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.     

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.     

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.     

Host-range and cold temperature tolerance of Floracarus perrepae Knihinicki & Boczek (Acari: Eriophyidae), a potential biological-control agent of Lygodium microphyllum (Pteridophyta: Lygodiaceae)

Abstract The leaf galling eriophyid mite Floracarus perrepae is a widespread and damaging herbivore of Old World climbing fern, Lygodium microphyllum , across its native range in tropical and subtropical Asia and Australia; and was therefore selected as a candidate biological-control agent for the invasive fern in Florida, USA. The host testing of F. perrepae focused on Lygodium species from North America and the Neotropics, along with threatened or endangered ferns from Florida. Sporeling ferns were used for the initial no-choice screening and F. perrepae showed normal development on the Florida genotype of L. microphyllum with 33.0 ± 4.6 mites per marginal leaf roll gall, modest development on the North American native Lygodium palmatum with 29.0 ± 9.3 mites per roll, and minor development on six other fern species (4.0–0.3). Leaf rolls were induced only on Lygodium species and full rolls were common only on L. microphyllum . These same six non-target plant species were tested again as more mature plants in both no-choice and choice tests and F. perrepae developed only on L. microphyllum . Lethal minimum temperature and cold stress tests were also conducted on F. perrepae . These tests revealed that it would not likely establish north of USDA hardiness zone 8a (−9.5 to −12.2°C), which means it would not overlap with L. palmatum from temperate North America. Our studies conclude that F. perrepae is specific to L. microphyllum , poses little or no risk to native or cultivated ferns in North and South America, and therefore should be considered for release in Florida.     

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.     

Palynology and intergeneric relationships in subtribe Hyoseridinae (CompositaerLactuceae)

Ceratopteris pteridoides is an inbreeding species. Intralocus homozygosity is deduced from the almost total absence of genetic load (recessive deleterious and lethal genes) in sporophytes from natural populations, and from gametangial sequences which indicate intragametophytic selfing. This mating system and the profound capacity of the species for vegetative reproduction, coupled with the small size and geographical isolation of many populations, strongly support the hypothesis of individual homozygosity. However, polymorphism is exhibited both within and between populations in specific, genetically mediated, gametophytic characteristics. The population genetic structure of C. pteridoides is similar to other inbreeding species.     

Genetic diversity,sexual condition,and microhabitat preference determine mating patterns in Sphagnum (Sphagnaceae) peat‐mosses

In bryophytes, the possibility of intragametophytic selfing creates complex mating patterns that are not possible in seed plants, although relatively little is known about patterns of inbreeding in natural populations. In the peat‐moss genus Sphagnum, taxa are generally bisexual (gametophytes produce both sperm and egg) or unisexual (gametes produced by separate male and female plants). We sampled populations of 14 species, aiming to assess inbreeding variation and inbreeding depression in sporophytes, and to evaluate correlations between sexual expression, mating systems, and microhabitat preferences. We sampled maternal gametophytes and their attached sporophytes at 12–19 microsatellite loci. Bisexual species exhibited higher levels of inbreeding than unisexual species but did generally engage in some outcrossing. Inbreeding depression did not appear to be common in either unisexual or bisexual species. Genetic diversity was higher in populations of unisexual species compared to populations of bisexual species. We found a significant association between species microhabitat preference and population genetic diversity: species preferring hummocks (high above water table) had populations with lower diversity than species inhabiting hollows (at the water table). We also found a significant interaction between sexual condition, microhabitat preference, and inbreeding coefficients, suggesting a vital role for species ecology in determining mating patterns in Sphagnum populations. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 96–113.     

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.     

Octothrips lygodii sp. n. (Thysanoptera: Thripidae) damaging weedy Lygodium ferns in south-eastern Asia, with notes on other Thripidae reported from ferns

Abstract Weedy Lygodium ferns have become a serious fire hazard in Florida, introduced from the Old World Tropics. During joint CSIRO/USDA studies on potential biological control agents, a thrips species was found widely in south-eastern Asia causing severe damage to L. microphyllum . Described here as a new species, Octothrips lygodii , this is the second species of a poorly known genus that has been based on a single damaged male from Papua New Guinea. The genus is redefined, the two species distinguished, and notes given on the range of Thripidae reported from ferns worldwide.     

Isolation and identification of antheridiogens in the ferns, Lygodium microphyllum and Lygodium reticulatum.

Antheridiogens in culture media of 6-week-old prothallia of two species of Schizaeaceous ferns, Lygodium microphyllum and Lygodium reticulatum, were analyzed by gas chromatography-mass spectrometry. In both species, the gibberellin A73 methyl ester (GA73-Me) was identified as the most abundant antheridiogen, and the methyl esters of GA9 and of several monohydroxy-GA73 derivatives were also detected. Since both species produced antheridiogens at a high level, they were classified into high-antheridiogen-producing ferns. The response to GA73-Me of gametophytes of both species is also discussed.     

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.     

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.     

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

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.     

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.     

