Longitudinal analysis on parasite diversity in honeybee colonies: new taxa, high frequency of mixed infections and seasonal patterns of variation

To evaluate the influence that parasites have on the losses of Apis mellifera it is essential to monitor their presence in the colonies over time. Here we analysed the occurrence of nosematids, trypanosomatids and neogregarines in five homogeneous colonies for up to 21 months until they collapsed. The study, which combined the use of several molecular markers with the application of a massive parallel sequencing technology, provided valuable insights into the epidemiology of these parasites: (I) it enabled the detection of parasite species rarely reported in honeybees (Nosema thomsoni, Crithidia bombi, Crithidia acanthocephali) and the identification of two novel taxa; (II) it revealed the existence of a high rate of co-infections (80% of the samples harboured more than one parasite species); (III) it uncovered an identical pattern of seasonal variation for nosematids and trypanosomatids, that was different from that of neogregarines; (IV) it showed that there were no significant differences in the fraction of positive samples, nor in the levels of species diversity, between interior and exterior bees; and (V) it unveiled that the variation in the number of parasite species was not directly linked with the failure of the colonies

Multilocus patterns of genetic variation across Cryptosporidium species suggest balancing selection at the gp60 locus

Cryptosporidium is an apicomplexan protozoan that parasites most vertebrates, including humans. Its gp60 gene is functionally involved in its attachement to host’s cells and its high level of genetic variation has made it the reference marker for sample typing in epidemiological studies. To understand the origin of such high diversity and to determine the extent to which this classification applies to the rest of the genome, we analysed the patterns of variation at gp60 and nine other nuclear loci in isolates of three Cryptosporidium species. Most loci showed low genetic polymorphism (πS < 1%) and similar levels of between-species divergence. Contrastingly, gp60 exhibited very different features: (i) it was nearly ten times more variable than the other loci, (ii) it displayed a significant excess of polymorphisms relative to between-species differences in a Maximum-likelihood Hudson-Kreitman-Aguadé test (iii) gp60 subtypes turned out to be much older than the species they were found in and (iv) showed a significant excess of polymorphic variants shared across species from random expectations. These observations suggest that this locus evolves under balancing selection, and specifically under negative frequency-dependent selection. Interestingly, genetic variation at the other loci cluster very well within the subtypes defined by gp60, which may provide new tools to understand the genome-wide patterns of genetic variation of the parasite in the wild. These results suggest that gp60 plays an active and essential role in the life cycle of the parasite and that genetic variation at this locus might be essential for the parasite’s long-term success