Abstract
Transposable elements are characterized by their ability to spread within a host genome. Many are also capable of crossing species boundaries to enter new genomes, a process known as horizontal transfer. Focusing mostly on animal transposable elements, we review the occurrence of horizontal transfer and examine the methods used to detect such transfers. We then discuss factors that affect the frequency of horizontal transfer, with emphasis on the mechanism and regulation of transposition. An intriguing feature of horizontal transfer is that its frequency differs among transposable element families. Evidence summarized in this review indicates that this pattern is due to fundamental differences between Class I and Class II elements. There appears to be a gradient in the incidence of horizontal transfer that reflects the presence of DNA intermediates during transposition. Furthermore, horizontal transfer seems to predominate among families for which copy number is controlled predominantly by self-regulatory mechanisms that limit transposition. We contend that these differences play a major role in the observed predominance of horizontal transfer among Class II transposable elements.
Highlights
Transposable elements include a diverse array of DNA sequences that share an inherent property of mobility
A more dramatic example of the potential for increase in copy number accompanying horizontal transfer was found for the mariner element in some host species
P elements are significantly more active in D. melanogaster, reaching a higher copy number per cell than in D. simulans. These results suggest that, in addition to the transposase, host-specific factors are necessary to support P element mobility at a sufficient level to ensure its spread and subsequent persistence in a species, once it has been introduced by horizontal transfer
Summary
Transposable elements include a diverse array of DNA sequences that share an inherent property of mobility. A second example in which horizontal transfer of the canonical P element has been supported by all three detection methods was reported recently involving a member of the willistoni species group (possibly D. nebulosa) and a distantly related species, D. mediopunctata (Loreto et al, 2001; Figure 1).
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