Factors influencing reproductive success in the clonal moss, Hylocomium splendens

Female reproductive success in the unisexual perennial clonal moss Hylocomium splendens was examined by recording, if the segment was reproductive [produced sporophyte(s)] or not, together with several distance-to-male and male density variables, and segment size. This was done for every female segment in a population over a 5 year study period. A high fraction of the population could be sexed because we monitored the population in situ for 5 years, and thereafter harvested the population for electrophoretic analysis from which the clonal identity and expressed sex could be deduced. Fertilization distances in H. splendens were short, indicated by the fact that as many as 85% of the female segments with sporophytes were situated within a distance of 5.0 cm from the nearest male. The longest distance measured between a sporophytic female and the closest male was 11.6 cm. However, analysed within a generalized linear modelling (GLM) framework, the year was the best single predictor for the presence of H. splendens sporophyte although female-segment size and distance to the closest situated male were also strongly significant. The two latter factors explained larger fractions of variation in sporophyte presence in a GLM model with three predictors than in single-predictor models. This is because (i) the large variation in sporophyte production among years partly obscures the strong general increase in sporophyte production with increasing female-segment size and vitality, and (ii) the between-year variation and the size obscure the effect of the distance to the most proximate male. To our knowledge, this study is the first to incorporate into one model the relative importance of several factors for bryophyte reproductive success. Our results demonstrate the value of multiple-predictor approaches in studies of reproductive success.     

Population biology of the clonal moss Hylocomium splendens in Norwegian boreal spruce forests. 5. Vertical dynamics of individual shoot segments

Demographic information was obtained for 12 528 mature segments (for which dry weight was estimated and vertical position in the bryophyte carpet recorded) and 3109 regenerated growing points for the perennial clonal moss Hylocomium splendens, recorded in Norwegian boreal spruce forests during a 6‐year period. Branching frequency varied with vertical position in the bryophyte carpet. Termination risk (probability of producing no offspring) was highest (44%) for buried segments, lowest (12%) for segments at intermediate vertical positions, and also high (26%) for emergent segments (due to increasing exposure to external mortality agents). Segment size increased from low levels in the bryophyte carpet to a maximum ca 2–10 mm below the top of the bryophyte carpet. This intermediate level was interpreted as the optimal compromise between incoming radiation (attenuating downwards) and microclimatic moisture conditions (improving downwards). Size‐corrected fitness, the number of offspring emerging from a mature segment within one year after maturation after allowance for differences in size, was lower for buried and emergent segments than for segments at intermediate positions. Small emergent segments were apparently liable to suffer from vitality reductions due to desiccation. The vertical position of a daughter segment depended on that of its parent segment, but also showed considerable stochastic variation. Burial acted as a strong sink for small segments regardless of vertical position. No evidence was found for species‐specific differences in the way pleurocarpous bryophytes interact, but reduced vertical mobility of H. splendens when growing among acrocarps indicated that growth‐form is an important determinant of bryophyte interactions. Evidence was found for vertical layering of the bryophyte carpet according to dominant type of interactions among individuals: none (environmental stress) above and at top, facilitation [a (+, +) interaction] at intermediate levels because of favourable water relationships in closed stands, and amensalism [a (0, ?) interaction] from higher‐situated segments that deprive lower‐situated segments access to light at lower relative levels. The intensity of amensalism increased downwards in the bryophyte carpet as indicated by a reinforced size hierarchy. The tendency for small H. splendens segments to become buried and lost from the population by amensalism is likely to represent a general mechanism for interactions between bryophyte species and succession in bryophyte‐dominated stands. Population effects of climatic and local environmental factors (favourability vs stress), disturbance and apparently random events are discussed with reference to their impact on the relative sizes of subpopulations acting as sources (due to facilitation) and sinks (due to amensalism).     

The time of fruiting of Ulota Bruchii Hornsch. and U. crispa Brid.

