RNA editing and the evolution of parasites.

The kmeloplaslid flagellates, together \Villi lllcil SlSl¢l ~lOUp (d euglenoid>, represenl ]lie, earlies| extan* }incage of eukaryotIc c~rgani>ms conlainnlg milocbondria (]). \Xzilbu: file kinen~plaslids, there arc two maj<~l groups, ll~e p,~olly studied bodonidscrypn-4qids, which con>is, of both ttce-lix m~ and parasllit cells, and :be bcuel kilo\vii irypallc~>onlalids, w]licll are i4~hFalc parasiles (2). Perhaps }~ecause oi lhe anllqmty ot tilt. trypallosonlalid ]ineagt, these c~'ll> possess several unique geneilc lea ~ral, Ihut there is disagreemenl on the nature ot the pr imary parasitic host. The "mverlel~rale firsl" model f ie, i l) slate,,, ibm lhe :nili>d parasitism wa.ill *be gul o| preCamlhrian inverlelhlale>. Coev,>lution of parasite and bos* would have led lo a wide di>tril~ution of u-ypanc>omatids ii7 insect> and leeches. In lhi> l]leol'y, di~t'netic life cycles (H]lelnalln~ invcrlcbralc and \'erlebrate bosts'l evolved In:el as a result O{ the acquisition b\' son:t hen:ipterans and dipterans of l]:e abihly m feed ,in the blood lion ~f leeches and bemalophagou, arthropod> led 'it/~hc appearance ot a dige. hel le life cycle. Monogenetic parasitewould represent cells that secondarily losl l i l t ability to live in the vertebrate host. In /lnv will tile "invertel~rale first" hyp o l b c s l S , cons l l 'HCtS Of a l l unrooted t r e e f rOl l l mimchondrial ribos,mlal RNA (rRNA) sequences were arbitrarily rooted in the Crithidia branch (14). However, more recell* phylogenelic reconstruciions with nuclear rRNA sequences and with Euglena as an outgroup yielded a tree with an identical lopolog': , but w i t h a rooi in the Tryl>anu.,,,ma branch (15-17). Ill this tree, the Bodonid-cryptobiid clade, represented by the tree-living Bodo caudams and the fish p,rasile Trypanoplasrna borreli, constitute> all early divergmg sister group to tile lrypan.somatids. In the trypanosomatid branch, digenetic or eanisms do not fi.~rm a scparale clade, suggesting either several i l l -