Microevolution, low clonal diversity and genetic affinities of parthenogenetic sitobion aphids in new zealand

In sharp contrast to their southeast Asian and European counterparts, Sitobion miscanthi and S. near fragariae aphids in Australia exhibit a complete absence of sexual reproduction. This demands an explanation within the context of the evolution and maintenance of sex and parthenogenesis. Accordingly, we executed a genetic analysis of the two species in neighbouring New Zealand. Microsatellites and single-stranded conformation polymorphism/sequence analysis of the nuclear gene elongation factor 1alpha were used to identify aphid clones and confirm species identification, respectively. Karyotypic variation was also investigated. The New Zealand fauna showed few (nonrecombining) genotypes and appears to have received migrants from both Australia and Asia. Other genotypes have apparently arisen in situ in New Zealand, exhibiting stepwise mutation of microsatellite alleles and also karyotypic change. Thus, these data represent rare evidence of evolution within wild-living parthenogenetic lineages. Karyotypic changes appear to occur at a rate even greater than that of microsatellite evolution. Strong geographical partitioning of genotypes/karyotypes was found, with certain ones predominating over large areas. These data suggest that clonal selection could be important in the distribution and patterning of genetic variation. We present a model to explain the genetic patterns, with particular reference to the absence of sexual reproduction in Sitobion aphids in New Zealand and Australia.     

Patterns, sources and ecological implications of clonal diversity in apomictic Ranunculus carpaticola (Ranunculus auricomus complex, Ranunculaceae)

Sources and implications of genetic diversity in agamic complexes are still under debate. Population studies (amplified fragment length polymorphisms, microsatellites) and karyological methods (Feulgen DNA image densitometry and flow cytometry) were employed for characterization of genetic diversity and ploidy levels of 10 populations of Ranunculus carpaticola in central Slovakia. Whereas two diploid populations showed high levels of genetic diversity, as expected for sexual reproduction, eight populations are hexaploid and harbour lower degrees of genotypic variation, but maintain high levels of heterozygosity at many loci, as is typical for apomicts. Polyploid populations consist either of a single AFLP genotype or of one dominant and a few deviating genotypes. genotype/genodive and character incompatibility analyses suggest that genotypic variation within apomictic populations is caused by mutations, but in one population probably also by recombination. This local facultative sexuality may have a great impact on regional genotypic diversity. Two microsatellite loci discriminated genotypes separated by the accumulation of few mutations ('clone mates') within each AFLP clone. Genetic diversity is partitioned mainly among apomictic populations and is not geographically structured, which may be due to facultative sexuality and/or multiple colonizations of sites by different clones. Habitat differentiation and a tendency to inhabit artificial meadows is more pronounced in apomictic than in sexual populations. We hypothesize that maintenance of genetic diversity and superior colonizing abilities of apomicts in temporally and spatially heterogeneous environments are important for their distributional success.     

Environmentally related patterns of reproductive modes in the aphid Myzus persicae and the predominance of two 'superclones' in Victoria, Australia

Asexual organisms that naturally coexist with sexual relatives may hold the key to understanding the maintenance of sex and recombination, a long-standing problem in evolutionary biology. This situation applies to the peach-potato aphid, Myzus persicae, in southeastern Australia where cyclical parthenogens form mixed populations with obligate parthenogens. We collected M. persicae from several areas across Victoria, genotyped them at seven microsatellite loci and experimentally determined their reproductive mode. The geographic distribution of reproductive modes was correlated with two environmental variables that differentially affect obligate and cyclical parthenogens; obligate parthenogens were less frequent in areas with cold winters because they cannot produce frost-resistant eggs while cyclical parthenogens were limited by the availability of their primary host, peach, on which sexual reproduction takes place. Clonal diversity increased with the proportion of cyclical parthenogens in a sample because they tended to have unique microsatellite genotypes, whereas many obligate parthenogens were copies of the same genotype. Two obligately asexual genotypes stood out as being very abundant and widespread, one constituting 24% and the other 17.4% of the entire collection. Both of these highly successful genotypes were present in the majority of all collection sites. Genetic population structure was weak, albeit significant, with a multilocus FST of only 0.021 when samples were reduced to only one representative of each genotype. Interestingly, obligate parthenogens were, on average, more heterozygous and exhibited larger allele size differences between the two alleles at individual loci than cyclical parthenogens. This striking pattern could result from hybridization, for which we have no evidence, or may reflect the previously proposed model of biased mutational divergence of microsatellite alleles within asexual aphid lineages.     

