Reproductive biology of the Pteridophyta. II. Theoretical considerations

Because of the homothallic nature of many pteridophytes, two categories of mating are possible: intragametophytic selling (the origin of both gametes from a single gametophyte) and inter-gametophytic mating (the origin of each gamete from a different gametophyte).Various morphological and genetical criteria (placement of the gametangia on the thallus, their sequence of ontogeny, the capacity for simple polyembryony and genetic self-incompatibility) can be used to indicate the relative probability of intragametophytic selfing or intergametophytic mating. Only the former has genetic significance (i.e.complete homozygosity); if the latter is evidenced, then detailed studies of population variability are required to ascertain the breeding system. Three types of reproductive systems involve the gametophyte generation: intragametophytic selfing, intergametophytic mating and apogamy. Apogamy generally offers the shortest gametophyte generation and the least evolutionary potential, intergametophytic mating systems generally have the longest gametophyte generation and the greatest evolutionary potential, and intragametophytic mating systems are intermediate in both respectS. It is envisioned that the interaction between gametophyte ecology and evolutionary potential is important in the evolution of a taxon's reproductive system.     

Reproductive biology of the Pteridophyta 1. General considerations and a study of Onoclea sensibilis L.

Many pteridophytes are capable of two levels of selling, intra and intergametophytic. Since intragametophytio selling (the origin of both gametes from one gametophyte) results in zygotic homozygosity, this introduces a factor in pteridophyte evolution seldom contemplated. Morphological and genetic techniques are presented which may give an estimate of the frequency of intragametophytic selfing in nature.
Morphological criteria such as the sequence of the development and the spatial arrangement of the gametangia may be indicative of the importance of intragametophytic selfing in the mating system. Estimates of the genetic load (sporophytio lethals) in a population are related to the frequency of the occurrence of intragametophytio selfing.
Onoclea sensibilis L. morphologically has a mating system which gives a low probability of intragametophytio selfing. A genetic load was found in this species.     

THE DISTRIBUTION OF SELFING RATES IN HOMOSPOROUS FERNS

The models of Lande and Schemske predict that among species in which the selfing rate is largely under genetic control and not subject to tremendous environmental variation, the distribution of selfing rates should be bimodal. When this prediction was tested empirically using data from the literature for species of angiosperms and gymnosperms, the distribution of outcrossing rates for all species was clearly bimodal. To provide another empirical test of the prediction, we analyzed mating-system data for 20 species of Pteridophyta (ferns). Homosporous ferns and their allies are unique among vascular plants because three types of mating are possible: intragametophytic selfing (selfing of an individual gametophyte); intergametophytic selfing (analogous to selfing in seed plants); and intergametophytic crossing (analogous to outcrossing in seed plants). The distribution of intragametophytic selfing rates among species of homosporous ferns is clearly uneven. Most species of homosporous ferns would be classified as extreme outcrossers. In contrast, a few species are nearly exclusively inbreeding. In only a few populations of Dryopteris expansa and Hemionitis palmata and a single population of Blechnum spicant do we see convincing evidence of a mixed mating system. The uneven distribution of selfing rates we observed for homosporous ferns, coupled with a corresponding bimodality of the magnitude of genetic load, strongly supports the model.     

Sporophyte formation in experimentally-induced unisexual female and bisexual gametophytes ofLygodium japonicum

Unisexual female and male and bisexual gametophytes were experimentally induced inLygodium japonicum. A single bisexual gametophyte was isolated in a dish and a female gametophyte was paired with a male one to allow intragametophytic selfing and intergametophytic mating, respectively. About 30% of the females formed sporophytes but no bisexual gametophytes formed them.     

The reproductive biology of the invasive ferns Lygodium microphyllum and L. japonicum (Schizaeaceae): implications for invasive potential

The effect of culture system and population source on sexual expression and sporophyte production was examined for two invasive fern species in Florida, USA, Lygodium microphyllum and L. japonicum (Schizaeaceae). Both species are currently spreading through Florida. Long-distance dispersal of ferns is thought to rely on successful intragametophytic selfing. Given the rate of spread observed in both Lygodium species, we hypothesized that both species are capable of intragametophytic selfing. To test this hypothesis, gametophytes of both species were grown in vitro as isolates, pairs, and groups. Both species were capable of intragametophytic selfing; 78% of L. microphyllum isolates produced sporophytes and over 90% of the L. japonicum isolates produced sporophytes. Lygodium microphyllum also displayed the ability to reproduce via intergametophytic crossing, facilitated by an antheridiogen pheromone. Sporophyte production was rapid across mating systems for both species, an advantage in Florida's wet and dry seasonal cycles. The high intragametophytic selfing rate achieved by both species has likely facilitated their ability to colonize and spread through Florida. The mixed mating system observed in L. microphyllum appears to give this species the ability to invade distant habitats and then adapt to local conditions.     

