SIEVE-PORE DEVELOPMENT IN SOME LEPTOSPORANGIATE FERNS

Sieve-pore development was examined in four species of leptosporangiate ferns: Phlebodium aureum, Platycerium bifurcatum, Pityrogramma calomelanos, and Regnellidium diphyllum. In all four species, sieve-pore development is initiated with the formation of a barrel-shaped periplasmodesmatal region—bounded by a narrow, electron-dense band—around the prepore plasmodesma. This phase is followed by the deposition of callose, or of a calloselike substance, around the prepore plasmodesma. With deposition of this substance, the periplasmodesmatal region is no longer discernible, but an electron-dense band continues to border the pore site. Perforation of the pore site involves removal of the calloselike substance and widening of the plasmodesmatal canal, both processes occurring more or less uniformly along the entire length of the pore site. During widening of the plasmodesmatal canal, numerous membranous elements appear within the pore. The number of membranous elements within the pore appears to decrease with increasing age of the sieve element. The location of feeding aphid (Myzus persicae) mouth-parts indicates that the conducting sieve elements in Pellaea viridis are mature cells interconnected by membrane-containing, callose-free sieve pores.     

蕨类植物配子体发育的研究

研究了福建观音座莲、华南紫萁、粗齿紫萁、粗齿桫椤、溪边凤尾蕨、娱蚣草、狭翅铁角蕨、乌毛蕨等21种庆类植物的配子体发育过程。结果如下：(1)对孢子和配子体性状的UPG—MA法聚类分析结果表明,秦仁昌分类系统中亲缘关系近的种类在孢子和配子体性状方面相似性也高,但也有少数例外的情况;(2)蕨类植物的配子体发育有许多共性,但也存在许多不同点;(3)配子体发育中出现的某些性状是稳定的,可以作为分类的依据。     

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.     

ALTERNATION OF GENERATIONS IN FERNS: MECHANISMS AND SIGNIFICANCE

Arguments against the compiling of generalized life cycles summarizing alternation of generations in ferns are presented, and some common misconceptions about breeding systems addressed. What little is known or can be deduced about time frames, mechanisms and significance of alternation events in the lives of two species: bracken fern ( Pteridium ) and Killarney fern ( Trichomanes speciosum ) is presented. Evidence is provided that gametophytes may play a more important role in survival of both these species than previously suspected, and the need for more long-term studies and experiments/measurements of ferns in natural conditions/populations is stressed.     

COMPLEX PALEOZOIC FILICALES IN THE EVOLUTIONARY RADIATION OF FERNS

Filicalean frond segments bearing indusiate sori with gradate development have been discovered in Pennsylvanian deposits of eastern North America. The specimens combine characters of Schizaeaceae, Gleicheniaceae, Cyatheaceae and Hymenophyllaceae, and together with data from other sources, indicate that the Filicales were in the midst of an evolutionary radiation during the late Paleozoic. Evidence from fossil and extant species prompts the hypothesis that filicalean ferns originated near the base of the Carboniferous, and have undergone three major evolutionary radiations.     

MUTATION IN FERNS GROWING IN AN ENVIRONMENT CONTAMINATED WITH POLYCHLORINATED BIPHENYLS

Polychlorinated biphenyls (PCBs) are ubiquitous and persistent environmental contaminants. The Housatonic River in Massachusetts is heavily contaminated with PCBs in the vicinity of the city of Pittsfield. The ostrich fern, Matteuccia struthiopteris, forms extensive populations in the flood plain of this river. The frequency of somatic mutations was determined in these fern populations as well as control populations growing in uncontaminated environments in the Connecticut River and its tributaries. The frequency of somatic mutations in the Housatonic River populations was 5.26–6.14 times the controls. Whether similar genetic damage has occurred to other inhabitants of the Housatonic environment is unknown.     

GENETIC LOAD AND THE MATING SYSTEM IN HOMOSPOROUS FERNS

The mutational genetic load was calculated assuming mutation-selection-inbreeding equilibrium and applied to homosporous ferns. Diploid species with past inbreeding should have a low genetic load while outcrossers should have a high genetic load. These predictions are consistent with the bimodal pattern of genetic load found in 18 diploid homosporous fern species. The prediction that tetraploids should have a low genetic load is also consistent with estimates of genetic load in several species.     

LIGHT-DEPENDENT INHIBITION OF CELL DIVISION AND RHIZOID ELONGATION IN GEMMAE OF THE FERN,VITTARIA GRAMINIFOLIA

Gametophytes of the shoe-string fern Vittaria graminifolia produce linear, six-celled propagules called gemmae. The terminal cells of each gemma elongate into primary rhizoids in culture, and the inner body cells divide asymmetrically to produce prothallial or rhizoid initials. The initiation of both asymmetric cell division and rhizoid elongation is delayed by light intensities greater than 2 w/m². The maximal rates of cell division and rhizoid elongation are unaltered. A 24-hr pulse of high light intensity delays cell division and rhizoid elongation to the same extent, whenever applied during the first 3 d of culture. The model we propose for cell division hypothesizes the existence of a preparatory phase of finite duration prior to mitosis that is sensitive to light intensity. If a cell is irradiated by light intensities greater than 2 w/m² while in the preparatory phase, its entrance into mitosis is delayed. A similar model is proposed for the initiation of rhizoid elongation. Despite the fact that both cell division and rhizoid elongation are dependent on photosynthesis, direct measurements of CO₂-uptake rates show that the inhibitory effects of high light intensities are not due to an inhibition of photosynthesis.     

ELECTROPHORETIC EVIDENCE FOR INBREEDING IN THE FERN BOTRYCHIUM VIRGINIANUM (OPHIOGLOSSACEAE)

An electrophoretic investigation of Botrychium virginianum was conducted to determine the levels and distribution of genetic variation within and among populations of this species. A total of 18 loci representing seven enzymes was examined. For the four polymorphic loci, observed heterozygosity was substantially lower than expected heterozygosity. Values of F were extremely high, indicating a significant deviation from random mating, probably due to inbreeding. We suggest that the high inbreeding coefficients obtained result from a life cycle involving subterranean gametophytes which restrict sperm movement. The study also demonstrates the value of using F-statistics and gene diversity statistics to analyze genetic subdivision in a fern species. Despite the high chromosome number reported for B. virginianum (n = 90), there is no genetic evidence to support the contention that this species is highly polyploid.