Phylogeny of a rapidly evolving clade: the cichlid fishes of Lake Malawi, East Africa.
Lake Malawi contains a flock of >500 species of cichlid fish that have evolved from a common ancestor within the last million years. The rapid diversification of this group has been attributed to morphological adaptation and to sexual selection, but the relative timing and importance of these mechanisms is not known. A phylogeny of the group would help identify the role each mechanism has played in the evolution of the flock. Previous attempts to reconstruct the relationships among these taxa using molecular methods have been frustrated by the persistence of ancestral polymorphisms within species. Here we describe results from a DNA fingerprinting technique that overcomes this problem by examining thousands of polymorphisms distributed across the genome. The resulting dendrogram averages the evolutionary history of thousands of genes and should accurately reflect the evolutionary history of these species. Our tree resolves relationships among closely related Lake Malawi cichlids and provides insights into the pattern of speciation in this group. We demonstrate that adaptive divergence in trophic morphology played an important role during the early history of the lake. Subsequent species diversity has arisen with little change in trophic morphology, which suggests that other forces are responsible for the continued speciation of these fishes. (+info
Typing of Salmonella enterica serotype paratyphi C isolates from various countries by plasmid profiles and pulsed-field gel electrophoresis.
Pulsed-field gel electrophoresis (PFGE) of 61 Salmonella enterica serotype Paratyphi C isolates from six countries gave five distinct clusters. Twenty-four isolates from five countries were susceptible to 10 antimicrobials tested and gave similar restriction endonuclease digest patterns of the 38-MDa plasmid. In contrast, plasmid and PFGE profiles of 37 multidrug-resistant isolates from Zaire were different from those from other countries. (+info
Natural experimental models: the global search for biomedical paradigms among traditional, modernizing, and modern populations.
During the past four decades, biomedical scientists have slowly begun to recognize the unique opportunities for studying biomedical processes, disease etiology, and mechanisms of pathogenesis in populations with unusual genetic structures, physiological characteristics, focal endemic disease, or special circumstances. Such populations greatly extend our research capabilities and provide a natural laboratory for studying relationships among biobehavioral, genetic, and ecological processes that are involved in the development of disease. The models presented illustrate three different types of natural experiments: those occurring in traditionally living, modernizing, and modern populations. The examples are drawn from current research that involves population mechanisms of adaptation among East African Turkana pastoralists; a search for etiology and mechanisms of pathogenesis of an emerging disease among the Yakut people of Siberia; and psychosocial stress, hypertension, and cardiovascular disease in women working outside the home in New York City and among subpopulations in Hawaii. The models in general, and the examples in specific, represent natural laboratories in which relatively small intrapopulation differences and large interpopulation differences can be used to evaluate health and disease outcomes. (+info
Epidemiological uses of a population model for the tick Rhipicephalus appendiculatus.
The spatial and temporal risk of tick-borne disease depends fundamentally on the distribution, abundance and seasonal dynamics of the vector ticks. The latter factor exerts a major quantitative influence on the transmission dynamics of tick-borne parasites. The population model for Rhipicephalus appendiculatus applies throughout the range of this tick in eastern Africa, and predicts all three fundamental risk factors on the basis of the local temperature and rainfall conditions. Satellite imagery can provide more detailed, real-time measures of environmental conditions over extensive areas than climatic data. There is preliminary evidence to suggest that the population model could be driven by satellite-derived surrogates of its climatic predictors, thus providing wide-scale predictive risk maps of theileriosis. (+info
The origin and age of haplochromine fishes in Lake Victoria, east Africa.
According to a widely held view, the more than 300 species of haplochromine cichlid fishes in Lake Victoria (LV), East Africa, originated from a single founder species in less than 12,000 years. This view, however, does not follow from the published geological and molecular evidence. The former does indeed suggest that the LV basin dried out less than 15,000 years ago, but it does not provide any information about the species that re-colonized the new lake or that remained in the rivers draining the area. The molecular evidence is inconclusive with respect to the origin of the LV haplochromines because cichlids from critical regions around LV were not adequately sampled; and as far as the age of the LV haplochromines is concerned, it in fact led to an estimate of 250,000-750,000 years old. In the present study, mitochondrial DNA (control region) variation was determined by heteroduplex and sequencing analyses of more than 670 specimens collected at widely distributed East African riverine and lacustrine localities. The analyses revealed the existence of seven haplogroups (I-VII) distinguishable by characteristic substitutions. All endemic LV samples tested fell into one of these haplogroups (V) which, however, was also found to be present at various other localities, both riverine and lacustrine, outside LV. Within this haplogroup, four subgroups (VA through VD) could be distinguished, two of which (VB and VC) were represented in LV and at other localities. The great majority of the LV haplochromine species could be classified as belonging to the VC subgroup, which was found only in LV and in the rivers draining into it. Hence, while the endemic haplochromine species of LV could not have originated from a single founding population, the lake does harbour a large species flock which probably arose in situ. (+info
Long term results of glaucoma surgery among participants in an east African population survey.
