Asexual Components Research Articles

Population genetic structure of the sheath blight pathogen Rhizoctonia solani AG-1 IA from rice fields in China, Japan and the Philippines

Sheath blight, caused by the fungal pathogen Rhizoctonia solani AG-1 IA is one of the most important rice diseases worldwide. The objetives of this study was to determine the predominant reproductive system and the genetic structure of 18 rice-infecting populations of R. solani sampled from China, Japan and the Philippines, the most important rice production countries in Asia. Knowledge about the population genetic structure of the pathogen in Asia is useful in identifying sources of infection and formulating sustainable management strategies for rice sheath blight. From a total of 717 isolates, 423 unique multilocus genotypes were detected based on nine microsatellite loci. The three country populations of R. solani AG-1 IA exhibited a mixed reproductive system, which included both sexual and asexual components. A moderate degree of clonality indicated that the asexual sclerotia represent important source of inoculum. Population subdivision varied within and among countries, fitting the isolation by distance model. While no subdivision was found among populations within Japan or within the Philippines, subdivision was detected among populations within China. Historic migration indicated high influx of immigrants from Japan into Northern, Central and Eastern China populations. Southern China contributed a high number of immigrants to the populations from the Philippines.

Population Genetic Variation in the Tree Fern Alsophila spinulosa (Cyatheaceae): Effects of Reproductive Strategy

BackgroundEssentially all ferns can perform both sexual and asexual reproduction. Their populations represent suitable study objects to test the population genetic effects of different reproductive systems. Using the diploid homosporous fern Alsophila spinulosa as an example species, the main purpose of this study was to assess the relative impact of sexual and asexual reproduction on the level and structure of population genetic variation.Methodology/Principal FindingsInter-simple sequence repeats analysis was conducted on 140 individuals collected from seven populations (HSG, LCH, BPC, MPG, GX, LD, and ZHG) in China. Seventy-four polymorphic bands discriminated a total of 127 multilocus genotypes. Character compatibility analysis revealed that 50.0 to 70.0% of the genotypes had to be deleted in order to obtain a tree-like structure in the data set from populations HSG, LCH, MPG, BPC, GX, and LD; and there was a gradual decrease of conflict in the data set when genotypes with the highest incompatibility counts were successively deleted. In contrast, in population ZHG, only 33.3% of genotypes had to be removed to achieve complete compatibility in the data set, which showed a sharp decline in incompatibility upon the deletion of those genotypes. All populations examined possessed similar levels of genetic variation. Population ZHG was not found to be more differentiated than the other populations.Conclusions/SignificanceSexual recombination is the predominant source of genetic variation in most of the examined populations of A. spinulosa. However, somatic mutation contributes most to the genetic variation in population ZHG. This change of the primary mode of reproduction does not cause a significant difference in the population genetic composition. Character compatibility analysis represents an effective approach to separate the role of sexual and asexual components in shaping the genetic pattern of fern populations.

Population genetic consequences of the reproductive system in the liverwort Mannia fragrans

Ecological factors affecting reproduction and dispersal are particularly important in determining genetic structure of plant populations. Polyoicous reproductive system is not rare in bryophytes; however, to date, nothing is known about its functioning and possible population genetic effects. Using the liverwort Mannia fragrans as a model species, the main aims of this study were to separate the relative importance of the components of the polyoicous reproductive system and to assess its consequences on the genetic structure of populations. High sex expression rates increasing with patch size and strongly female-biased sex ratios were detected. Additional input into clonal growth after production of sex organs was found in males compared to females. Similar clonal traits of the rare bisexual and asexual plants and preference toward newly colonized patches suggest that selection prefers colonizers that first develop organs of both sexes, hence ensuring sexual reproduction when no partner is present. Despite frequent spore production, ISSR markers revealed low genetic diversity, probably resulting from the effective clonal propagation of the species and frequent crossing between genetically identical plants. The presence of numerous rare alleles and unique recombinant haplotypes indicates occasional recombination and mutation. Effective spreading of new haplotypes is probably hampered however by large spore size. Since populations are small and isolated, such haplotypes are probably continuously eliminated by genetic drift. These results suggest that although both sexual and asexual reproductions seem to be effective, asexual components of the reproductive system play a greater role in shaping the genetic composition of the populations.

Are sexual or asexual events determining the genetic structure of populations in the liverwort Mannia fragrans?