Mating systems and genetic load in pioneer and non-pioneer Hawaiian Pteridophyta *

Sporophytes from natural populations of ferns occupying pioneer lava and mature rainforest habitats on the island of Hawaii, Hawaiian Islands, were investigated to determine their mating system and frequency of recessive lethal genes (genetic load). Species dominant in pioneer lava habitats were found to have intragametophytic mating systems and to be devoid of lethal genotypes. Species from intermediate and mature rain-forest habitats exhibited complex intergametophytic mating systems and higher levels of genetic load. It is suggested that natural selection has favoured intragametophytic mating and homozygosity in species of less diverse and less competitive pioneer habitats and intergametophytic mating and heterozygosity in species of more mature habitats.     

Growth, sex determination and reproduction of Dryopteris filix-mas (L.) Schott gametophytes under varying nutritional conditions

KORPELAINEN, H., 1994. Growth, sex determination and reproduction of Dryopteris filix-mas (L.) Schott gametophytes under varying nutritional conditions. Gametophyte isolates originating from two populations of Dryopteris filix-mas (L.) Schott were grown in culture media containing different amounts of nutrients. Both nutrition and source population significantly affected gametophyte growth, sex, reproduction and mortality. Taking into account the most optimal nutritional condition for the selfing of gametophytes originating from individual source sporophytes, the proportions of hermaphrodites reproducing by intragametophytic selfing in the two populations varied from 33 to 96% and from 54 to 100%, respectively. It is emphasized that when examining the amount of genetic load only hermaphrodites, not all individuals, should be included, and genetic load should be estimated from the growth experiments where the intensity of reproduction is at the maximum. It was detected that hermaphroditic gametophytes are considerably larger than males or asexuals. It follows that gametophyte size is decisive in sex determination. It is suggested that the effect of antheridiogen hormones, which is considered to be an important factor in gametophyte sex determination, is indirect. Antheridiogens would actually affect size, and size would influence sex expression. The ecology of fern mating systems, and the different genetic and nongenetic factors which promote intergametophytic matings are discussed.     

Reproductive biology of the fernPhegopteris decursive-pinnata

It has been revealed that gametophytes of diploid plants ofPhegopteris decursive-pinnata have a low capability for intragametophytic selfing (Masuyama, 1979). In the present study, intergametophytic mating tests were conducted for the self-sterile gametophytes of four diploids to demonstrate the genetic factors responsible for such a low capability for selfing. The results of the tests indicated that the gametophytes carried two or more kinds of recessive embryonic lethal factors which were non-allelic with each other and that the occurrence frequency of the gametophytes with an identical recessive lethal factor was 13% to 27% in the gametophyte families of these four diploids The karyological study of a diploid sporophyte suggested not the tetraploid but the diploid constitution of somatic chromosomes. Based on these data, the diploid inheritance of two or three special deleterious genes with a synergistic interaction responsible for the embryonic lethality was hypothesized to elucidate the self-sterility in the diploids of this species.     

Self-fertilization in mosses: a comparison of heterozygote deficiency between species with combined versus separate sexes

Self-fertilization is a key difference of adaptive significance between species with combined versus separate sexes. In haploid-dominant species such as mosses and ferns, species with either combined or separate sexes (monoicous and dioicous, respectively) have the potential to self-fertilize (intergametophytic selfing), but being monoicous allows an additional mode of selfing (intragametophytic selfing). We used allozyme electrophoresis to estimate deviations from expected levels of heterozygosity under Hardy-Weinberg equilibrium to infer selfing rates in 10 moss species from 36 New Zealand populations. We found that while there were deficiencies of heterozygotes compared to expectation in both monoicous and dioicous mosses, monoicous species had significantly higher levels of heterozygote deficiency than dioicous species (F(IS)=0.89+/-0.12 and 0.41+/-0.11, respectively). Estimated selfing rates suggest that selfing occurs frequently in monoicous populations, and rarely in dioicous populations. However, in two dioicous species (Polytrichadelphus magellanicus and Breutelia pendula), we found significant indications of mixed mating or biparental inbreeding in a handful of populations. These data provide the first analysis of heterozygote deficiency and selfing among haploid-dominant species with breeding system variation, and we discuss our results with respect to the consequences of inbreeding depression and the evolution of breeding systems.     

