The completion of genomic sequences of physiologically important genes frequently revealsnon-coding genetic elements such as tandem repeats (micro- and minisatellites) that areoften more polymorphic than nearby coding sequences. We obtained the complete genomicsequences of three hormone genes in sea bass Dicentrarchus labrax: growthhormone (dlGH), somatolactin (dlSL) and insulin-likegrowth factor-1 (dlIGF-1), including 5′- and 3′-untranslated regions.Mini- and microsatellites were discovered in both flanking and intron regions. Some werepartially conserved across Perciformes. To assess the usefulness and relevance of thesegene-associated markers for understanding population structure, an investigation was madeon genetic diversity and differentiation at four of them in (i) five wildpopulations from the North Sea, the Bay of Biscay and the Western Mediterranean, and(ii) two samples of hatchery-bred individuals from afreshwater-acclimation experiment. Gene and allelic diversities were lower in culturedindividuals than in wild ones. Significant genetic differentiation was demonstratedbetween Bay of Biscay + North Sea and Mediterranean populations(F st > 0.06, p < 0.001),primarily due to dlGH-associated markers. Significant geneticdifferentiation was also detected among the Atlantic and North Sea samples, but restrictedto the locus associated with dlSL. Significant genetic differentiationwas also found among experimental individuals before and after a salinity challenge(F st ≈ 0.05, p < 0.001), but was dueto dlSL and dlIGF-1 loci. Gene-associated markers provedto be more efficient than formerly used anonymous microsatellite markers in providing aclear picture of genetic differentiation.

Plasmodium malariae, a protozoan parasite that causes malaria in humans, has a global distribution in tropical and subtropical regions and is commonly found in sympatry with other Plasmodium species of humans. Little is known about the genetics or population structure of P. malariae. In the present study, we describe polymorphic genetic markers for P. malariae and present the first molecular epidemiological data for this parasite. Six microsatellite or minisatellite markers were validated using 76 P. malariae samples from a diverse geographical range. The repeat unit length varied from 2 to17 bp, and up to 10 different alleles per locus were detected. Multiple genotypes of P. malariae were detected in 33 of 70 samples from humans with naturally acquired infection. Heterozygosity was calculated to be between 0·236 and 0·811. Allelic diversity was reduced for samples from South America and, at some loci, in samples from Thailand compared with those from Malawi. The number of unique multilocus genotypes defined using the 6 markers was significantly greater in Malawi than in Thailand, even when data from single genotype infections were used. There was a significant reduction in the multiplicity of infection in symptomatic infections compared with asymptomatic ones, suggesting that clinical episodes are usually caused by the expansion of a single genotype.

Analysis of natural populations of Trypanosoma brucei has shown that there is linkage disequilibrium between alleles at pairs of loci in isolates taken from the field. This disequilibrium can occur as a result of a low frequency of genetic exchange, the masking of frequent genetic exchange by the rapid expansion of a few genotypes or by the treatment of 2 (or more) genetically isolated populations as a single population. We have analysed stocks from 2 geographically separate locations using 3 minisatellite markers to determine the frequencies of the alleles in each area and the frequency and nature of the multilocus genotypes. The results show that many alleles and multilocus genotypes are unique to each geographical location, supporting the conclusion that these populations are genetically isolated with limited or no gene flow between them. This geographical substructuring needs to be taken into account in considering the origins of the linkage disequilibrium in a number of populations.