Abstract

For obscure reasons, sporophytes of the dioicous Hamatocaulis vernicosus are currently unknown in France. With the aim of understanding this failure of sporophyte production we investigated (1) sexual phenology, (2) sex ratio pattern at different spatial scales, (3) limitation of sporophyte formation by the availability of only one sex, (4) limitation of sporophyte formation caused by the distance between male and female, (5) location of sporophytes and (6) the consevation implications of this failure of sporophyte production. Principal results show that phenology cannot explain the failure of sporophyte production. At the regional scale, sporophyte formation is prevented by the fact that several mountain ranges have single-sex populations. The production of spore capsules relies on a strict combination of factors at the local scale: occurrence of sexually expressed mixed-sex colonies, a short distance between male and female individuals and favourable environmental factors (light grazing, high water table). In the Massif Central of France spore production is unlikely because of current unfavourable conditions. Fertility patterns appeared useful in a conservation context.     

Gametophyte contribution to sporophyte growth on the basis of carbon gain in the fern <Emphasis Type="Italic">Thelypteris palustris</Emphasis>: effect of gametophyte organic-matter production on sporophytes

At an early stage of growth gametophytes support the sporophytes of ferns. Young sporophytes become independent of gametophytes when the first leaves develop. Although large fern gametophytes produce multiple archegonia simultaneously, only one sporophyte is typically established on one gametophyte. The number of sporophytes is believed to be controlled in two possible directions, from gametophyte to sporophyte or from preceding sporophyte to another sporophyte. To investigate the effects of gametophytes on their sporophytes, we studied the relationship between organic matter production by gametophytes and the growth of young sporophytes of Thelypteris palustris. We cut gametophytes in half (CGs) to reduce the gametophytes’ production of matter. There was no significant difference between the growth of sporophytes on intact gametophytes (IGs) and that on CGs. According to our estimates, based on the rate of organic matter production, the large gametophyte was able to produce two or more sporophytes. The resources required for CGs to make similar-sized sporophytes was twice that for IGs. In polyembryony each of the multiple sporophytes was similar in size to the single sporophytes. Resource limitation does not seem to explain why fern gametophytes establish single sporophytes.     

A Bryophyte flora of Kent. I

Abstract

Gender expression and sporophyte frequency of Frullania tamarisci were compared between epiphytic woodland and exposed coastal colonies in an Atlantic region of western Britain. Corticolous colonies exhibited very high sporophyte frequency in mixed-sex colonies. Samples from well-grown epiphytic patches commonly had intimate mixtures of highly fertile female and male shoot networks, and fertilisation reduced gynoecia number. In marked contrast, colonies in exposed coastal habitats were found to be sub-fertile or non sex-expressing and sporophytes were rare. There was a distinct female bias among colonies on the coast where spatial segregation of the sexes and male scarcity appear to limit sporophyte frequency. It is suggested that the close association of males and sporophytic females in epiphytic colonies may be influenced by within-colony sporophyte frequency.     

Family affiliation,sex ratio and sporophyte frequency in unisexual mosses

Patterns of sex expression and sex ratios are key features of the life histories of organisms. Bryophytes are the only haploid‐dominant land plants. In contrast with seed plants, more than half of bryophyte species are dioecious, with rare sexual expression and sporophyte formation and a commonly female‐biased sex ratio. We asked whether variation in sex expression, sex ratio and sporophyte frequency in ten dioecious pleurocarpous wetland mosses of two different families was best explained by assuming that character states evolved: (1) in ancestors within the respective families or (2) at the species level as a response to recent habitat conditions. Lasso regression shrinkage identified relationships between family membership and sex ratio and sporophyte frequency, whereas environmental conditions were not correlated with any investigated reproductive trait. Sex ratio and sporophyte frequency were correlated with each other. Our results suggest that ancestry is more important than the current environment in explaining reproductive patterns at and above the species level in the studied wetland mosses, and that mechanisms controlling sex ratio and sporophyte frequency are phylogenetically conserved. Obviously, ancestry should be considered in the study of reproductive character state variation in plants. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174 , 163–172.     