Climate effects on life cycle variation and population genetic architecture of the black bean aphid, Aphis fabae

Aphid species may exhibit different reproductive modes ranging from cyclical to obligate parthenogenesis. The distribution of life cycle variation in aphids is generally determined by ecological forces, mainly climate, because only sexually produced diapausing eggs can survive harsh winters or periods of absence of suitable host plants. Aphids are thus interesting models to investigate intrinsic and environmental factors shaping the competition among sexual and asexual lineages. We conducted a Europe-wide sampling of black bean aphids, Aphis fabae, and combined population genetic analyses based on microsatellite data with an experimental determination of life cycle strategies. Aphids were collected from broad beans (Vicia faba) as well as some Chenopodiaceae, but we detected no genetic differentiation between aphids from different host plants. Consistent with model predictions, life cycle variation was related to climate, with aphids from areas with cold winters investing more in sexual reproduction than aphids from areas with mild winters. Accordingly, only populations from mild areas exhibited a clear genetic signature of clonal reproduction. These differences arise despite substantial gene flow over large distances, which was evident from a very low geographic population structure and a lack of isolation-by-distance among 18 sites across distances of more than 1000 km. There was virtually no genetic differentiation between aphids with different reproductive modes, suggesting that new asexual lineages are formed continuously. Indeed, a surprising number of A. fabae genotypes even from colder climates produced some parthenogenetic offspring under simulated winter conditions. From this we predict that a shift to predominantly asexual reproduction could take place rapidly under climate warming.     

Grain aphid population structure: no effect of fungal infections in a 2-year field study in Denmark

1 Sitobion avenae (F.) is a serious pest in Danish cereal crops. To understand the population genetic structure, aphids were sampled in seven different winter wheat (Triticum sativum Lamarck) fields throughout Denmark. The aphids were genotyped with seven microsatellite markers. In total, 2075 aphids were collected and 1203 of these were genotyped. 2 The Danish S. avenae populations displayed very high genotypic diversity, high percentages of unique genotypes and low linkage disequilibria; this is likely to be a result of genetic recombination encompassed by their holocyclic lifestyle. The populations showed very limited differentiation and no sign of isolation by distance. Almost all the genetic variation was ascribed within the populations rather than between populations, probably due to a high migration rate at approximate 10% per generation. 3 Seasonal changes in clonal diversity and distribution of asexual summer generations of S. avenae within the infestation period in a single winter wheat field were followed over two consecutive years by weekly sampling from 60 plots each of 20 × 20 m. Clonal diversity was high in all samples with no dominant clonal lineages and no significant difference in the genotypic diversity between weeks or between years. However, a temporal genetic differentiation effect, throughout the infestation, suggests that selective factors or high temporal migration play an important role in shaping the genetic structure S. avenae. 4 Analyses of fungal infected and uninfected aphids were performed to test whether some clonal linage were more often infected by fungi from the Entomophthorales under field conditions. In total, 54 progeny from aphids with Entomophthorales were genotyped and compared with 422 uninfected aphid genotypes. The Entomophthorales‐infected aphid genotypes did not cluster out together, suggesting that these fungal pathogens did not affect the population differentiation or clonal distribution of S. avenae in a Danish agroecosystem. 5 Our findings indicate that S. avenae populations can be controlled using conservation biological control     

Genetic structure of an aphid studied using microsatellites: cyclic parthenogenesis, differentiated lineages and host specialization