Reproductive ecological traits such as success of fertilization, partitioning of sexes, the relative success of sexual versus asexual reproduction and dispersal distances are likely to considerably influence genetic structure within and among plant populations. In the liverwort Mannia fragrans both sexual and asexual reproduction can be frequently observed: sporophytes are produced abundantly every year and asexual propagation by fragmentation of thalli is also common. The aim of this study was to use ecological and molecular methods (ISSR markers) to separate the role of sexual and asexual components in shaping the partitioning of genetic variability within and among populations. In addition to genetic analyses conducted seasonally, sex expression and fertilization rates, sex ratios, regeneration from vegetative fragments and outcrossing was estimated in 3 populations of the species. Sex expression rates were high and, in spite of strongly female biased sex ratios, high fertilization rates were detected. However, capacity for regeneration from fragments was also high. Despite frequent spore production genetic diversity was low within populations. This is probably the result of the predominance of asexual reproduction s. 1. including crossing between genetically identical plants. Although recombination and mutation occasionally generates new haplotypes, these have little chance to spread because of the large spores mainly falling into their own patch, where chances for germination are low. Due to small size and isolation of the populations, genetic drift is likely to eliminate these haplotypes. Remote populations differed significantly, each being dominated by a few clones, reflecting negligible gene flow among them. Differences among individual populations can partly be related to differences in their reproductive behaviour and degree of isolation.

Macrogeographic patterns of breeding system diversity in the Daphnia pulex group. I. Breeding systems of Canadian populations

Efforts to deduce the evolutionary significance of sexual reproduction from the taxonomic and ecological distributions of asexuals rely on the presumption that reversions of asexual taxa to sexual reproduction are rare. There is growing evidence, however, that reversions may be likely as long as asexuals are diploid, and some sexual populations persist. However, few systems have been studied in sufficient detail to provide insight into the nature of interactions between the sexual and asexual components of a single taxon. The cladoceran crustaceans Daphnia pulex and D. pulicaria both show variation in breeding system with some populations reproducing by cyclic and others by obligate parthenogenesis. The present study used allozymic analysis to characterize the breeding systems of both species across Canada. Although F1 hybrids were prevalent between the species and invariably reproduced by obligate parthenogenesis, each parent species showed regional variation in its breeding system. D. pulex reproduces by obligate parthenogenesis in eastern Canada but switches to cyclic parthenogenesis in the west with an abrupt transition coincident with the forest/prairie ecotone. D. pulicaria shows a divergent pattern, with populations on the priaries reproducing by obligate parthenogenesis, while those in eastern Canada and the far west are cyclic parthenogens. The results make clear that breeding systems in both taxa tend to be uniform over large areas. The discordance in breeding system employed by each species at specific sites suggests that the patterns of breeding system variation are not linked to environmental factors, but are instead a consequence of the interplay between historical factors, such as dispersion patterns from glacial refugia and the sites of origin of mutations promoting the transition to asexuality.

Molecular evidence for multiple origins of hybridogenetic fish clones (Poeciliidae:Poeciliopsis).

Hybrid matings between the sexual species Poeciliopsis monacha and Poeciliopsis lucida produced a series of diploid all-female lineages of P. monacha-lucida that inhabit the Río Fuerte of northwestern Mexico. Restriction site analyses of mitochondrial DNA (mtDNA) clearly revealed that P. monacha was the maternal ancestor of these hybrids. The high level of mtDNA diversity in P. monacha was mirrored by similarly high levels in P. monacha-lucida; thus hybridizations giving rise to unisexual lineages have occurred many times. However, mtDNA variability among P. monacha-lucida lineages revealed a geographical component. Apparently the opportunity for the establishment of unisexual lineages varies among tributaries of the Río Fuerte. We hypothesize that a dynamic complex of sexual and clonal fishes appear to participate in a feedback process that maintains genetic diversity in both the sexual and asexual components.

The partitioning of net energy resources in two populations of Solidago canadensis during a single developmental cycle in southern Ontario

The partitioning of net energy resources among component organs was measured in two populations of vegetatively produced Solidago canadensis L. individuals on two abandoned pasture sites in southern Ontario. One population was invading a grass-dominated site of comparatively recent abandonment while the other population was well established on a site which was dominated by perennial forbs and which typified a late stage of secondary herbaceous succession. Early in the season there were differences in the proportional contribution of leaf and stem tissue to total new growth, but allometric analysis showed these differences to be a function of developmental stage. New roots emerged earlier in the invading population, which subsequently maintained a higher new root – shoot ratio. The relative growth of new roots was the same in both populations between July and mid-August. Reproductive effort (RE), consisting of both sexual and asexual components, was similar in both populations. Asexual RE was very small (1.5%), but rhizome length was longer in the invading population. It is suggested that rhizome length is determined principally by the size of the photosynthetic apparatus in late season when rhizome extension and leaf senescence are occurring simultaneously. An explanation for differential senescence rates is offered based on the size and density of Solidago clumps and the influence of these characteristics on light attenuation in the canopy.