Worm Breeder's Gazette 12(5): 15 (February 1, 1993)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have sequenced five Tc1 -liketransposons from zebrafish, trout and Atlantic salmon. The five elements are all between 1.6 kb and 1.7 kb in length, have inverted repeats of between 20 and 52 base pairs, end in TACAGT, and encode a single conceptual protein. The conceptual protein can be aligned with the open reading frame protein of Tc1 provided the 5' exon proposed by Plasterk and coworkers is included in the nematode sequence.
Tc1 -liketransposons with the above characteristics are now known from nematodes, arthropods, and vertebrates. What accounts for this widespread distribution? Are these elements horizontally transmissible and spreading from one species to another, or are they only vertically transmitted and very ancient?
We tested for vertical versus horizontal transmission by analyzing the phylogenetic relationships of the known Tc1 -liketransposons. These relationships were reconstructed from the amino acid sequences of the conceptual open reading frame proteins and from DNA sequences, using neighbor joining and maximum parsimony methods. A striking difference between the phylogenetic relationships of the transposons and those of the organisms hosting these elements would have supported the hypothesis of horizontal transmission. On the contrary, where sequences were sufficiently similar to establish definite relationships, the transposons fit into species-specific groups, with two exceptions: (1) Tcb2of C. briggsae was grouped with Tc1 of C. elegans rather than with Tcb1of C. briggsae, and (2) Bari-1 of Drosophila melanogaster grouped with Minos of D. hydei, whereas HB1 and HB2 of D. melanogaster grouped with Uhu of D. heteroneura. For closely related species such as these, such relationships could mean that multiple, variant families of Tc1 -liketransposons have been established earlier in shared ancestral lineages.
Similarity of the phylogenetic relationships of the Tc1 -liketransposons and the species that contain them does not exclude any horizontal transmission in the ancient establishment of the widespread Tc1 family. If such transmission had occurred at an early time it would not have been detected by this analysis. However, these results do appear to rule out recent, extensive spread between species in a virus-like manner. Apparently, Tc1 -liketransposons have been a feature of animal genomes since ancient times. This allows us to predict that Tc1 -likeelements or their remnants are likely to be ubiquitous components of animal genomes.
Despite their apparent ancient origin, some Tc1 -liketransposons remain active. The interesting question therefore arises as to what selective forces have maintained this transpositional activity since ancient times, and what role this activity has played, if any, in providing genomic variability during animal evolution.