In a previous study, samples of the grain aphid Sitobion avenae (F.) were collected from wheat and adjacent cocksfoot hosts in a population thought to be primarily parthenogenetic, and DNA from individual aphids was analysed with a multilocus technique. Here we have applied single-locus microsatellites and a mitochondrial DNA marker to a subset of the same DNA extracts, and have made several additional inferences about important genetic and population processes in S. avenae . Microsatellite analysis indicated very high levels of genic and genotypic variation. S. avenae fell into three genotypic groups inferred to be almost noninterbreeding, while analysis of linkage and Hardy-Weinberg equilibria suggested high levels of sexual recombination within each genotypic group. Host specialization was evident: one lineage was found only on wheat, and one (bearing many alleles inferred to be introgressed from the blackberry-grass aphid S. fragariae (Walker)) was found only on cocksfoot. The third group of interrelated genotypes was found commonly on both hosts. Although most genotypes were found only once, some were much more numerous in the sample than expected from the frequency of the alleles they contained. This, and rapid temporal changes in genotypic composition of samples, indicates strong selective differences between genotypes and lineages. In the major genotypic group, the commonest genotypes were significantly more homozygous than were rare ones: thus these data may help to explain the frequent observation of homozygous excess in aphid allozymes. The genotype group showing S. avenae -like as well as S. fragariae -like alleles also carried S. fragariae -like mitochondrial DNA in at least 25/31 cases, indicating gender-asymmetrical hybridization.     

Reproductive mode and population genetic structure of the cereal aphid Sitobion avenae studied using phenotypic and microsatellite markers

As French populations of the aphid Sitobion avenae exhibit a range of reproductive modes, this species provides a good opportunity for studying the evolution of breeding system variation. The present analysis combined ecological and genetic investigations into the spatial distribution of variation in reproductive mode. Reproductive mode was characterized in 277 lineages of S. avenae from France, and these aphids were scored for five microsatellite loci. The analyses revealed strong geographical partitioning of breeding systems, with obligate asexuals mostly restricted to the south of France, while lineages producing sexual forms were more common in the north. Contrary to what might be anticipated for organisms with frequent parthenogenesis, there was substantial genic and genotypic diversity, even in the obligately asexual lineages. More than 120 different genotypes were detected among the 277 aphid lineages, with an average of 5.9 alleles per locus (range four to 16) and heterozygosity of 56.7%. As with previous studies of allozyme variation in aphids, most loci showed heterozygote deficits, and disequilibrium was common among allelic variants at different loci, even after removal of replicate copies of genotypes that might have been derived through clonal reproduction. Our results suggest that selection is important in structuring reproductive systems and genetic variation in French S. avenae. Canonical correspondence analysis was employed to examine the associations between genotypic and phenotypic variables, enabling the identification of alleles correlated with life-history traits.     

微卫星标记及其在蚜虫种群生物学研究中的应用

微卫星（SSR）是以1～6个碱基为重复单位组成的简单串联重复序列,具有丰度高、多态性高、共显性遗传、选择中性和可重复性好等优点。微卫星标记技术可分为三种类型：基于杂交的SSR指纹,基于PCR的SSR指纹和单位点SSR的PCR扩增。前两者不需要知道基因组序列信息,属于多位点标记;后者则需要前期工作,属于单位点标记,即通常所指的微卫星标记。到目前为止,共在14种蚜中筛选到141个微卫星位点,并发表了相关的引物序列,这给今后的相关研究提供了丰富的共享资源。业已证明微卫星引物在邻近种之间有一定的通用性。微卫星标记严格遵守孟德尔遗传规律,已经被用来推断某些蚜虫有性生殖的情况。同种蚜虫中可能同时存在有性系和无性系,微卫星研究证明有性系比无性系有更高的遗传多样性,无性系通常表现出杂合过剩和连锁不平衡。蚜虫的迁飞规律适合用微卫星标记加以研究,已有的研究显示在具有高度迁飞特性的种类中,地理种群间的遗传分化程度低,基因频率的相似性高。存在广泛分布的相同的多位点基因型是迁飞性蚜虫的另一个重要特征。在我国,微卫星应用于蚜虫生物学研究的工作还较少,鉴于该种标记的优良特性和巨大的潜力,本文建议今后相关的研究应该首先考虑微卫星标记。     

