Anssi Saura

The effect of serpentine on the population structure of Silene dioica (Caryophyllaceae)

Serpentine soils are rich in heavy metals and have a distinctive flora. Silene dioica is a member of the Scandinavian serpentine plant community but is also widespread outside serpentine soils. To study the population genetic consequences of serpentine stress and the origin and evolution of serpentine populations we analyzed the isozyme genetic structure of S. dioica. Seventeen populations located in the mountains of Västerbotten and Jämtland, central Sweden, were investigated by starch gel enzyme electrophoresis. About one half of the populations grow in serpentine soils and the rest on adjacent non‐serpentine sites. Analyses of allele frequencies show that both serpentine and non‐serpentine populations in the northern part of the studied area (Västerbotten) are genetically similar. Evidently serpentine does not exert strong selection acting upon isozyme loci. In the south (Jämtland), however, the serpentine populations exhibit genetic differentiation. This allozyme divergence is probably not due to direct selection but rather represents the effects of isolation and genetic drift. The results suggest that S. dioica has colonized serpentine repeatedly and that the tolerant populations have a multiple origin.

Gene Flow and Pollinator Behaviour in Silene dioica Populations

Silene dioica (L.) Clairv. (Caryophyllaceae) is an insect pollinated and obligately cross-fertilising herb commonly found on serpentine, i.e. soils rich in heavy metals, and on cultivated meadows. Groups of Silene populations were studied in two areas in the Scandinavian mountains. In a northern area Silene populations grow in an open pine and birch forest while in the south, populations are surrounded by a continuous and dense spruce forest. Gene flow, estimated on the basis of allele frequencies, is highly restricted in the south in comparison to the north. This has led to an extensive genetic differentiation among populations. The pollinator guilds (Thricops flies, syrphid flies and bumblebees) are similar in the northern and southern populations and can therefore not account for the difference in gene flow. The flowering in nearby populations overlaps broadly and is also not the reason for the genetic differentiation in the south. The differentiation is more likely due to vegetation that limits pollinator movement. The level of gene flow differs in the two habitats both in the north and in the south. The gene flow is restricted among serpentine populations but common among meadow populations. Pollen dispersal, and in particular, anthropogenic seed dispersal mediate gene flow among meadows. Serpentine populations are unaffected by human activity. Pollinators are therefore the main agent of gene flow on serpentine. This study shows the importance of the surrounding vegetation in influencing gene flow patterns among populations as well as that habitat fragmentation affects the genetic properties of plant populations. In addition, we have also observed a restricted gene flow within populations. Enzyme allele frequencies show an excess of homozygotes within most of the populations. This can be explained through limited pollen dispersal and differences in male and female flowering density.

Dispersal and clonal diversity of North-European parthenogenetic earthworms

At least 15 earthworm species are known to reproduce parthenogenetically. Most of these retain meiosis but the chromosome set is duplicated before meiosis; alternatively there is mitosis instead of meiosis. In both cases the offspring are genetic copies of the parent worm. Parthenogens are always polyploid. Parthenogenesis is associated with a dispersal advantage: a single propagule suffices to establish a new population. We have studied the clone pool structure and dispersal of ecologically dissimilar polyploid parthenogenetic lumbricids in northern Europe using enzyme electrophoresis. The anthropochorous Octolasion cyaneum has a very low number of clones in populations that are located far away from each other. The opposite is the eurytopic Dendrobaena octaedra that has a wide array of clones in each population. The ripicolous Eiseniella tetraedra disperses with flowing water and possibly also through zoochory. On subarctic North-European mountains its clone pool decreases with increasing elevation. At the top there are a few but persistent clones. Small brooks carry propagules downstream, so that at the mouths of brooks clone pools are more diverse than higher up; again larger rivers carry clones downstream. Clone dispersal is relatively free in a freely flowing river, while dams stop propagules in harnessed rivers. The mouths of rivers have high E. tetraedra clone diversity. Clones disperse from these clone centers to islands formed through land uplift along the northern Baltic Sea. The annual turnover of clones is high on these islands. A survey of epigeic and endogeic parthenogens on the Åland islands which serve as stepping stones between Estonia, Finland and Sweden shows an invasion route of clones across the Baltic Sea. Anthropochory (Aporrectodea rosea and Octolasion cyaneum) and hydrochory (E.tetraedra and Dendrobaena octaedra) seem to play important roles in the clone pool formation on the Åland islands. Quite recently an exotic parthenogen Dichogaster bolaui has found a curious habitat in human settlements viz., the sewer pipe system. Many clonal earthworms show significant morphological and morphometric diversity in and between sample localities but we have failed to associate this variation with the clonal variability. It seems that local factors modify the morphometrics and morphology ultimately determined by the genotype of parthenogenetic earthworms.

