Out of the forest: past and present range expansion of a parthenogenetic weevil pest, or how to colonize the world successfully

Previous research revealed complex diversification patterns in the parthenogenetic weevil Naupactus cervinus. To understand the origin of clonal diversity and successful spreading of this weevil, we investigated its geographic origin and possible dispersal routes and whether parthenogens can persist in habitats under unsuitable environmental conditions. This study is based on samples taken throughout a broad area of the species' range. We used both mitochondrial and nuclear markers and applied phylogenetic and network analyses to infer possible relationships between haplotypes. Bayesian phylogeographic analyses and ecological niche modeling were used to investigate the processes that shaped genetic diversity and enabled the colonization of new geographic areas. Southeastern Brazil emerges as the original distribution area of N. cervinus. We detected two range expansions, one along natural corridors during the Pleistocene and the other in countries outside South America during recent times. Isolation due to climate shifts during the early Pleistocene led to diversification in two divergent clades, which probably survived in different refugia of the Paranaense Forest and the Paraná River delta. The origin of the clonal diversity was probably a complex process including mutational diversification, hybridization, and secondary colonization. The establishment of N. cervinus in areas outside its native range may indicate adaptation to drier and cooler conditions. Parthenogenesis would be advantageous for the colonization of new environments by preventing the breakup of successful gene combinations. As in other insect pests, the present distribution of N. cervinus results from both its evolutionary history and its recent history related to human activities.

Population genetics of bisexual and unisexual populations of the scaly-winged bark louse Echmepteryx hageni (Insecta: Psocoptera)

The scaly-winged bark louse, Echmepteryx hageni, exhibits a unique pattern of co-existence of apparently differnt reproductive modes. Unisexuality is widespread in eastern North America, while sexual populations are restricted to isolated rock out-croppings in southern Illinois and eastern Kentucky. Three of the four nuclear loci examined show greater genetic diversity in the unisexual form compared to the sexual form of E. hageni, in accordance with the pattern previously shown in mitochondrial genetic data. Neutrality tests of the nuclear loci indicate a consistent signal of demographic expansion in asexual populations, but not in sexual populations. There was evidence of inbreeding in the isolated sexual populations at three of the nuclear loci, and one locus had signs of gene specific balancing selection. However, there is no significant genetic differentiation between bisexual and unisexual populations, possibly due to the greater effective population size of nuclear loci relative to mitochondrial loci. The mitochondrial differentiation of E. hageni populations in the northwestern part of their range (Minnesota and Wisconsin) was also not reflected in the nuclear data. We present three hypotheses that may explain the disparity in observed nuclear and mitochondrial genetic diversity between the reproductive forms of E. hageni.

Polyploid races, genetic structure and morphological features of earthworm Aporrectodea rosea (Savigny, 1826) (Oligochaeta, Lumbricidae) in Ukraine

Four chromosomal races (2n=36, 3n=54, 6n=108, 8n=144) and 96 clones have been revealed among 224 specimens of the earthworm Aporrectodea rosea over the territory of Ukraine by means of karyological analysis and biochemical genetic marking. Each population has been showed by several clones at least; moreover the clones from different places have never been identical. The clones in the range of one population can be identified with the set of quantitative and qualitative parameters.

Standardizing methods to address clonality in population studies

Although clonal species are dominant in many habitats, from unicellular organisms to plants and animals, ecological and particularly evolutionary studies on clonal species have been strongly limited by the difficulty in assessing the number, size and longevity of genetic individuals within a population. The development of molecular markers has allowed progress in this area, and although allozymes remain of limited use due to their typically low level of polymorphism, more polymorphic markers have been discovered during the last decades, supplying powerful tools to overcome the problem of clonality assessment. However, population genetics studies on clonal organisms lack a standardized framework to assess clonality, and to adapt conventional data analyses to account for the potential bias due to the possible replication of the same individuals in the sampling. Moreover, existing studies used a variety of indices to describe clonal diversity and structure such that comparison among studies is difficult at best. We emphasize the need for standardizing studies on clonal organisms, and particularly on clonal plants, in order to clarify the way clonality is taken into account in sampling designs and data analysis, and to allow further comparison of results reported in distinct studies. In order to provide a first step towards a standardized framework to address clonality in population studies, we review, on the basis of a thorough revision of the literature on population structure of clonal plants and of a complementary revision on other clonal organisms, the indices and statistics used so far to estimate genotypic or clonal diversity and to describe clonal structure in plants. We examine their advantages and weaknesses as well as various conceptual issues associated with statistical analyses of population genetics data on clonal organisms. We do so by testing them on results from simulations, as well as on two empirical data sets of microsatellites of the seagrasses Posidonia oceanica and Cymodocea nodosa. Finally, we also propose a selection of new indices and methods to estimate clonal diversity and describe clonal structure in a way that should facilitate comparison between future studies on clonal plants, most of which may be of interest for clonal organisms in general.

The population genetics of clonal and partially clonal diploids

The consequences of variable rates of clonal reproduction on the population genetics of neutral markers are explored in diploid organisms within a subdivided population (island model). We use both analytical and stochastic simulation approaches. High rates of clonal reproduction will positively affect heterozygosity. As a consequence, nearly twice as many alleles per locus can be maintained and population differentiation estimated as F(ST) value is strongly decreased in purely clonal populations as compared to purely sexual ones. With increasing clonal reproduction, effective population size first slowly increases and then points toward extreme values when the reproductive system tends toward strict clonality. This reflects the fact that polymorphism is protected within individuals due to fixed heterozygosity. Contrarily, genotypic diversity smoothly decreases with increasing rates of clonal reproduction. Asexual populations thus maintain higher genetic diversity at each single locus but a lower number of different genotypes. Mixed clonal/sexual reproduction is nearly indistinguishable from strict sexual reproduction as long as the proportion of clonal reproduction is not strongly predominant for all quantities investigated, except for genotypic diversities (both at individual loci and over multiple loci).