Home Economics American Trypanosomiasis Chagas Disease, Second Edition: One Hundred Years of Research
Examples of specific questions
At the species level, most of the conflicts between traditional and modern concepts of species were treated using the subjective argument of “large” or “low” genetic and/or phenetic distances. Thus, in the current literature, the molecular approach to species resolution is widely inspired by the evolutionary concept of species, as well as by the subjective step of the phylogenetic (PHG 2) concept asking whether two monophyletic clades are two different species.
R. prolixus and R. robustus
The prolixus—robustus pair received a phylogenetic recognition as distinct monophyletic groups,54’55 but the problem has been treated mostly by considering the degree of genetic distances (see previous sections). The two species present the particularity of no111,112 or very low nuclear DNA divergence,36’55 but relatively high mitochondrial55 genetic distances. Phenotypic distances have been found to be significant.92,93 Thus, the recognition of monophyletic clades was not sufficient to consider them as separate species, it was the amount of sequence divergence of mitochondrial DNA and the known ecological adaptations which were the arguments to label them as distinct species. Thus far, only molecular markers may help to separate the members of this complex.113 The morphological characters that would help discriminating them are still to be discovered, although some morphometric propositions have been made.93,114
T. infestans, T. platensis, T. delpontei
In the more abundant genus Triatoma, many species have been revisited by using molecular tools, first by using isoenzymes, , , then by DNA analyses.
The infestans—platensis—delpontei question has been addressed again by considering genetic distances. Contrary to the prolixus—robustus pair, they do not show significant mtDNA divergence,58,64 but show a consistent, although low, genetic distance derived from MLEE,80 or ITS-2 and ITS-181 as well as cytogenetic differences.118 The genetic differences at the nuclear level were among the arguments to decide about their species status,31 although in this case the ecological specialization also provided a very strong argument.21,80
T. infestans, T. melanosoma
The infestans—melanosoma pair has been explored by geometric morphometrics119 and DNA analyses. Triatoma melanosoma was originally described as a subspecies of T. infestans and later raised to species rank.120,121 It differs only by the black color of its exoskeleton from T. infestans, with which it is interfertile. This melanic characteristic appears to be recessive in crossbreeds. , Analyses of Cyt b and ITS-2 and ITS-181 considered that genetic differences were not sufficient to justify two different taxa and thus supported the synonymizing of T. melanosoma with T. infestans after morphometric comparison.119
The "dark morphs" of T. infestans
The “dark morphs” of T. infestans123 have been examined by RAPD,124 by APH patterns125 and morphometric characters.119 Low or lack of phenotypic and genetic distances converged to the conclusion of intraspecific variation. Recently, ITS sequences showed that the dark morph haplotype (T. inf-CH5A) was identical to the majority at ITS-1 and differed from the most dispersed T. infestans H2 by only one transversion at ITS-2 level. In the phylogenetic reconstruction, the dark morph appeared in the branch grouping all non-Andean T. infestans haplotypes. These results suggested that this melanic form did not need any taxonomic distinction.81
T. sordida, T. garciabesi
The Triatoma sordida species is morphologically close to T. guasayana and T. patagonica, but its variable morphology seems to cover different species. Triatoma garciabesi has been revalidated mainly on basis of genetic differences with T. sordida.58,126 In Bolivia, the species has been shown through MLEE to contain two sibling species88 but because of the apparent lack of morphological difference, no new name was created in this situation. This group has been revisited recently using chromosome markers and COI sequences, and more lineages (called “chromosomal taxa”) have been distinguished, including a new cryptic species.127