Experimental studies on mating systems and genetic load in Onoclea sensibilis L. (Aspleniaceae: Athyrioideae)

Sporophytes from natural populations of Onoclea sensibilis L. from eastern North America and Japan were investigated to determine their mating system and the frequency of recessive deleterious genes (genetic load). Morphological and physiological criteria in the gametophyte generation indicate that this species has a high probability of intergametophytic mating in nature. All sporophytes exhibited some degree of heterozygosity for genetic load, expressed either as zygotic lethals or leaky lethals. Effects of antheridogen on gametophyte sex ratios and polyembryony are described.     

Reproductive biology of the fernPhegopteris decursive-pinnata

Four diploid plants and four tetraploid plants ofPhegopteris decursive-pinnata were investigated for determination of the reproductive characteristics of their gametophytes and two major features were recognized. First, gametophytes of the diploids showed an ontogenetic sequence of gametangium formation which is unfavorable for intragametophytic selfing, whereas those of the tetraploids showed that favorable for intragametophytic selfing. Second, 41 to 72% of the isolated gametophytes of the diploids produced sporophytes in the intragmetophytic selfing tests, whereas all of the isolated gametrophytes of the tetraploids produced sporophytes in the tests. Based on these developmental and genetic features of gametophytes, the dissimilar mating systems of the diploids and the tetraploids of this species are discussed.     

POPULATION GENETICS OF INTRAGAMETOPHYTIC SELFING

Intragametophytic selfing is a mode of reproduction occurring in homosporous ferns where two gametes from the same haploid gametophyte form a completely homozygous sporophyte. The inbreeding equilibrium is derived for a population with partial intragametophytic selfing, selfing, and outcrossing. Procedures for directly estimating the extent of intragametophytic selfing and selfing using parent-offspring data are given. The conditions for a stable polymorphism from a heterozygous-advantage fitness model are more restrictive for partial intragametophytic selfing than for selfing. The rate of decay of gametic disequilibrium is slower for partial intragametophytic selfing than for selfing. Based on these findings, one would predict that plants with intragametophytic selfing would have less polymorphism for loci with a heterozygous advantage and more gametic disequilibrium between neutral loci than is expected for populations with an equivalent amount of selfing. Data from several studies are consistent with these predictions.     

Deleting species from model food webs

Although self-fertilization and its evolutionary consequences have been widely studied, the relative influence of genetic and environmental factors on the determination of mixed-mating systems remains poorly known. In 1999 and 2000, we surveyed the mating system, the population dynamics and some life-history traits of four populations of the freshwater snail Biomphalaria pfeifferi , the major intermediate host of Schistosoma mansoni in Africa, in two areas of Madagascar (Itasy and Antananarivo). We confirmed that B. pfeifferi is a predominant selfer, with selfing rates ranging between 80 and 100%. Temporal and geographical variation of the selfing rate was observed at both local and large spatial scale. Historical processes of colonization and invasion of B. pfeifferi in Madagascar could explain the geographical variation of the mating system observed at regional scale. Pure selfing has probably evolved in the two populations of Antananarivo area as a reproductive assurance strategy in a metapopulation where extinction is frequent and migration rare. The reproductive assurance hypothesis does not explain the spatio-temporal mating system variations observed in Itasy area. However genetic factors including inbreeding depression-the expression of which can be environmentally mediated-and metapopulation dynamics could influence the mating system in both populations sampled in Itasy and lead to different levels of evolutionary stable intermediate selfing rate in this region. Our results therefore highlight the influence of environmental heterogeneity and stochasticity on mating system.     