AIM: To evaluate the long term results of glaucoma surgery among people in East Africa. METHODS: Participants in a population based survey of eye disease prevalence were offered glaucoma surgery using standardised criteria. Either surgical iridectomy or trabeculectomy was carried out as indicated by a medical officer or by one of two ophthalmologists. Trabeculectomy methods included releasable sutures and mitomycin C in the majority of eyes. Subjects were examined during the first week and 2 months after surgery. Nearly 3 years later, re-examination was carried out in those who were still resident in the region. RESULTS: Among 46 people who were offered iridectomy, trabeculectomy, or combined cataract extraction/lens implant/trabeculectomy, 21 people underwent surgery (46%). Of the 21, 19 were re-examined at 3 years (90%), including 16/18 eyes after trabeculectomy. Among these, intraocular pressure (IOP) declined from 29.9 (SD 9.4) mm Hg to 14.7 (5.9) mm Hg, with 16 of 18 eyes (89%) achieving a reduction > 25%. Hypotony maculopathy, late bleb leak, and late endophthalmitis were not detected. Visually significant cataract developed in 5/15 re-examined eyes that underwent trabeculectomy alone (33%), possibly associated with pre-existing cataract and diagnosis of angle closure glaucoma, but not with mitomycin C use. CONCLUSIONS: Nearly half of those with glaucoma among residents of rural African villages accepted the offer of surgical therapy. While technical success was achieved at satisfactory levels, the development of cataract must be considered an important issue for application of glaucoma surgical therapy programmes. (+info
Foot-and-mouth disease type O viruses exhibit genetically and geographically distinct evolutionary lineages (topotypes).
Serotype O is the most prevalent of the seven serotypes of foot-and-mouth disease (FMD) virus and occurs in many parts of the world. The UPGMA method was used to construct a phylogenetic tree based on nucleotide sequences at the 3' end of the VP1 gene from 105 FMD type O viruses obtained from samples submitted to the OIE/FAO World Reference Laboratory for FMD. This analysis identified eight major genotypes when a value of 15% nucleotide difference was used as a cut-off. The validity of these groupings was tested on the complete VP1 gene sequences of 23 of these viruses by bootstrap resampling and construction of a neighbour-joining tree. These eight genetic lineages fell within geographical boundaries and we have used the term topotype to describe them. Using a large sequence database, the distribution of viruses belonging to each of the eight topotypes has been determined. These phylogenetically based epidemiological studies have also been used to identify viruses that have transgressed their normal ecological niches. Despite the high rate of mutation during replication of the FMD virus genome, the topotypes appear to represent evolutionary cul-de-sacs. (+info
Mitochondrial DNA sequences in ancient Australians: Implications for modern human origins.
DNA from ancient human remains provides perspectives on the origin of our species and the relationship between molecular and morphological variation. We report analysis of mtDNA from the remains of 10 ancient Australians. These include the morphologically gracile Lake Mungo 3 [ approximately 60 thousand years (ka) before present] and three other gracile individuals from Holocene deposits at Willandra Lakes (<10 ka), all within the skeletal range of living Australians, and six Pleistocene/early Holocene individuals (15 to <8 ka) from Kow Swamp with robust morphologies outside the skeletal range of contemporary indigenous Australians. Lake Mungo 3 is the oldest (Pleistocene) "anatomically modern" human from whom DNA has been recovered. His mtDNA belonged to a lineage that only survives as a segment inserted into chromosome 11 of the nuclear genome, which is now widespread among human populations. This lineage probably diverged before the most recent common ancestor of contemporary human mitochondrial genomes. This timing of divergence implies that the deepest known mtDNA lineage from an anatomically modern human occurred in Australia; analysis restricted to living humans places the deepest branches in East Africa. The other ancient Australian individuals we examined have mtDNA sequences descended from the most recent common ancestor of living humans. Our results indicate that anatomically modern humans were present in Australia before the complete fixation of the mtDNA lineage now found in all living people. Sequences from additional ancient humans may further challenge current concepts of modern human origins. (+info