Trypanosomatid Genomes Contain Several Subfamilies of ingi-Related Retroposons

Abstract

Retroposons are ubiquitous transposable elements found in the genomes of most eukaryotes, including trypanosomatids. The African and American trypanosomes (Trypanosoma brucei and Trypanosoma cruzi) contain long autonomous retroposons of the ingi clade (Tbingi and L1Tc, respectively) and short nonautonomous truncated versions (TbRIME and NARTc, respectively), as well as degenerate ingi-related retroposons devoid of coding capacity (DIREs). In contrast, Leishmania major contains only remnants of extinct retroposons (LmDIREs) and of short nonautonomous heterogeneous elements (LmSIDERs). We extend this comparative and evolutionary analysis of retroposons to the genomes of two other African trypanosomes (Trypanosoma congolense and Trypanosoma vivax) and another Leishmania sp. (Leishmania braziliensis). Three new potentially functional retroposons of the ingi clade have been identified: Tvingi in T. vivax and Tcoingi and L1Tco in T. congolense. T. congolense is the first trypanosomatid containing two classes of potentially active retroposons of the ingi clade. We analyzed sequences located upstream of these new long autonomous ingi-related elements, which code for the recognition site of the retroposon-encoded endonuclease. The closely related Tcoingi and Tvingi elements show the same conserved pattern, indicating that the Tcoingi- and Tvingi-encoded endonucleases share site specificity. Similarly, the conserved pattern previously identified upstream of L1Tc has also been detected at the same relative position upstream of L1Tco elements. A phylogenetic analysis of all ingi-related retroposons identified so far, including DIREs, clearly shows that several distinct subfamilies have emerged and coexisted, though in the course of trypanosomatid evolution, only a few have been maintained as active elements in modern trypanosomatid (sub)species.