Consensus features of microsatellite distribution: Microsatellite contents are universally correlated with recombination rates and are preferentially depressed by centromeres in multicellular eukaryotic genomes

Microsatellite DNA is highly polymorphic and informative, which makes its distribution pattern and its associations very valuable for marker applications and genomic research in evolution. Using computational and statistical approaches, based on database technology, we have demonstrated that microsatellite content is consistently and significantly 2 to 5 fold lower than the average chromosomal level in the centromeric and pericentromeric regions of the chromosomes of two plant species, Arabidopsis thaliana and Oryza sativa. We conducted a path coefficient analysis to compare the direct effect of microsatellites (from mono-nucleotide through to penta-nucleotide repeats) on recombination rates. The results revealed that tri- and penta-nucleotide microsatellites significantly influence recombination rates. In the human genome, tri-, tetra- and mono-nucleotide microsatellites, in decreasing order, make significant direct contributions to recombination rates, according to DECODE, GENTHON, and MARSHFIELD averages. Path coefficient analysis in rice and human genomes of the impact of di-nucleotide microsatellites of different motifs on recombination rates indicate that motifs with either A or T have an effect, resulting in increased recombination rates for microsatellites with motifs consisting of 50% A or T, such as AG, TC, CA, TG. Conversely, microsatellites with motifs consisting of only A & T or G & C, such as AT, TA, GC or CG, have decreased recombination rates. The extremely low microsatellite content in centromeric and pericentromeric regions, as well as the quantitative association of microsatellite sequences with the recombination rate at the genome level, suggests that purifying selection in genome evolution creates a balance between genomic polymorphisms and the biological function of sequences in a genome.     

Frequent non-reciprocal exchange in microsatellite-containing-DNA-regions of vertebrates

Microsatellites are DNA-fragments containing short repetitive motifs with 2–10 bp. They are highly variable in most species and distributed throughout the whole genome. It is broadly accepted that their high degree of variability is closely associated with mispairing of DNA-strands during the replication phase, termed slippage, although recombination is also observed. The aim of this study is to demonstrate evidence that non-reciprocal recombination processes changing the total genomic structure are common in microsatellites and flanking regions. We sequenced DNA fragments from birds in which microsatellites are located, and analyzed the structure of the microsatellites and their flanking regions. Additionally, other data and those from literature of three microsatellite regions of primates coding for the Ataxin-2, the Huntingtin and the TATA-box binding protein were analyzed. The structures of seven avian and three primate microsatellites support the hypothesis that non-reciprocal recombination is a common process that may also contribute considerably to the variation at microsatellite loci. We conclude that results of population genetic studies that are analyzed statistically with methods based on stepwise mutation models should be interpreted with caution if no detailed information on the allelic variation of microsatellites is available.     

Do reproductive tactics vary with habitat heterogeneity in the intertidal sea anemone Actinia tenebrosa?

Cnidarians display a diverse range of reproductive tactics including sexual and asexual modes of reproduction. Although few studies have looked for intraspecific variation in reproductive tactics, flexible expression of such life-history traits may be favoured in species that occupy a range of habitats. We tested this in the sea anemone Actinia tenebrosa by comparing cycles of reproductive activity and the mode of production of brooded larvae in local populations occupying boulder fields and stable rock platforms. We determined the mode of production of broods from eight rock platforms (separated by up to 1600 km) and two boulder shores on the south east coast of Australia using a combination of allozyme data and four newly characterised microsatellite markers.

We determined seasonal patterns of brooding and gonad development by monthly dissection of 15–30 adults from each of two boulder fields and two stable platforms. Previous genetic studies have shown that populations of A. tenebrosa on rock platforms can be highly clonal, whereas anemones on more heterogeneous boulder habitats display levels of genotypic diversity similar to that expected for sexual reproduction. We genotyped a total of 221 juveniles from 37 brooding adults including 11 broods and 80 juveniles from boulder shores. We did not detect any evidence of sexual production of broods. All brooded juveniles displayed identical multi-locus genotypes to their brood parent irrespective of habitat of origin or location, including 28 broods (200 juveniles) that were heterozygous at one or more locus. Similarly, we found that temporal patterns of gonad formation and brooding were consistent across habitats and locations. We detected 346 mature males, 234 non-reproductive or immature individuals, and no mature females within a total of 580 dissected individuals. These data suggest that the reproductive tactics of A. tenebrosa are essentially fixed and that variation in the genotypic diversity of populations may reflect variation in factors such as the input of sexually derived planktonically dispersed recruits or post-settlement processes. However, the apparent lack of females paradoxically implies that sexual reproduction, and hence recruitment, must be rare or no longer possible within some populations, and highlights the need for long-term studies of these populations.     

