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Population genetics of Cryptosporidium parvum

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Abstract

Among the apicomplexan parasites of medical interest, Cryptosporidium is the least studied. Consistent with other members of this phylum, the generally accepted life cycle of Cryptosporidium comprises a sexual phase during which gamete differentiation and fertilization occurs. This process can affect the population structure of this parasite because of the possibility of genetic recombination between genotypes. Experimental evidence has confirmed that recombinant genotypes can emerge from mixed infections. A recent study of Cryptosporidium parvum populations infecting humans and livestock in Aberdeenshire, UK, shows that recombination also takes place in nature. Distinct population structures were identified among C. parvum types (species) infecting humans and livestock.

Section snippets

The population biology of C. parvum

During the past 10 years, much effort has focused on the taxonomy of the genus Cryptosporidium, but a consensus of what constitutes a valid or named species [2] remains elusive. New species are still being described 3, 4 and the subdivision of C. parvum into two species, C. parvum and Cryptosporidium hominis, has recently been proposed to replace the Type 1 and Type 2 subspecies designation, respectively [5]. These newly defined species are being adopted rapidly. However, although suggested by

Differences in population structure

The study by Mallon et al. [12] demonstrates that genetic recombination occurs in nature, and that out-crossing is common among C. parvum Type 2 parasites. Together with evidence showing genetic recombination in experimental mouse infections [19], this study emphasizes the importance of this process for the epidemiology of cryptosporidiosis. The results highlight the differences in the population structure of Type 1 and Type 2 C. parvum, and uncover what appear to be human-specific Type 2

Future implications

The presence of different population structures in C. parvum originating from a restricted geographical area represents a new insight into the epidemiology of cryptosporidiosis. Future studies from different geographical locations, in particular from those with high prevalence of human cryptosporidiosis, will reveal whether the differences in Type 1 and Type 2 populations are maintained, and will expand our understanding of the population biology of C. parvum.

In contrast to malaria, which has

Glossary

Glossary

Linkage disequilibrium (LD):
The non-random association between alleles at different genetic loci. LD is a measure of association between alleles at any two loci. LD is low in randomly mating populations and high in clonal populations.
Microsatellite:
Repeat of a long (>5 bp) sequence motif in a given DNA sequence. Microsatellites are useful markers for genotyping because of their frequent length polymorphism originating from a variable number of repeat units.
Minisatellite:
Repeat of a short (1–5

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