Arbuscular mycorrhiza and water and nutrient supply differently impact seedling performance of dry woodland species with different acquisition strategies

Background: We recently demonstrated that dwarf male plants are much more common among mosses than hitherto thought. Dwarf plants, producing functional male sexual organs occur in 10–20% of the moss species worldwide.

Aims: We investigated how an inadvertent omission of dwarf males affects estimates of male sex expression rates and sex ratios.

Methods: We recorded frequencies of specimens with female-expressing plants, male-expressing normal-sized and/or dwarf plants, and sporophytes in herbarium material of five model species from Macaronesia. We analysed how frequency estimates of male occurrence and sex ratios were affected by excluding dwarf males.

Results: Occurrence of dwarf males was positively associated with sporophyte formation. When dwarf males were excluded, male mate availability was missed in 51–61% of specimens (in 74–76% of sporophyte-containing specimens). Specimen-level sex ratio was balanced in material with sporophytes in the three species exhibiting male dwarfism, but strongly female biased if dwarf males were omitted.

Conclusions: We provide the first quantitative evidence that sex-expressing dwarf males are essential for sexual reproduction and sporophyte production in mosses that form such males. Considering its abundance, male dwarfism therefore deserves more attention among plant biologists. Innovative approaches are required to further study the minute male plants.     

Enhanced sexual reproduction in bryophytes at high latitudes in the maritime Antarctic

Abstract

Contrary to the generally accepted hypothesis that bryophyte fertility decreases with increasing latitude and therefore climatic severity, a detailed study of bryophyte reproductive strategies at sites in the southern maritime Antarctic (68–72°S) has revealed that an unexpectedly high proportion of species is capable of producing sporophytes. Of the regional bryoflora, 43% (19 species; 17 mosses, 2 liverworts) in Marguerite Bay and 47% (17 species; 16 mosses, 1 liverwort) in Alexander Island are known to produce sporophytes, although the number fruiting at comparable latitudes on the colder and more arid Antarctic continent is less (33%). These numbers and proportions are much greater than were previously known at such relatively high southern polar latitudes. Sporophytes of several species are recorded for the first time within the Antarctic biome, while those of two liverworts (Lophozia excisa and Cephaloziella varians) are reported for the first time south of 62°S. High incidence of sporophyte production is attributed to locally favourable microclimatic conditions producing small-scale 'oases'. The large majority of fertile mosses are monoecious short acrocarps growing on rather calcareous soils. Spore production and size data for a number of species are compared with previously published studies of the same or closely related species from the northern maritime Antarctic and sub-Antarctic, but no consistent trends are revealed between species over the latitudinal gradient.     

The cost of realized sexual reproduction: assessing patterns of reproductive allocation and sporophyte abortion in a desert moss

The desert moss Syntrichia caninervis exhibits one of the most skewed sex ratios in the plant kingdom, with female individuals far outnumbering male individuals (exceeding 14♀:1♂). The "cost of sex hypothesis" derives from allocational theory and predicts that the sex which is most expensive should be the rarer sex. This hypothesis, which, as considered here represents the realized cost of sexual reproduction, is contingent upon two assumptions that are explored: (1) that male sex expression is more expensive than female sex expression, and (2) that sexual reproduction is resource limited. Using inflorescence biomass and discounting sperm, male sex expression was found to be in the neighborhood of one order of magnitude more expensive than female sex expression, and this difference is reflected in higher numbers of gametangia per male inflorescence, presence of paraphyses in male inflorescences, and a much longer developmental time for male inflorescences. The realized cost of female reproduction from two communities dominated by S. caninervis was found to be lower than the realized cost of male sexual reproduction. Resource-limited reproduction was assessed by determining the frequency of sporophyte abortion, the age distribution of sporophyte abortions, and patterns of sporophyte abortion that may be density dependent. Among ten sexually reproducing populations, abortive sporophytes occurred at a frequency of 0.64. Abortive sporophytes averaged 8% the mass of mature sporophytes, and cohort sporophytes from the same individual female were found to abort in a density-dependent pattern. We conclude that the two assumptions, upon which the cost of sex hypothesis depends, are supported.     