Cytology of Asexual Animals

We review the cytological mechanisms underlying asexual reproduction, i.e. reproduction without fertilization, in animals. Asexuality or parthenogenesis has evolved many times and the cytological mechanisms to restore the parental chromosome number can vary between and even within species. In automictic or meiotic parthenogenesis, meiosis takes place but the chromosomal constitution of the mother is restored through one or several different mechanisms. Some of these mechanisms enforce homozygosity at all loci while some other mechanisms pass the genome of the mother intact to the offspring. In apomictic or mitotic parthenogenesis the eggs are formed through what is essentially a set of mitoses. Polyploidy, is in general incompatible with chromosomal sex determination and is a rare condition in animals. However, many asexual and hermaphroditic forms are polyploid to various degrees. Polyploidy is divided into allo- and autopolyploidy. In the former mode the chromosome sets are derived from two or more different species while in autopolyploidy the multiplication has taken place within one species. We discuss the evolutionary consequences of the different cytological mechanisms involved in asexual reproduction.

Meiosis and Its Deviations in Polyploid Animals

We review the different modes of meiosis and its deviations encountered in polyploid animals. Bisexual reproduction involving normal meiosis occurs in some allopolyploid frogs with variable degrees of polyploidy. Aberrant modes of bisexual reproduction include gynogenesis, where a sperm stimulates the egg to develop. The sperm may enter the egg but there is no fertilization and syngamy. In hybridogenesis, a genome is eliminated to produce haploid or diploid eggs or sperm. Ploidy can be elevated by fertilization with a haploid sperm in meiotic hybridogenesis, which elevates the ploidy of hybrid offspring such that they produce diploid gametes. Polyploids are then produced in the next generation. In kleptogenesis, females acquire full or partial genomes from their partners. In pre-equalizing hybrid meiosis, one genome is transmitted in the Mendelian fashion, while the other is transmitted clonally. Parthenogenetic animals have a very wide range of mechanisms for restoring or maintaining the mothers ploidy level, including gamete duplication, terminal fusion, central fusion, fusion of the first polar nucleus with the product of the first division, and premeiotic duplication followed by a normal meiosis. In apomictic parthenogenesis, meiosis is replaced by what is effectively mitotic cell division. The above modes have different evolutionary consequences, which are discussed. See also the sister article by Grandont et al. in this themed issue.

Low clonal diversity and morphometrics in the parthenogenetic earthworm Octolasion cyaneum (Sav.): The 7th international symposium on earthworm ecology · Cardiff · Wales · 2002

Enzyme electrophoresis indicated that the cosmopolitan parthenogenetic earthworm Octolasion cyaneum (Sav.) has a very low clonal variability in populations located far from each other in Europe and Australia. In northern Europe, namely Finland, Sweden and Estonia, only one or two clones were found at a site. Specimens from Germany, England and Australia also had a low clonal heterogeneity. A sample from Switzerland showed, however, a higher clone pool diversity in comparison to that of the other sites. In general, the ratio number of clones/number of worms studied was higher in the southern than in the northern sample localities. This is best explained by assuming that there is a low-frequency flow of immigrating propagules from the south to the north. We propose that the low heterogeneity of the mainly anthropochorously spreading O. cyaneum is rather due to founder effect than to competitive exclusion of less adapted clones. Morphometric variability measured in 0. cyaneum adults showed some statistically significant differences between the localities but no clear-cut geographical trends were found. The differences in morphometric characters are best attributed to local factors.