Genetic diversity and population structure in the outcrossing populations of Equisetum arvense and E. hyemale (Equisetaceae)

An allozyme examination was conducted to study the mating systems and genetic differentiation of populations of Equisetum arvense and E. hyemale. The study revealed that the rate of intragametophytic selfing in these homosporous pteridophytes is very low, i.e., on average 0.020 and 0.019, respectively, despite the potential hermaproditism and selfing of the gametophytes. Most populations consisted of numerous genotypes, and the average heterozygosities of E. arvense and E. hyemale equalled 0.092 and 0.134, respectively. The commonly observed excess of the heterozygote genotypes indicates that there are interclonal differences in the frequency of vegetative reproduction. The level of genetic divergence among populations was considerable even within a limited geographic area. It is suggested that the life history of Equisetum, characterized by the inefficiency of spore germination and gametophyte reproduction in noncolonizing situations, limits the level of gene flow and leads to a great genetic divergence between populations.     

ESTIMATED RATES OF INTRAGAMETOPHYTIC SELFING IN LYCOPODS

Homosporous pteridophytes are characterized by the production of free-living, potentially bisexual gametophytes. Because of the close proximity of archegonia and antheridia on the same thallus, it has been assumed that high rates of intragametophytic self-fertilization would predominate in natural populations of homosporous pteridophytes. Using enzyme electrophoresis we determined sporophytic genotype frequencies for natural populations of three lycopod species, Lycopodium clavatum, L. annotinum, and Huperzia miyoshiana. Based on these genotype frequencies and the estimation procedures of Holsinger (1987), the estimated rates of intragametophytic selfing in these species are extremely low. Estimated selfing rates were greater than 0.000 in only two of 13 populations of L. clavatum, one of six populations of L. annotinum, and one of four populations of H. miyoshiana. Despite the potential for intragametophytic self-fertilization, the gametophytes of these three lycopod species predominantly cross-fertilize, although the mechanism(s) promoting intergametophytic matings are unknown. These results are similar to those obtained for homosporous ferns and Equisetum arvense. It is therefore clear that most homosporous pteridophyte species investigated do not exhibit high rates of intragametophytic self-fertilization; in contrast, intergametophytic matings predominate.     

MATING SYSTEM AND DISTRIBUTION OF GENETIC VARIATION IN GYMNOCARPIUM DRYOPTERIS SSP. DISJUNCTUM

Levels and distribution of genetic variation were analyzed in 15 populations of the homosporous fern Gymnocarpium dryopteris ssp. disjunctum to evaluate the mating system and the genetic structure of this taxon. Estimated rates of intragametophytic selling were 0.000 in all populations, signifying that all sporophytes examined arose via intergametophytic matings. Furthermore, values of F, the fixation index, ranged from –0.171 to 0.038, indicating that all populations approach Hardy-Weinberg equilibrium. Therefore, not only is intragametophytic selling rare in G. dryopteris ssp. disjunctum, but intergametophytic selling is also uncommon. Factors promoting outcrossing in G. dryopteris ssp. disjunctum include an antheridiogen system, an ontogenetic sequence that favors intergametophytic matings, and high levels of genetic load. Analysis of F-statistics indicated statistically significant genetic differentiation among populations despite a mean genetic identity of 0.973 and high levels of interpopulational gene flow (Nm = 3.41 to 4.09). These patterns may reflect levels of gene flow that prevent significant interpopulational divergence while permitting slight genetic differentiation among populations.     

植物交酸系统的进化、资源分配对策与遗传多样性

影响植物自交率进化的选择力量主要体现在两个方面：当外来花粉量不足时,自交可以提高植物的结实率,即雌性适合度（繁殖保障）;而如果进行自交的花粉比异交花粉更易获得使胚珠受精的机会,那么自交也可以提高植物的雄性适合度（自动选择优势）。但是,鉴别什么时候是繁殖保障、什么时候是自动选择优势导致了自交的进化却是极其困难的。花粉贴现降低了自交植物通过异交花粉途径获得的适合度,即减弱了自动选择优势,而近交衰退既减少了自动选择优势也减少了繁残给自交者带来的利益。具有不同交配系统的植物种群将具有不同的资源分配对策。理论研究已经说明,自交率增加将减少植物对雄性功能的资源分配比例,但将使繁殖分配加大,而且在一定条件下交配系统在改变甚至可以导致植物生活史发生剧烈变化,即从多年生变为一年生。文献中支持自交减少植物雄性投入的证据有很多,但是对繁殖分配与自交率的关系目前还没有系统的研究,资源分配理论可以解释植物繁育系统的多样性,尤其是能够3说明为什么大多数植物都是雌雄同体的,自交对植物种群遗传结构的影响是减少种群内的遗传变异,增加种群间的遗传分化,长期以来人们一直猜测,自交者可能会丢掉一些长期进化的潜能,目前这个假说得到了一些支持。     