Amplified fragment length polymorphism (AFLP) markers reveal that population structure of triploid dandelions (Taraxacum officinale) exhibits both clonality and recombination

Highly variable amplified fragment length polymorphism (AFLP) fingerprints of triploid apomictic dandelions obtained from three localities in an area where diploids are lacking were analysed to infer the predominant modes of reproduction. The distribution of markers was analysed using character compatibility to infer whether many genotypes agree with a tree-like structure in the data set. The presence of incompatible character state combinations (matrix incompatibility; MI) was used as a measure of genetic exchange. The detection of overrepresented genotypes, of which some were widespread, confirmed asexual reproduction. Not all genotypes were overrepresented; approximately half of the genotypes in the three localities were found only once. Because, in terms of genotype frequencies, only a part of the genetic variation is described, more important aspects of the molecular data such as relationships between markers or genotypes have been studied. The analysis of character compatibility indicated a disagreement of the data with a clonal structure. Nearly all genotypes contributed to MI and this contribution varied considerably among genotypes in each sampled locality. A gradual decrease of matrix incompatibility upon successive deletion of genotypes showing the highest contribution to MI indicated that marker distribution of virtually all genotypes disagreed with a tree-like structure in the data. This result suggested that many genotypes were separated by one or more sexual generations. Consistent with this conclusion was the fact that markers that show a low probability of contributing to MI are different in every sampled locality, which is most easily explained as the result of recombination. Apparently, asexual reproduction has resulted in overrepresented, widespread genotypes but sexual recombination has also substantially contributed to genetic variation in the sites studied.     

Reconstruction of microsatellite mutation history reveals a strong and consistent deletion bias in invasive clonal snails,Potamopyrgus antipodarum

Direct observations of mutations and comparative analyses suggest that nuclear microsatellites show a tendency to expand, with reports of deletion biases limited to very long alleles or a few loci in multilocus studies. Here we investigate microsatellite evolution in clonal snails, Potamopyrgus antipodarum, since their introduction to Britain in the 19th century, using an analysis based on minimum spanning networks of multilocus microsatellite genotypes. British populations consist of a small number of highly distinct genotype groups with very few outlying genotypes, suggesting clonal lineages containing minor variation generated by mutation. Network patterns suggest that a single introduced genotype was the ancestor of all extant variation and also provide support for wholly apomictic reproduction within the most common clonal lineage (group A). Microsatellites within group A showed a strong tendency to delete repeats, with an overall bias exceeding 88%, irrespective of the exact method used to infer mutations. This highly unusual pattern of deletion bias is consistent across populations and loci and is unrelated to allele size. We suggest that for persistence of microsatellites in this clone, some change in the mutation mechanism must have occurred in relatively recent evolutionary time. Possible causes of such a change in mechanism are discussed.     

Genetic variation and structure of the endangered Lady Fern Athyrium viridescentipes based on ubiquitous genotyping

To clarify the genetic status and provide effective information for the conservation of Athyrium viridescentipes, a critically endangered fern species with only 103 individuals remaining in the wild, we conducted ubiquitous genotyping to determine the genotypes of all remnant individuals of the target species. We analyzed the genetic variation of the 103 known individuals in four populations by using 13 microsatellite loci. The genotypes of single spores from a sporophytic individual were also determined in order to reveal the breeding system of this species. The level of allelic variation in A. viridescentipes was significantly lower than that of closely related Athyrium species. The genetic composition of the four populations was rather similar. Sixty-nine individuals (67%) possessed an identical pattern in the allele combinations at 13 microsatellite loci. The mean pairwise F (ST) among four populations was 0.018. The segregated pattern of alleles, determined by single-spore genotyping, revealed that allelic recombination occurs through meiosis. The results indicate that this species contains a low level of genetic variation, has low population differentiation, and maintains populations by sexual reproduction. These findings could lead to more effective conservation programs, the selection of the most appropriate individuals for ex situ conservation efforts, and separate management of extant populations.     

