Zsófia Hock

Efficient purging of deleterious mutations in plants with haploid selfing

In diploid organisms, selfing reduces the efficiency of selection in removing deleterious mutations from a population. This need not be the case for all organisms. Some plants, for example, undergo an extreme form of selfing known as intragametophytic selfing, which immediately exposes all recessive deleterious mutations in a parental genome to selective purging. Here, we ask how effectively deleterious mutations are removed from such plants. Specifically, we study the extent to which deleterious mutations accumulate in a predominantly selfing and a predominantly outcrossing pair of moss species, using genome-wide transcriptome data. We find that the selfing species purge significantly more nonsynonymous mutations, as well as a greater proportion of radical amino acid changes which alter physicochemical properties of amino acids. Moreover, their purging of deleterious mutation is especially strong in conserved regions of protein-coding genes. Our observations show that selfing need not impede but can even accelerate the removal of deleterious mutations, and do so on a genome-wide scale.

Phorophyte preferences of epiphytic bryophytes in a stream valley in the Carpathian Basin

Abstract Epiphytic bryophytes were studied on two tree species with different bark characteristics (Alnus glutinosa and Carpinus betulus) in two vertical ranges (0–60 and 60–200 cm height above ground on the trunk) in a stream valley in the Carpathian Basin (N.W. Hungary). There is a significant difference in the species composition of the trees. Dicranum montanum and Platygyrium repens occurred significantly more often on Alnus glutinosa, whereas Frullania dilatata and Radula complanata were more often on Carpinus betulus in both vertical ranges. Orthotrichum species were typical only in the vertical range 60–200 cm on Carpinus betulus, whereas Plagiothecium nemorale and Metzgeria furcata were typical of Alnus glutinosa and Carpinus betulus, respectively, in the vertical range 0–60 cm. Although most of the bryophyte species have significant phorophyte preferences, Hypnum cupressiforme (in both vertical ranges) and Isothecium alopecuroides (in vertical range 0–60 cm) are indifferent in this respect. Our findings showed clearly that phorophyte preference and vertical position on the trunk are not independent variables. We investigated the independence of the frequencies and abundances of bryophyte species in the two vertical ranges separately on the two tree species. The species form four different groups: (1) species with greater abundance in the higher vertical range; (2) species with smaller abundance in the higher vertical range; (3) indifferent species with nearly uniform abundances in both vertical ranges; (4) species present only in one vertical range. It was found that even a single bryophyte species can show different vertical preferences depending on tree species. No significant relationships were found between the abundances of bryophytes and distance from the stream, slope of the bark surface and trunk circumference. Species-specific traits appear to explain individual phorophyte preferences; however, in several cases chemically mediated effects of the bark appeared to be important.

Seasonal variation in the bryophyte diaspore bank of open grasslands on dolomite rock

Abstract Seasonal changes in the diaspore bank of bryophytes and background processes were investigated in two open grasslands on dolomite rock in Hungary by means of soil samples. To test the longevity of propagules and to collect information about their potential dormancy, a part of each sample was stored air-dried in the laboratory for periods of half a year and 1 year. In contrast to results of earlier studies, temporal variation was detected in the number of propagules stored in the soil of three species (Bryum argenteum, Bryum erythrocarpum agg. and Weissia controversa). There is evidence of dormancy in vegetative gemmae of species of the Bryum erythrocarpum complex and Weissia controversa. Dormancy is also hypothesized in spores of Phascum cuspidatum. We suggest that dormancy in bryophyte propagules is less rare than had hitherto been assumed. Probably this is due mainly to the relatively small number of investigations of species living in habitats with periodically unfavourable moisture and temperature conditions.

Selection is no more efficient in haploid than in diploid life stages of an angiosperm and a moss.

The masking hypothesis predicts that selection is more efficient in haploids than in diploids, because dominant alleles can mask the deleterious effects of recessive alleles in diploids. However, gene expression breadth and noise can potentially counteract the effect of masking on the rate at which genes evolve. Land plants are ideal to ask whether masking, expression breadth, or expression noise dominate in their influence on the rate of molecular evolution, because they have a biphasic life cycle in which the duration and complexity of the haploid and diploid phase varies among organisms. Here, we generate and compile genome-wide gene expression, sequence divergence, and polymorphism data for Arabidopsis thaliana and for the moss Funaria hygrometrica to show that the evolutionary rates of haploid- and diploid-specific genes contradict the masking hypothesis. Haploid-specific genes do not evolve more slowly than diploid-specific genes in either organism. Our data suggest that gene expression breadth influence the evolutionary rate of phase-specific genes more strongly than masking. Our observations have implications for the role of haploid life stages in the purging of deleterious mutations, as well as for the evolution of ploidy.

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

Abstract 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.