

Such horizontal transfer may occur between members of the same species (e.g. the transfer of a plasmid between two closely related strains of Escherichia coli) or over major taxonomic distances (e.g. the transfer of a Ti-plasmid from bacteria to plant cells). Horizontal gene transfer over long distances depends on carriers that cross the boundaries from one species to another. Viruses, plasmids and transposons are all involved in such sideways movement of genes and have been discussed in their own chapters (see Chs. 15-17). Retroviruses, in particular, are capable of inserting themselves into the chromosomes of animals, picking up genes and moving them into another animal species.

One well-described example of horizontal transfer in animals concerns the type-C virogene shared by baboons and all other Old World monkeys. The type-C virogene was present in the common ancestor of these monkeys, about 30 million years ago, and since then has diverged in sequence just like any other normal monkey gene. Related sequences are also found in a few species of cats. Only the smaller cats of North Africa and Europe possess the baboon type-C virogene. American, Asian and Sub-Saharan African cats all lack this sequence. Therefore, the original cat ancestor did not have this type-C virogene. Furthermore, the sequence found in North African cats resembles that of baboons more closely than the sequences in monkeys closer to the ancestral stem (see Fig. 20.28). This suggests that about 5-10 million years ago a retro-virus carried the type-C virogene horizontally from the ancestor of modern baboons to the ancestor of small North African cats. The domestic European pussycat originally came from Egypt, so it also carries the type-C virogene. However, other cats that diverged more than 10 million years ago lack these sequences.

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