An ultrastructural study of the relationship between the mite Floracarus perrepae Knihinicki & Boczek (Acariformes: Eriophyidae) and the fern Lygodium microphyllum (Lygodiaceae)

Abstract  The ultrastructure of the mite Floracarus perrepae was investigated in relation to its host, Lygodium microphyllum , the Old World climbing fern. Floracarus perrepae has been suggested as a means of biological control for the fern, which is an aggressive weed in tropical areas. Feeding by the mite induces a change in the size of epidermal cells, and cell division is stimulated by mite feeding, causing the leaf margin to curl over into a roll with two to three windings. The enlarged epidermal layer greatly increases its cytoplasmic contents, which become a nutritive tissue for the mite and its progeny. Damage by the mite ultimately debilitates the fern. The structure and depth of stylet penetration by the mite, and the thickness of the epidermal cell wall of L. microphyllum , do not appear to account for the mite's differential ability to induce leaf rolling in its co-adapted host from south-east Queensland but not in the invasive genotype of the fern in Florida.     

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.     

Limited field establishment of a weed biocontrol agent, Floracarus perrepae (Acariformes: Eriophyidae), against Old World climbing fern in Florida--a possible role of mite resistant plant genotypes

The leaflet galling mite Floracarus perrepae Knihinicki & Boczek was released on Lygodium microphyllum (Cav.) in 63 plots in Florida from 2008 to 2009. Mites transferred onto field plants in 34 plots, but failed to establish populations in the majority of plots. Leaflet galls were observed in only six plots, and in only two plots did mite populations persist for >12 mo. Rates of mite transfer onto field plants were similar for methods using direct transfer of galls versus approaches using passive transfer of mites from infested plants. Often leaflets on some L. microphyllum plants were heavily galled by F. perrepae, whereas leaflets on intertwined stems of other L. microphyllum plants were ungalled but exhibited a characteristic browning and scorching of the leaflet tips. Living mites were consistently present on the undersurface of scorched leaflet tips on ungalled plants, suggesting that this damage might be caused by mite feeding on L. microphyllum genotypes that did not support induction of leaflet galls. Plant nutritional status did not account for differences in galling response, because there were no differences in leaflet nitrogen between galled and ungalled stems. We review those factors known to affect the colonization of biological control agents, and discuss how they may have contributed to the lower than expected rate of F. perrepae establishment.     

ABNORMAL REDUCTIONAL AND NONREDUCTIONAL MEIOSIS IN CERATOPTERIS: ALTERNATIVES TO HOMOZYGOSITY AND HYBRID STERILITY IN HOMOSPOROUS FERNS

Natural and synthesized hybrids of Ceratopteris were investigated to determine the effect of hybridization on the genetic system. Studies indicated that the hybrids exhibited massive spore abortion and pairing abnormalities at meiotic prophase, characteristic of “sterile diploids and triploids” reported in hybridization studies of other fern genera. However, a small percentage of viable spores also was produced by the hybrids. Cytological investigations indicated the presence of previously unreported meiotic adaptations that allowed the production of unreduced spores and reduced spores exhibiting chromatid heterozygosity. The reduced spores allow haploid gametophytes to form heterozygous zygotes in spite of intragametophytic selfing. The unreduced spores were shown to be responsible for the fertility of the “sterile” hybrid and allowed the subsequent production of up to three generations of sporophytes. The literature suggests that these meiotic adaptations are present in other fern genera and may play a significant role in evolution through hybridization.     

Laboratory host range testing of Neomusotima conspurcatalis (Lepidoptera: Crambidae) – a potential biological control agent of the invasive weed,Old World climbing fern,Lygodium microphyllum (Lygodiaceae)

Old World climbing fern, Lygodium microphyllum, is a serious invasive weed in south Florida. Development of biological control is vital for sustainable management of L. microphyllum. Neomusotima conspurcatalis was discovered in Hong Kong in 1997 and was subsequently found causing feeding damage on L. microphyllum in much of its native range in Asia. Quarantine testing of N. conspurcatalis used 37 non-Lygodium fern species representative of New World genera of cultivated ferns and fern allies, one gymnosperm, three crop species, six Lygodium species, and the primary host L. microphyllum. No significant oviposition or feeding was observed on any of the 41 non-Lygodium species evaluated. Oviposition and feeding occurred on all Lygodium species, but amounts were low and usually significantly less than observed on L. microphyllum. The exception was L. japonicum, which was preferred as an oviposition host. Neomusotima conspurcatalis was only able to complete development on L. japonicum and L. palmatum, but survival on these species was only half that occurring on L. microphyllum. Neomusotima conspurcatalis is a Lygodium specialist. Lygodium japonicum is an invasive weed in the United States. Lygodium palmatum is restricted to areas of the United States where freezing winter temperatures would be lethal to N. conspurcatalis. It was concluded that N. conspurcatalis would pose no threat to native or cultivated plants in North America or the Caribbean and should be considered for biocontrol of L. microphyllum. A release petition was submitted in 2005. An USDA-APHIS release permit for N. conspurcatalis was issued in 2007.