Freshwater trematodes differ from marine trematodes in patterns connected with division of labor.

Background: Prior research suggests that trematode rediae, a developmental stage of trematode parasites that reproduce clonally within a snail host, show evidence of division of labor (DOL). Single-species infections often have two morphologically distinct groups: small rediae, the 'soldiers', are active, aggressive, and do not appear to reproduce; large rediae, the 'reproductives', are larger, sluggish, and full of offspring. Most data supporting DOL come from trematodes infecting marine snails, while data from freshwater trematodes are more limited and generally do not supported DOL. The shorter lifespan typical of freshwater snails may partially explain this difference: defending a short-lived host at the expense of reproduction likely provides few advantages. Here, we present data from sixty-one colonies spanning twenty species of freshwater trematode exploring morphological and behavioral patterns commonly reported from marine trematodes believed to have DOL. Methods: Trematode rediae were obtained from sixty-one infected snails collected in central Vermont, USA. A portion of the COI gene was sequenced to make tentative species identifications ('COI species'). Samples of rediae were photographed, observed, and measured to look for DOL-associated patterns including a bimodal size distribution, absence of embryos in small rediae, and pronounced appendages and enlarged pharynges (mouthparts) in small rediae. Additional rediae were used to compare activity levels and likelihood to attack heterospecific trematodes in large vs. small rediae. Results: Many of the tests for DOL-associated patterns showed mixed results, even among colonies of the same COI species. However, we note a few consistent patterns. First, small rediae of most colonies appeared capable of reproduction, and we saw no indication (admittedly based on a small sample size and possibly insufficient attack trial methodology) that small rediae were more active or aggressive. This differs from patterns reported from most marine trematodes. Second, the small rediae of most colonies had larger pharynges relative to their body size than large rediae, consistent with marine trematodes. We also observed that colonies of three sampled COI species appear to produce a group of large rediae that have distinctly large pharynges. Conclusions: We conclude that these freshwater species likely do not have a group of specialized non-reproductive soldiers because small rediae of at least some colonies in almost every species do appear to produce embryos. We cannot rule out the possibility that small rediae act as a temporary soldier caste. We are intrigued by the presence of rediae with enlarged pharynges in some species and propose that they may serve an adaptive role, possibly similar to the defensive role of small 'soldier' rediae of marine trematodes. Large-pharynx rediae have been documented in other species previously, and we encourage future efforts to study these large-pharynx rediae.

Population structure in African Drosophila melanogaster revealed by microsatellite analysis.

Tropical sub-Saharan regions are considered to be the geographical origin of Drosophila melanogaster. Starting from there, the species colonized the rest of the world after the last glaciation about 10 000 years ago. Consistent with this demographic scenario, African populations have been shown to harbour higher levels of microsatellite and sequence variation than cosmopolitan populations. Nevertheless, limited information is available on the genetic structure of African populations. We used X chromosomal microsatellite variation to study the population structure of D. melanogaster populations using 13 sampling sites in North, West and East Africa. These populations were compared to six European and one North American population. Significant population structure was found among African D. melanogaster populations. Using a Bayesian method for inferring population structure we detected two distinct groups of populations among African D. melanogaster. Interestingly, the comparison to cosmopolitan D. melanogaster populations indicated that one of the divergent African groups is closely related to cosmopolitan flies. Low, but significant levels of differentiation were observed for sub-Saharan D. melanogaster populations from West and East Africa.

Allele excess at neutrally evolving microsatellites and the implications for tests of neutrality.

Skews in the observed allele-frequency spectrum are frequently viewed as an indication of non-neutral evolution. Recent surveys of microsatellite variability have used an excess of alleles as a statistical approach to infer positive selection. Using neutral coalescent simulations we demonstrate that the mean numbers of alleles expected under the stepwise-mutation model and infinite-allele model deviate from the observed numbers of alleles. The magnitude of this difference is dependent on the sample size, mutation rates (theta-values) and observed gene diversities. Moreover, we show that the number of observed alleles differs among loci with the same observed gene diversity but different mutation rates (theta-values). We propose that a reliable test statistic based on allele excess must determine the confidence interval by computer simulations conditional on the observed gene diversity and theta-values. As the latter are notoriously difficult to obtain for experimental data, we suggest that other statistics, such as lnRV, may be better suited to the identification of microsatellite loci subject to selection.

Two distinct modes of microsatellite mutation processes: evidence from the complete genomic sequences of nine species.

We surveyed microsatellite distribution in 10 completely sequenced genomes. Using a permutation-based statistic, we assessed for all 10 genomes whether the microsatellite distribution significantly differed from expectations. Consistent with previous reports, we observed a highly significant excess of long microsatellites. Focusing on short microsatellites containing only a few repeat units, we demonstrate that this repeat class is significantly underrepresented in most genomes. This pattern was observed across different repeat types. Computer simulations indicated that neither base substitutions nor a combination of length-dependent slippage and base substitutions could explain the observed pattern of microsatellite distribution. When we introduced one additional mutation process, a length-independent slippage (indel slippage) operating at repeats with few repetitions, our computer simulations captured the observed pattern of microsatellite distribution.

Nonneutral admixture of immigrant genotypes in African Drosophila melanogaster populations from Zimbabwe.

Drosophila melanogaster originated in Africa and colonized the rest of the world only recently (approximately 10,000 to 15,000 years ago). Using 151 microsatellite loci, we investigated patterns of gene flow between African D. melanogaster populations representing presumptive ancestral variation and recently colonized European populations. Although we detected almost no evidence for alleles of non-African ancestry in a rural D. melanogaster population from Zimbabwe, an urban population from Zimbabwe showed evidence for admixture. Interestingly, the degree of admixture differed among chromosomes. X chromosomes of both rural and urban populations showed almost no non-African ancestry, but the third chromosome in the urban population showed up to 70% of non-African alleles. When chromosomes were broken into contingent microsatellite blocks, even higher estimates of admixture and significant heterogeneity in admixture was observed among these blocks. The discrepancy between the X chromosome and the third chromosome is not consistent with a neutral admixture hypothesis. The higher number of European alleles on the third chromosome could be due to stronger selection against foreign alleles on the X chromosome or to more introgression of (beneficial) alleles on the third chromosome.