How to establish a mutually beneficial relationship between a transposon and its host: lessons from Tam3 in Antirrhinum.

The transposon Tam3 of Antirrhinum (snapdragon) has acquired properties that distinguish it from other transposons. Mobile DNA, commonly referred to as a transposable element or transposon, is considered to be synonymous with a selfish factor. That is, a transposable element increases in copy number and moves copies of itself independently of the survival of the host organism. Therefore, the host collectively regulates the transposition activities of most transposable elements in its genome by epigenetic means. However, our analyses of the structure and behavior of Tam3, as shown by the following five results, provide evidence that it does not behave in a selfish manner in relation to the host. 1) Active transposable elements normally increase the abundance of their non-autonomous elements, whereas Tam3 is known to have no non-autonomous elements, and a limited number of around 10 copies of autonomous elements present in the genome have been isolated as active copies. 2) Tam3 does not transpose at 25 ℃, which is the optimal growth temperature for Antirrhinum. Transposition of Tam3 occurs only at low temperatures of about 15 ℃, which is stressful for Antirrhinum. 3) Few strains of Antirrhinum have been found to contain genes that specifically suppress Tam3 transposition. 4) Most of the Tam3 insertions found in Antirrhinum genes do not affect the host genome, and the expression of these host genes is not completely suppressed. 5) Transcription and translation of the Tam3 transposase gene are not epigenetically regulated by the host. These five experimental results constitute evidence that Tam3 retains features that are dissimilar to those of many other transposons and that it does not behave in a selfish manner that is detrimental to the survival of the host. In this review, we consider what kinds of behavior are required if transposons are to establish a mutually beneficial relationship with their hosts, with reference to Tam3.