Reproductive biology and population dynamics of wild gametophytes ofEquisetum

This is d study of the reproductive biology of wild gametophytes of Equisetum arvense L., E. fluviatile L., and E. palustre L., which form extensive populations during the summer months on previously inundated bare mud along the margins of reservoirs and lakes in Northern England and Wales. The garnetophytes have all the characteristics of pioneer species: they rapidly attain sexual maturity and are adversely affected by competition. Early in the summer mixtures of male and female gametophytes are produced. The former are smaller and their frequency (which is always less) diminishes throughout the season as a result of bryophytic competition. The rarity of bisexual, initially female, gametophytes, in wild populations is related to the absence of metabolite accumulation which mediates this change in rulture. Significantly different sex ratios between populations and species underlines the labile nature of the sex-determining mechanism in Equisetum. The high frequency of females bearing sporophytes indicates that intergarnetophytic fertilization is highly effective in nature. Absence of any correlation between the incidence of fertilized females and the proportions of males, in conjunction with a consideration of the male gamete dispersal distance, suggests that sporophyte formation is restricted by the availability of ripe archegonia. Sexual reproduction in Equisetum is probably limited by the narrow range of conditions under which gametophytes can become established rather than availability of water for fertilization. The majority of females, which bear one sporophyte, are smaller than unfertilized or polyembryonic females. Their small size results from nutrient demands and allelopathic compounds from the sporophytes which also probably prevent the establishment of gametophytes within mature stands of the parent species. Correlations between female diameters and frequencies of males suggest that gametophytes are more likely to produce archegonia under favourable conditions. The natural reproductive biology of Equisetum is in accord with predictions based on an understanding of the mechanism of sex determination in axenic culture. Several striking parallels between sexuality in Equisetum and dioecious angiosperms are revealed. The absence of winter flooding at two of the gametophyte sites led to the establishment of mature rtands of Equisetum, which produced cones after 3–4 years; two hybrids, E. fluviatile×E. arvense and E. fluviatile×E. palustre, were detected.     

Ploidy chimeras induced in haploid sporophytes of <Emphasis Type="Italic">Osmunda claytoniana</Emphasis> and <Emphasis Type="Italic">Osmunda japonica</Emphasis>

Haploid sporophytes of Osmunda claytoniana (2n = x = 22) were apogamously produced from calli when cultivated on a hormone-free medium. Flow cytometric analysis showed that ploidy chimeras were spontaneously produced in a haploid sporophyte of O. claytoniana and those of O. japonica that were obtained in the previous study. In the haploid sporophyte of O. claytoniana, a diploid pinnule and a partially diploid terminal segment were produced in a haploid pinna. In O. japonica, a haploid sporophyte yielded a diploid pinna in a haploid frond, and another haploid sporophyte yielded a diploid pinnule in a haploid pinna. Diploid chimeras were large in size and could be readily distinguished from other haploid parts of the fronds. It is likely that the chimeras were produced clonally from a single diploid cell that established chromosome doubling.     

