Haplotype distribution of five nuclear genes based on network genealogies and Bayesian inference indicates that Trypanosoma cruzi hybrid strains are polyphyletic.

Chagas disease is still a major public health problem in Latin America. Its causative agent, Trypanosoma cruzi, can be typed into three major groups, T. cruzi I, T. cruzi II and hybrids. These groups each have specific genetic characteristics and epidemiological distributions. Several highly virulent strains are found in the hybrid group; their origin is still a matter of debate. The null hypothesis is that the hybrids are of polyphyletic origin, evolving independently from various hybridization events. The alternative hypothesis is that all extant hybrid strains originated from a single hybridization event. We sequenced both alleles of genes encoding EF-1alpha, actin and SSU rDNA of 26 T. cruzi strains and DHFR-TS and TR of 12 strains. This information was used for network genealogy analysis and Bayesian phylogenies. We found T. cruzi I and T. cruzi II to be monophyletic and that all hybrids had different combinations of T. cruzi I and T. cruzi II haplotypes plus hybrid-specific haplotypes. Bootstrap values (networks) and posterior probabilities (Bayesian phylogenies) of clades supporting the monophyly of hybrids were far below the 95% confidence interval, indicating that the hybrid group is polyphyletic. We hypothesize that T. cruzi I and T. cruzi II are two different species and that the hybrids are extant representatives of independent events of genome hybridization, which sporadically have sufficient fitness to impact on the epidemiology of Chagas disease.

Maximum-likelihood divergence date estimates based on rRNA gene sequences suggest two scenarios of Trypanosoma cruzi intraspecific evolution.

The phylogenetic relationships of Trypanosoma cruzi strains were inferred using maximum-likelihood from complete 18S rDNA sequences and D7-24Salpha rDNA regions from 20 representative strains of T. cruzi. For this we sequenced the 18S rDNA of 14 strains and the D7-24Salpha rDNA of four strains and aligned them to previously published sequences. Phylogenies inferred from these data sets identified four groups, named Riboclades 1, 2, 3, and 4, and a basal dichotomy that separated Riboclade 1 from Riboclades 2, 3, and 4. Substitution models and other parameters were optimized by hierarchical likelihood tests, and our analysis of the 18S rDNA molecular clock by the likelihood ratio test suggests that a taxa subset encompassing all 2,150 positions in the alignment supports rate constancy among lineages. The present analysis supports the notion that divergence dates of T. cruzi Riboclades can be estimated from 18S rDNA sequences and therefore, we present alternative evolutionary scenarios based on two different views of T. cruzi intraspecific divergence. The first assumes a faster evolutionary rate, which suggests that the divergence between T. cruzi I and II and the extant strains occurred in the Tertiary period (37-18 MYA). The other, which supports the hypothesis that the divergence between T. cruzi I and II occurred in the Cretaceous period (144-65 MYA) and the divergence of the extant strains occurred in the Tertiary period of the Cenozoic era (65-1.8 MYA), is consistent with our previously proposed hypothesis of divergence by geographical isolation and mammalian host coevolution.

Effect of allylic CH(3-n)F(n) groups (n = 1-3) on pi-facial diastereoselection.

[structure: see text]. Michael addition of various enolates toward gamma-CH(3-n)F(n)-alpha,beta-unsaturated ketones (n = 1-3) was proven to smoothly furnish the 1,4-adducts with high si face selectivities which monotonously decreased by reduction in the number of fluorines. Although the Felkin-Anh model correctly anticipates the present stereochemical outcome only with E-acceptors, the hyperconjugative stabilization of transition states by electron donation from the allylic substituents (the Cieplak rule) successfully explains the pi-facial preference of both acceptors at least in a qualitative level.