Intragametophytic selfing does not reduce reproduction inDryopteris filix-max

Gametophytes ofDryopteries filix-mas (L.) Schott were grown as isolates and as pairs of related and unrelated individuals. Reproductive efforts were generally highest among isolates, which reproduce by intragametophytic selfing only. Reproductive success ofD. filixmas may depend on the abundance of resources rather than the actual mating system employed. The estimates of the amount of genetic load for the two populations examined were low, 10% and 16%, respectively. The great numbers and the high viability of sporophytes produced by intragametophytic selfing indicate thatD. filix-mas does not experience serious inbreeding depression.     

Density affects gametophyte growth and sexual expression of Osmunda cinnamomea (Osmundaceae: Pteridophyta)

BACKGROUND AND AIMS: To understand how gametophyte densities affect the sexual expression and sizes of Osmunda cinnamomea and to provide information on the density of growth needed to favour successful reproduction, fresh spores were sown at various densities and subsequent gametophyte growth was studied. METHODS: Spores were sown and cultured in the laboratory. Subsequent gameophytes at different population densities were sampled and their sexual expression and sizes were recorded. KEY RESULTS: One-year-old multispore cultures of the fern O. cinnamomea demonstrated that population density affected gametophyte growth and sexual expression. As density increased, gametophytes became significantly smaller and more slender. Female and asexual gametophytes dominated in populations of low and high densities, respectively. At intermediate population densities, hermaphroditic and male gametophytes were dominant. Female gametophytes were larger than gametophytes of all other types. Hermaphroditic gametophytes were larger than male gametophytes, which were larger than asexual gametophytes. Large gametophytes were wide-cordate, whereas smaller ones tended to be narrow-spathulate. CONCLUSIONS: Gametophyte size of O. cinnamomea is negatively related to the population density, which significantly affects gametophytes' sexual expression. The presence of unisexual and bisexual gametophytes at intermediate densities indicates that both intergametophytic and intragametophytic selfing may occur.     

Mixed mating system in the fern Asplenium scolopendrium: implications for colonization potential

Background and Aims

Human-mediated environmental change is increasing selection pressure for the capacity in plants to colonize new areas. Habitat fragmentation combined with climate change, in general, forces species to colonize areas over longer distances. Mating systems and genetic load are important determinants of the establishment and long-term survival of new populations. Here, the mating system of Asplenium scolopendrium, a diploid homosporous fern species, is examined in relation to colonization processes.

Methods

A common environment experiment was conducted with 13 pairs of sporophytes, each from a different site. Together they constitute at least nine distinct genotypes, representing an estimated approx. 95 % of the non-private intraspecific genetic variation in Europe. Sporophyte production was recorded for gametophytes derived from each parent sporophyte. Gametophytes were grown in vitro in three different ways: (I) in isolation, (II) with a gametophyte from a different sporophyte within the same site or (III) with a partner from a different site.

Key Results

Sporophyte production was highest in among-site crosses (III), intermediate in within-site crosses (II) and was lowest in isolated gametophytes (I), strongly indicating inbreeding depression. However, intragametophytic selfing was observed in most of the genotypes tested (eight out of nine).

Conclusions

The results imply a mixed mating system in A. scolopendrium, with outcrossing when possible and occasional selfing when needed. Occasional intragametophytic selfing facilitates the successful colonization of new sites from a single spore. The resulting sporophyte, which will be completely homozygous, will shed large amounts of spores over time. Each year this creates a bed of gametophytes in the vicinity of the parent. Any unrelated spore which arrives is then selectively favoured to reproduce and contribute its genes to the new population. Thus, while selfing facilitates initial colonization success, inbreeding depression promotes genetically diverse populations through outcrossing. The results provide further evidence against the overly simple dichotomous distinction of fern species as either selfing or outcrossing.