Disentangling complex fine‐scale ecological patterns by path modelling using GLMM and GIS

Question: How can statistical modelling tools (GLMM) and GIS be used as an aid in understanding complex ecological patterns? This general question was approached by using bryophyte demography data as an example. More specifically, we asked what is the contribution of terrain shape to explaining the performance and fate of plant individuals, controlling for all other known relationships? Location: Norway. Methods: Information on demography was obtained for 140 populations of the perennial clonal bryophyte Hylocomium splendens in Norway spruce forests during an 11‐year period (1992‐2002). Performance (size and branching pattern) was recorded for mature segments and fate was recorded for growing points. Positions of each of the more than 30 000 recorded bryophyte ramets were coupled with (micro‐) topographic characteristics (slope and convexity) derived from fine‐scale digital elevation models in a GIS framework. Carefully planned sequences of generalised linear mixed models (GLMM) were performed to test predictions from a conceptual path model. Results: We demonstrate strong dependence of size on branching, fate and on vertical position in the bryophyte carpet, and an effect of vertical position on branching pattern. Micro‐topography contributed to explaining plant performance by four different mechanisms: (1) a direct effect of slope on the segment's vertical position in the carpet; (2‐3) direct effects of both slope and convexity on fates of individuals via controls on risk of burial; and (4) an indirect effect of convexity on branching pattern via a direct effect on size. No indication of a direct effect of terrain on branching was found. Conclusions: Our study exemplifies the usefulness of GLMM for disentangling complex ecological relationships. Specifically, we recognise micro‐topography as a potentially important factor for plant demography in general and for performance and fate of individuals in particular.     

CONTROL OF SEX RATIOS IN HAPLOID POPULATIONS OF THE MOSS,CERATODON PURPUREUS

Female biased sex ratios occur in a number of unrelated mosses. Such ratios refer to the relative numbers of male and female gametophytes in moss populations and are therefore more comparable to the numbers of pollen grains and ovules in populations of seed plants than to the numbers of male (microsporangiate) and female (megasporangiate) sporophytes. A survey of 11 populations of the moss, Ceratodon purpureus, showed that sex ratios are heterogeneous, but that female biases occur in more than half the populations. One hundred and sixty single spore isolates representing 40 sporophytes from one population demonstrated that female gametophytes outnumbered males by a ratio of 3:2 at the time of germination. Female gametophytic clones formed significantly more biomass than male clones, and individual female shoots were more robust. Male clones, however, produced more numerous stems. These sexually dimorphic traits may be related to life history differences between male and female gametophytes since females must provide nutritional support to the “parasitic” sporophyte generation, a burden that males do not share.     

REPRODUCTIVE LONGEVITY OF DRIFTING KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) IN MONTEREY BAY,USA1

Drifting Macrocystis pyrifera (L.) C. Agardh sporophytes have long been viewed as the primary long‐distance dispersal vector; yet, few data exist that support the ability of reproductive viable sporophytes to actually travel the presumed hundreds to thousands of kilometers. This study addressed the reproductive longevity of experimental and naturally occurring M. pyrifera drifters. Temporal variability in sporophyte size and reproduction was estimated for experimental drifting sporophytes that were tethered to surface buoys and compared with attached plants (controls). Reproductive viability was also studied for beach‐cast drifters (BCD), and naturally drifting sporophytes observed during field surveys in Monterey Bay. Detached drifting sporophytes were tracked with radio transmitters to follow drifter trajectories and to measure drifting speed. Experimental drifters (ED) experienced a 74% reduction in frond length after 35 days, a 76% reduction in average frond number after 70 days, and a reduction in average sorus area by 83% after 28 days. Although zoospore production was reduced following detachment, sporophytes remained fertile with high zoospore germination success as long as sori were present (125 days). Zoospore production and germination success for natural and BCD was similar to ED. The average displacement of radio‐tagged drifters was 7.12 km·day^?1, suggesting that a sporophyte adrift for 125 days disperses viable propagules (zoospores) over 890 km (±363). Dispersal of propagules is important for population restoration, distribution, and genetic diversity. Such dispersal distances are long enough to connect potentially all Northern Hemisphere Macrocystis populations across a generational timescale and may facilitate inter‐hemispheric gene flow.     

