Table 1.

Possible coevolved mechanisms to mitigate reduction in host fitness

Mechanism	Examples
Insertion bias for noncoding regions	Preferential insertion in regulatory regions (1); nested retrotransposons in maize (2); clustered I elements (27); mdg1 in Drosophila (28, 29)
Pre-mRNA splicing	Splicing from the maize wx gene (18); various genes in Drosophila (57), C. elegans (59), and plants (58)
Tissue specificity of transposition	Repression of P element transposition in somatic cells at the level of RNA processing (60)
TE copy number regulation	Methylation of maize Ac, Spm, and Mu by host factors (63, 64); type I and type II P element-encoded repressors (13, 68, 70–72); Ac dosage effects (16); Spm repressor action (73–75)
Regulators of mutant phenotype expression	Transposase-dependent expression of mutant phenotype caused by insertion of Spm in maize (73); masking of mutant phenotypes by alleles of host suppressor genes in Drosophila (40)