The genetic outcomes of sex and recombination in long-term functionally parthenogenetic lineages of Australian Sitobion aphids

The typical life cycle of an aphid is cyclical parthenogenesis which involves the alternation of sexual and asexual reproduction. However, aphid life cycles, even within a species, can encompass everything on a continuum from obligate sexuality, through facultative sexuality to obligate asexuality. Loss of the sexual cycle in aphids is frequently associated with the introduction of a new pest and can occur for a number of environmental and genetic reasons. Here we investigate loss of sexual function in Sitobion aphids in Australia. Specifically, we aimed to determine whether an absence of sexual reproduction in Australian Sitobion results from genetic loss of sexual function or environmental constraints in the introduced range. We addressed our aims by performing a series of breeding experiments. We found that some lineages have genetically lost sexual function while others retain sexual function and appear environmentally constrained to asexuality. Further, in our crosses, using autosomal and X-linked microsatellite markers, we identified processes deviating from normal Mendelian segregation. We observed strong deviations in X chromosome transmission through the sexual cycle. Additionally, when progeny genotypes were examined across multiple loci simultaneously we found that some multilocus genotypes are significantly over-represented in the sample and that levels of heterozygosity were much higher than expected at almost all loci. This study demonstrates that strong biases in the transmission of X chromosomes through the sexual cycle are likely to be widespread in aphids. The mechanisms underlying these patterns are not clear. We discuss several possible alternatives, including mutation accumulation during periods of functional asexuality and genetic imprinting.     

The Pattern of Genetic Variability in Apomictic Clones of Taraxacum officinale Indicates the Alternation of Asexual and Sexual Histories of Apomicts

Dandelions (genus Taraxacum) comprise a group of sexual diploids and apomictic polyploids with a complicated reticular evolution. Apomixis (clonal reproduction through seeds) in this genus is considered to be obligate, and therefore represent a good model for studying the role of asexual reproduction in microevolutionary processes of apomictic genera. In our study, a total of 187 apomictic individuals composing a set of nine microspecies (sampled across wide geographic area in Europe) were genotyped for six microsatellite loci and for 162 amplified fragment length polymorphism (AFLP) markers. Our results indicated that significant genetic similarity existed within accessions with low numbers of genotypes. Genotypic variability was high among accessions but low within accessions. Clustering methods discriminated individuals into nine groups corresponding to their phenotypes. Furthermore, two groups of apomictic genotypes were observed, which suggests that they had different asexual histories. A matrix compatibility test suggests that most of the variability within accession groups was mutational in origin. However, the presence of recombination was also detected. The accumulation of mutations in asexual clones leads to the establishment of a network of clone mates. However, this study suggests that the clones primarily originated from the hybridisation between sexual and apomicts.     

Segregation of microsatellite alleles and residual heterozygosity at single loci in homozygous androgenetic common carp (Cyprinus carpio L.).

Thirty-three androgenetic progeny groups of common carp were analysed using 11 microsatellite markers to (i) verify the homozygous status of the 566 androgenetic individuals, (ii) analyse the microsatellite allele segregation, and (iii) study the possible association of microsatellite alleles with phenotypic traits. In total, 92% of the androgenetic individuals proved to be homozygous at all 11 loci. Forty-three of the 47 heterozygous individuals were heterozygous at a single locus only. This heterozygosity was probably due to DNA fragments caused by UV irradiation of the eggs. although the maternal origin of the fragments could not be proved beyond doubt. Screening with 11 microsatellites also revealed two linkage groups, a segregation distortion at two microsatellite loci, and the possible association of some microsatellites with mass, length, stress-related plasma cortisol levels, and basal plasma glucose levels. The success of the linkage and association study could be explained by a low recombination frequency due to high chiasma interference. This would imply a relatively short genetic map for common carp.     