GROWTH INHIBITION IN GAMETOPHYTES AND OAT COLEOPTILES BY THELYPTERIN A AND B RELEASED FROM ROOTS OF THE FERN THELYPTERIS NORMALIS

Gametophytes, when grown in the immediate vicinity of a Thelypteris normalis sporophyte—in soil or in sterile culture on agar—showed a reduced number of cells and an altered gross morphology. This is attributed to the action of the thelypterins, which are inhibitors released from T. normalis sporophytes. Growth inhibition of the gametophytes was greatest when thelypterins were added during early stages of gametophyte development. Removal of thelypterin A permitted resumption of growth. Thelypterin A noncompetitively inhibited auxin-enhanced elongation of A vena coleoptiles. The growth of T. normalis root segments was not affected by thelypterins. The growth of young sporophytes of T. normalis was inhibited by mature sporophytes.     

Developing sporophytes transition from an inducible to a constitutive ecological strategy of desiccation tolerance in the moss Aloina ambigua: effects of desiccation on fitness

Background and Aims Two ecological strategies of desiccation tolerance exist in plants, constitutive and inducible. Because of difficulties in culturing sporophytes, very little is known about desiccation tolerance in this generation and how desiccation affects sexual fitness.Methods Cultured sporophytes and vegetative shoots from a single genotype of the moss Aloina ambigua raised in the laboratory were tested for their strategy of desiccation tolerance by desiccating the shoot–sporophyte complex and vegetative shoots at different intensities, and comparing outcomes with those of undried shoot–sporophyte complexes and vegetative shoots. By using a dehardened clonal line, the effects of field, age and genetic variance among plants were removed.Key Results The gametophyte and embryonic sporophyte were found to employ a predominantly inducible strategy of desiccation tolerance, while the post-embryonic sporophyte was found to employ a moderately constitutive strategy of desiccation tolerance. Further, desiccation reduced sporophyte fitness, as measured by sporophyte mass, seta length and capsule size. However, the effects of desiccation on sporophyte fitness were reduced if the stress occurred during embryonic development as opposed to postembryonic desiccation.Conclusions The effects of desiccation on dehardened sporophytes of a bryophyte are shown for the first time. The transition from one desiccation tolerance strategy to the other in a single structure or generation is shown for only the second time in plants and for the first time in bryophytes. Finding degrees of inducible strategies of desiccation tolerance in different life phases prompts the formulation of a continuum hypothesis of ecological desiccation tolerance in mosses, where desiccation tolerance is not an either/or phenomenon, but varies in degree along a gradient of ecological inducibility.     

Substratum preference,spore output and temporal variation in sporophyte production of the epixylic moss Buxbaumia viridis

Abstract

Occupancy and sporophyte numbers of Buxbaumia viridis on patches of decaying wood were investigated during a 4-year period (1996-99). Temporal variation in sporophyte number was investigated in relation to precipitation. Spore number per sporophyte was counted and spore number per substratum patch area and forest area were calculated. To predict the occupancy of B. viridis on decaying logs and stumps in a forest, the patch size (suitable wood area) was the most important variable. Decomposition stage was included in the model as a quality factor, expressing the higher and more stable humidity in late wood-decay stages. The suitable wood area of each patch was determined first and foremost by the stage of decomposition and diameter of the decaying wood. Only 16% of all wood patches considered suitable for production of sporophytes were occupied. The probability of a patch being occupied increased linearly with patch size to approximately 7 dm²; above this size the probability of being colonized was close to one. Both the number of sporophytes and the number of occupied patches were correlated with precipitation amounts during the summer months, with a reduction of occupied patches of 73% in the dry year 1999, compared with mean values for 1996-98. Spore number per sporophyte was correlated with length and width of the capsule, and varied between 1.4 and 9.0 million, with a mean value of 6.0 million. It is suggested that the rarity of B. viridis is caused by a low probability of patches being occupied because of their short longevity and small size, together with the facts that the species is dioicous, short-lived, sensitive to desiccation of the substratum, and has a gametophyte that is so minute it cannot compete with larger bryophytes.     

