The microsporidian parasite Nosema ceranae is a common pathogen of the Western honeybee (Apis mellifera) whose variable virulence could be related to its genetic polymorphism and/or its polyphenism responding to environmental cues. Since the genotyping of N. ceranae based on unique marker sequences had been unsuccessful, we tested whether a multilocus approach, assessing the diversity of ten genetic markers – encoding nine proteins and the small ribosomal RNA subunit – allowed the discrimination between N. ceranae variants isolated from single A. mellifera individuals in four distant locations. High nucleotide diversity and allele content were observed for all genes. Most importantly, the diversity was mainly present within parasite populations isolated from single honeybee individuals. In contrast the absence of isolate differentiation precluded any taxa discrimination, even through a multilocus approach, but suggested that similar populations of parasites seem to infect honeybees in distant locations. As statistical evolutionary analyses showed that the allele frequency is under selective pressure, we discuss the origin and consequences of N. ceranae heterozygosity in a single host and lack of population divergence in the context of the parasite natural and evolutionary history.

Variation in the ant species associated with myrmecophytes (plants that provide their associated ants with nesting space, and sometimes with a complete diet) has been noted at both the regional and local levels, with plant distribution generally wider than that of the ants (Fonseca & Ganade 1996). This is the case for Cordia nodosa Lamark (Boraginaceae, subfamily Ehretioideae) whose most frequent associate ant species in Peru are Allomerus demararae (Wheeler) and Azteca spp. (Yu & Pierce 1998), while Azteca sp.1 and Allomerus octoarticulatus are the most frequent in Amazonian Brazil and French Guiana, respectively (Fowler 1993, Solano et al. 2003). Cordia nodosa plants are understorey treelets mostly less than 2 m tall, but taller individuals can be found. Their domatia are swollen, hollow stem nodes that form as the growing shoot tip invaginates through a subapical pore that then closes over when the domatium is mature (Yu 2001). So, plant-ants must reopen the chamber where the pore used to be, thus forming a kind of prostoma.

Bromeliads are common as epiphytes in warm neotropical forests (Benzing 1990). Species native to relatively wet forests impound water in a central cup and/or in seperate leaf axils. These tanks receive enough leaf litter and rainwater to support aquatic life (Maguire 1971). In these complex microcosms many groups of freshwater organisms ranging from algae, fungi, bacteria and protozoa through insects to frogs are represented and constitute considerable animal populations (Frank 1983, Laessle 1961, Maguire 1971, Picado 1913, Richardson 1999). Nutrients originating from the decomposition of litter and animal waste are absorbed by specialized trichomes on the bases of the leaves which form the tanks (Benzing 1980, 1990).