Triatoma brasiliensis is composed of at least four geographic populations (brasiliensis, melanica, macromelasoma, and juazeiro) that have distinct chromatic, morphologic, biologic, ecologic patterns, and genetic composition. This group has been considered using different approaches, exactly in the spirit of the evolutionary concept of species. Reciprocal crosses revealed genetic incompatibility between melanica and brasiliensis samples.100 Thus according to the biological and the Hennigian concepts of species, the group contains certainly two species. A large Cyt b study indicated that the four geographic populations of T. brasiliensis were genetically distinct.128 The conclusion was reached to consider brasiliensis-macromelasoma, melanica, and juazeiro forms as three separate species. No taxonomic status was concluded for the brasiliensis and macromelasoma forms despite their representing distinct evolutionary lineages.128 Geometric morphometric analyses, morphological comparisons with laboratory hybrids and ecological considerations led to suggest the hypothesis of T. macromelasoma as a historical hybrid (homoploid hybridization). , 9
Hypsa et al.56 presented the Argentinian species T. eratyrusiformis as belonging to the same monophyletic group as M. spinolai, supporting its inclusion in the genus as Mepraia eratyrusiformis. Combined cytogenetic and nuclear rDNA and mtDNA markers indicate that populations within the Mepraia genus can be divided into two separate lineages with specific status supported by the level of divergence observed between their nuclear and mitochondrial sequences which correspond to recently diverged species,130 with similar nucleotide divergence to that found between the closely related species from the infestans subcomplex.81 This phylogenetic approach supported the recently described M. gajardoi131; unfortunately, its validity is questioned since an ingroup species, M. eratyrusiformis, was used as an
Morphometric variation132 and later chromosomal variation and genome size89 suggested the existence of cryptic species of Triatoma dimidiata. The T. dimidiata complex of species has been suspected also thanks to a phylogenetic (PHG 2) approach.67 According to ITS-2, T. dimidiata populations from Yucatan part of Guatemala and northern Honduras belong to a cryptic species ( = T. spp. affinis dimidiata61), different from T. dimidiata.61 Cyt b and ITS-2 confirmed their specific status and extended their distribution to Belize.133 This distinction agrees with results from multidisciplinary studies using RAPD-PCR, genital structures, morphometrics, APH, cuticular hydrocarbons, and chromosomes ’ and has been confirmed by ND1 and pseudogenic sequences of 5.8S + ITS-2 analyses, and by the absence of introgressed sequences in overlapping zones.34-36 Recently, more emphasis has been brought to the cryptic subdivision of T. dimidiata based on mtDNA distances and phylogeography, recognizing the previous studies but suggesting now the existence of at least four separate species.31 Moreover, in disagreement with previous estimations, Monterio et al.31 added T. hegneri as a fifth species to the T. dimidiata complex.
Because of the lack of clear morphological diagnostic characters, no new names have been proposed, but some possible speciation mechanisms have been explored, highlighting the possible role of geographic isolation and ecological adaptation in the process of T. dimidiata cryptic speciation. Evidence for niche differentiation among cryptic species was observed using the ecological niche modeling approach (ENM).136 In Colombia, the genetic heterogeneity among Colombian populations, as observed with microsatellites, correlated with the ecoepidemiological and morphological traits observed in this species across regions.38 The population structure, as examined through ND4, COI and ITS-2 variation, supported a significant association between genetic divergence and the ecogeographical location of Colombian population groups.39
T. hegneri is a species originally described from the Mexican island of Cozumel. Although chromatically distinguishable from most forms of T. dimidiata,1 it is known to produce fertile hybrids when experimentally crossed with T. dimidiata (R.E. Ryckman, unpublished in Bargues et al.61). The two ITS-2 haplotypes of T. hegneri differ by only three mutations from haplotypes of T. dimidiata from Mexico and Guatemala. This reduced number of nucleotide differences and the location of T. hegneri haplotypes within the phylogenetic clade of T. dimidiata did not support its taxonomic status. These data suggested that it is an intraspecific morphological variation of T. dimidiata.61 In contradiction with these studies, relatively high mtDNA genetic distances were disclosed by Monteiro et al.,37 who suggested T. hegneri as a valid species. Thus, T. hegneri is another example where similar, objective genetic approaches produced different, subjective interpretations.
|< Prev||CONTENTS||Next >|