Sex‐linked microsatellite marker detected in the female gametophytes of Undaria pinnatifida (Phaeophyta)

In our microsatellite analysis of three male and three female gametophytes of Undaria pinnatifida (Harv.) Suringar, a microsatellite marker (part of the locus Up‐AC‐2A8, GenBank accession no. AY738602.1) was only polymerase chain reaction‐amplified in three female gametophytes. This putative female‐specific marker was further tested by the use of 32 male and 21 female gametophytes maintained in the Marine Biological Culture Collection Centre, China. In addition, three sporophytes were included for confirmation. Results showed that the marker was present in all of the female gametophytes and sporophyte cultures, but absent in all of the male gametophytes. To our knowledge, this is the first sex‐related marker ever reported in U. pinnatifida. The discovery of this marker will accelerate gender identification and shed light on our understanding of the mechanisms of sex determination at a molecular level in this commercially important seaweed.     

Life history characteristic ofCyrtomium falcatum around the natural northern boundary in Hokkaido,with reference to the alternation of generations

To understand the life history characteristic for expanding the distributional area to colder climates, developmental age structure of population ofCyrtomium falcatum was observed along southwestern coasts of Hokkaido at the natural northern boundary of its distribution, with reference to the alternation of generations. The length and number of pinna of fertile leaves ofCyrtomium falcatum decrease towards the northern part of Japan. In southwestern Hokkaido, typically dwarf fertile leaves and gametophytes were observed growing together on cliffs nearby the sea. To estimate the developmental ages of small and dwarf leaves, the number of venation (NV: branching number of vein from midrib) of leaves was counted on each sporophyte. The sporophyte with leaves at the simple pinna stage ranging from 0–25NV, is predominant in the population of southwestern coasts of Hokkaido. The fertility of the sporophyte seems to be achieved more than five years after the germination. The gametophytes were also observed at the location to be almost equal in number to sporophytes. The number of gametophytes and sporophytes decreases with advancement of developmental stages. In the same location at Okushiri Isl. with slight gradiency of humidity, the gametophyte is predominant on the drier cliff, while the sporophyte is predominant on the humid hole. The population ofCyrtomium falcatum at the natural northern boundary in Hokkaido, seems to have the life history characteristic with alternation of generations. Contribution No. 2557 from the Inst. of Low Temp. Sci.     

STUDIES ON THE GENETIC CONTROL OF RESISTANT SPORANGIUM FORMATION IN ALLOMYCES

The diploid sporophyte of the phycomycetous fungus Allomyces arbuscula bears two types of sporangia: thin-walled, colorless, ephemeral zoosporangia (ZS) and thick-walled, dark-brown, resistant sporangia (RS). Normal wild-type cultures (strain Portugal IE) under standard conditions produce approximately 90% of their total sporangia as RS. These RS give the cultures a dark-brown color. A mutant was induced with UV irradiation in which the ratio of ZS to RS was shifted so that only 20% of the total sporangia are RS. These cultures are a pale, tan color. Hybrids between the mutants and wild-types produce ca. 65% RS and are also intermediate in the color of the culture. Meiotic segregation in the RS of the hybrid sporophytes gives gametophytes half of which when selfed produce mutant sporophytes and half of which produce wild-type sporophytes. The shift from RS to ZS formation is thus considered to be the result of a one-gene mutation at a locus ‘R.’ The haploid gametophytes of wild-type strains have in addition to male and female gametangia a small number (2-4%) of RS. In mutant gametophytes the percent RS has dropped to 0.1-0.2%. The proposed genotypes at the ‘R’ locus in Allomyces arbuscula are: wild-type sporophytes (RR), hybrid sporophytes (Rr), mutant sporophytes (rr), wild-type gametophytes (R) and mutant gametophytes (r).