Clonal and morphological variation in marginal populations of parthenogenetic earthworms Octolasion tyrtaeum and O. cyaneum (Oligochaeta, Lumbricidae) from eastern Fennoscandia

Abstract Clone pool diversity was studied in two parthenogenetic Octolasion species in South Finland belonging to the region of Fennoscandia where the species occur at the northern margin of their European range. In spontaneously dispersing O. tyrtaeum we recorded 24 clones (238 individuals/8 localities), but in anthropochorously dispersing O. cyaneum only two clones (134 ind./4 locs.) were found. In O. tyrtaeum the clone pools of the East ( = sample region) and the North were more similar to each other than to that in the West. Clone groups showed a clinal distribution pattern, which suggests that they may be adapted to regional environments. The low clonal heterogeneity of O. cyaneum is attributed to a small founder population and clonal drift. Morphological variability showed that characters related to reproductive organs vary extremely little in the two species. Considering somatic characters, segment number seemed to be mostly controlled by the genotype, whereas body size varied mostly according to t...

Differences in environmental temperature, ethanol and sucrose associated with enzyme activity and weight changes in Drosophila melanogaster

Activity changes of three enzymes (ADH, ODH and AOX) of Drosophila melanogaster were followed under different environmental conditions. The influences of ethanol, starvation (no carbohydrates in the medium) and ethanol stress during starvation were studied at both 18 and 26 degrees C. Two strains that were monomorphic for different alleles at the Odh and Aldox loci but otherwise identical were used. The investigated environmental conditions affected ADH induction by exogenous ethanol differently in the two strains. The different allozymes of ODH and AOX also responded differently to the treatments. We observed that the sucrose content of the medium on which ethanol exposure took place and the temperature strongly affected the responses within any single strain. Correlations were estimated among the three enzymes in the larval and adult stages of each strain separately. At both temperatures, differences between strains were observed in the patterns of associations of the response variables, in the larval, but not in the adult stages.

MODE OF SWARMING IN RELATION TO REPRODUCTIVE ISOLATION IN MAYFLIES

Mayfly males swarm, that is they fly in a fixed pattern by a specific object, the swarm marker. Females orientate to the same markers. Leptophlebia marginata mayflies were observed to orientate to two kinds of objects in a single locality in central Finland: to trees and to horizontal pale objects on the ground; when dispersed or moved to the other type of marker, they returned to their former orientation. Tree swarming is by far the most common mode of swarming, but some horizontally orientating populations were found. Sympatric populations are genetically and morphologically distinct, whereas other populations appear to have some gene flow between the swarming types. The tree‐swarming mode appears to be primitive and the horizontal mode derived; wind rather than predation is the factor favoring swarming close to the ground. Swarming constitutes an effective mechanism of premating isolation in mayflies.

Chromosome evolution in Neotropical butterflies

We list the chromosome numbers for 65 species of Neotropical Hesperiidae and 104 species or subspecies of Pieridae. In Hesperiidae the tribe Pyrrhopygini have a modal n = 28, Eudaminae and Pyrgini a modal n = 31, while Hesperiinae have n = around 29. Among Pieridae, Coliadinae have a strong modal n = 31 and among Pierinae Anthocharidini are almost fixed for n = 15 while Pierini vary with n = 26 as the most common chromosome number. Dismorphiinae show wide variation. We discuss these results in the context of chromosome numbers of over 1400 Neotropical butterfly species and subspecies derived from about 3000 populations published here and in earlier papers of a series. The overall results show that many Neotropical groups are characterized by karyotype instability with several derived modal numbers or none at all, while almost all taxa of Lepidoptera studied from the other parts of the world have one of n = 29-31 as modal numbers. Possibly chromosome number changes become fixed in the course of speciation driven by biotic interactions. Population subdivision and structuring facilitate karyotype change. Factors that stabilize chromosome numbers include hybridization among species sharing the same number, migration, sexual selection and possibly the distribution of chromosomes within the nucleus.