Genetic architecture of sexual and asexual populations of the aphid Rhopalosiphum padi based on allozyme and microsatellite markers

Cyclical parthenogens, including aphids, are attractive models for comparing the genetic outcomes of sexual and asexual reproduction, which determine their respective evolutionary advantages. In this study, we examined how reproductive mode shapes genetic structure of sexual (cyclically parthenogenetic) and asexual (obligately parthenogenetic) populations of the aphid Rhopalosiphum padi by comparing microsatellite and allozyme data sets. Allozymes showed little polymorphism, confirming earlier studies with these markers. In contrast, microsatellite loci were highly polymorphic and showed patterns very discordant from allozyme loci. In particular, microsatellites revealed strong heterozygote excess in asexual populations, whereas allozymes showed heterozygote deficits. Various hypotheses are explored that could account for the conflicting results of these two types of genetic markers. A strong differentiation between reproductive modes was found with both types of markers. Microsatellites indicated that sexual populations have high allelic polymorphism and heterozygote deficits (possibly because of population subdivision, inbreeding or selection). Little geographical differentiation was found among sexual populations confirming the large dispersal ability of this aphid. In contrast, asexual populations showed less allelic polymorphism but high heterozygosity at most loci. Two alternative hypotheses are proposed to explain this heterozygosity excess: allele sequence divergence during long-term asexuality or hybrid origin of asexual lineages. Clonal diversity of asexual lineages of R. padi was substantial suggesting that they could have frozen genetic diversity from the pool of sexual lineages. Several widespread asexual genotypes were found to persist through time, as already seen in other aphid species, a feature seemingly consistent with the general-purpose genotype hypothesis.     

Segregation of autosomes during spermatogenesis in the peach-potato aphid (Myzus persicae) (Sulzer) (Hemiptera: Aphididae)

Most aphids are cyclic parthenogens, so are ideal models in studies of the mechanisms and consequences of sex and recombination. However, owing to a shortage of physical and genetic markers, there have been few studies of the most fundamental genetic processes in these organisms. For example, it is not known whether autosomal segregation during male spermatogenesis is in Mendelian proportions: we address that question here. The aphid Myzus persicae has a typical karyotype of 2n = 12 in females (XX), while males are XO (2n = 11). During male meiosis, only the spermatocytes with an X chromosome are viable. We hypothesized that assortment of autosomes might be non-random because chromosomal imprinting leading to elimination of the paternal autosomes is seen in the closely related coccoids. In other aphid models, we have observed segregation distortions at single microsatellite loci (Wilson, 2000). Such distortions may have nothing to do with 'selfish' behaviour, but may be caused by mutation accumulation causing fitness differentials. Thus single-locus distortions might be predicted to be more likely to be detected via the male lines of clones that have lost the ability to reproduce sexually (male-producing obligate parthenogenesis (androcyclic)). Using microsatellites we show that genetic imprinting or selfish autosome behaviour does not occur in male M. persicae. Generally, loci segregated in Mendelian proportions in both sexes of cyclically parthenogenetic (holocyclic) clones. However, in androcyclic clones, segregation distortions consistently involved the same two autosomes. This is consistent with linkage of markers to deleterious mutations associated with a loss of sexual reproduction.     

Microsatellite variability in wild populations of the house mouse is not influenced by differences in chromosomal recombination rates

Differences in recombination rates along the chromosomes can influence the evolution of neutral loci via hitchhiking effects. Generally, these effects should be stronger in regions of low recombination than in regions of high recombination. Detailed information on physical and genetic maps in the house mouse now allows an assessment of the correlation between neutral variability and recombination rates at given chromosomal regions. We chose 29 microsatellite loci from chromosomal regions which show differences in recombination rates and tested their variability in samples from five wild populations of Mus musculus musculus and M. m. domesticus . Our results provide no evidence for a correlation between microsatellite variability and recombination rates. This suggests that the high average mutation rate of microsatellites in mammals counterbalances the effects of long-range hitchhiking in the mouse genome.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 629–635.