Mobilization Of The Active Transposons MPing And Pong In Rice By Introgression From Wild Rice (Zizania Latifolia Griseb.)

Hybridization between different species plays an important role in plant genome evolution, as well as is a widely used approach for crop improvement. McClintock has predicted that plant wide hybridization constitutes a''genomic shock''whereby cryptic transposable elements may be activated. However, direct experimental evidence showing a causal relationship between plant wide hybridization and transposon mobilization has not yet been reported. The miniature-Ping (mPing) is a recently isolated active miniature inverted-repeat transposable element from rice, which is mobilized by tissue culture andγ-ray irradiation. We show herein that mPing, together with its putative transposase-encoding partner, Pong, is mobilized in three homologous recombinant inbred lines (RILs), derived from hybridization between rice (cultivar Matsumae) and wild rice ( Zizania latifolia Griseb.) by a novel approach called"repeated pollination". Thus, we have presented direct evidence that is consistent with McClintock's insight by demonstrating a causal link between wide hybridization and transposon mobilization in rice.It has been shown that mPing and Pong often excise imprecisely leaving footprints. Here, we report an atypical behavior of mPing/Pong mobilization in these lines, i.e., the exclusive absence of footprints after excision. To test if this unusual phenomenon was due to difference in the elicitors or a genotype-dependent trait, we subjected the three RILs and their parent, Matsumae, to tissue culture. We found that interestingly, tissue culture was capable of mobilizaing mPing and Pong only in the parental cultivar Matsumae. Sequencing analysis showed that excision of mPing and Pong hadn't left any footprint triggered by tissue culture. This strongly suggested that leaving footprints or not by mPing/Pong excision in rice is largely determined by the host genotype.To further study the phenomenon, that mPing could be mobilized in several rice recombinant inbred lines (RILs) derived from introgressive hybridization between rice and wild rice (Zizania latifolia Griseb.), we investigated activity of the same element in an independent plant system, i.e., a somatic hybrid of rice and Zizania. We found that mPing, together with its putative transposase-encoding partner, Pong, also underwent transpositional activation in a highly asymmetric somatic nuclear hybrid confirming that stochastic integration of Zizania chromatin can induce transpositional activation of mPing and Pong. Moreover, we found that mPing transposition was accompanied by massive loss of its original copies in the somatic hybrid, a phenomenon hitherto not described..Sequence-based mapping of identified mPing de novo insertions from RILs indicated all being in unique or low-copy regions, conform with the targeting propensity of mPing. Expression analysis by semi-quantitative RT-PCR on a subset of selected genes near which mPing had inserted in one or more of the RILs indicated that at least for some genes, mPing could markedly influence expression. Thus we can conclude that mPing and/or Pong could be mobilized in wide hybridization-derived RILs, which not only cause conventional insertional mutagenesis, but may also impact expression of adjacent genes, and hence, conceivably plays an important role in plant genome evolution and crop breeding.