EVIDENCE FOR GENETIC HETEROZYGOSITY IN A HOMOSPOROUS FERN

Homosporous fern sporophytes from natural populations exhibited heterozygous electrophoretic patterns for several enzyme systems. Genetic tests utilizing individual gametophytes demonstrate that the observed heterozygosity is coded by alleles at single loci. This simple procedure makes it possible to distinguish segregating from fixed or phenotypic heterozygosity, previously a problem in homosporous vascular plants.     

SPERMATOGENESIS IN A HOMOSPOROUS FERN,ONOCLEA SENSIBILIS

A detailed study of spermatogenesis in a homosporous fern, Onoclea sensibilis L., is presented from the formation of the first spermatogenous cell to the release of the sperm. Two different walls are deposited around the developing spermatids at specific developmental stages as opposed to one wall reported for other species. Most ultrastructural changes that occur in Onoclea during spermatid differentiation resemble those described in previous studies on other fern species, with the following exceptions: 1) A previously undescribed structure appears during midspermatid stage. This dense layer of amorphous material with a row of evenly spaced light areas occurs between the anterior portion of the mitochondrion associated with the multilayered structure and the anterior plasmalemma of the spermatid. 2) An early stage in blepharoplast formation resembles that which occurs in the heterosporous fern Marsilea, in contrast to that which has been reported in Platyzoma, the only other homosporous fern studied at this stage. 3) The osmiophilic crest does not form as early as reported in other ferns. 4) The cap cell of Onoclea is removed intact, rather than collapsing or forming a pore during sperm release. Observations are reported on the number of sperm per antheridium, the time course of spermatogenous cell mitosis, and of differentiation of spermatids into sperm. In Onoclea, an antheridium may contain either 32 or 64 sperm. Regardless of the final number of sperm, each has approximately the same volume.     

GENETIC VARIATION IN THE HOMOSPOROUS FERN PELLAEA ANDROMEDIFOLIA

Genetic variability was examined in nine sexual and three apogamous natural populations of the homosporous fern Pellaea andromedifolia by electrophoretic analysis of enzymes specified by eight loci. Genetic interpretations of heterozygous banding patterns were determined by segregational analysis of gametophytes. High levels of segregating heterozygosity characterized the sexually reproducing populations, and genotype frequencies at the five polymorphic loci were consistent with those expected under conditions of random intergametophytic mating. Multiple-banded patterns in the apogamously reproducing populations resembled those of heterozygous sexual individuals, but did not segregate. The results suggest that genetic variation in sexual homosporous vascular plants is produced by cross fertilization of genetically different gametes and may not result from pairing between homoeologous chromosomes carrying duplicated loci as previously thought.     

POPULATIONAL AND GENETIC STUDIES OF A HOMOSPOROUS FERN—OSMUNDA REGALIS

The mating system and the genetic system of the homosporous fern Osmunda regalis were investigated. Seven populations from western Massachusetts were sampled. All the sporophytes investigated were found to be heterozygous for zygotic lethals. Morphological studies of the gametophytes indicated an intergametophytic mating system when the gametophytes were spatially and chronologically situated to exchange male gametes. Genetic studies evidenced a genetic system based upon duplicate loci.     

POPULATION STRUCTURE AND ESTIMATES OF GENE FLOW IN THE HOMOSPOROUS FERN POLYSTICHUM MUNITUM

Levels and distribution of genetic variation were investigated in the homosporous fern, Polystichum munitum. Homosporous ferns differ from higher vascular plants in that they possess potentially bisexual gametophytes which can produce a completely homozygous sporophyte in a single generation. Because of this, it has long been maintained that ferns possess an inbreeding mating system, resulting in low levels of genetic variation and high levels of homozygosity within populations. The four populations sampled maintain high levels of genetic variation (P? = 0.542; H? = 0.111; ā = 2.23), comparable to that maintained by populations of outcrossing seed plants. The mean fixation index, F, for the four populations was 0.052, indicating no significant deviations from Hardy-Weinberg genotypic expectations. Polystichum munitum distributes most of its genetic variation within rather than among populations. Population-genetic structure was assessed by subdividing each of two large populations into 10 × 10-m subpopulations. Comparisons of genetic variation within and among subpopulations indicated little genetic substructure within either of the artificially subdivided populations. Estimates of interpopulational gene flow (Nm) are extremely high, comparable to those reported for gymnosperms. Statistical estimates of intragametophytic selling are very low, ranging from 0 to 3%. This study suggests that Polystichum munitum is an outcrossing species. Evidence from this and other investigations indicates that fern species do not typically self-fertilize and that mating systems in ferns vary as they do among species of seed plants.     

CHLOROPLASTIC AND CYTOSOLIC ISOZYMES OF THE HOMOSPOROUS FERN ATHYRIUM FILIX-FEMINA L

Intact chloroplasts isolated from mature leaf tissue of the homosporous fern Athyrium filixfemina were osmotically ruptured and subjected to starch gel electrophoresis in side by side comparisons with whole leaf extracts. The single enzyme activities of reportedly cytosolic [NADP]IDH and [NADP]ME were not expressed in the chloroplast fraction, and these were used as controls ensuring the cytosol-free quality of the chloroplast preparations. Isozymes F1,6DP-1, PGI-1, PGM-1, 6PGDH-1, ALDO-1, TPI-2, [NAD(P)]G3PDH-1, and [NAD(P)]G3PDH-2 are active in the chloroplast fraction, whereas Fl,6DP-2, PGI-2, PGM-2, 6PGDH-2, ALDO-2, and TPI-1 were lacking from the chloroplast fraction and are considered cytosolic. The single enzyme activities observed for AAT and SkDH, are chloroplastic. These data indicate that the two isozymes of certain enzymes in Athyrium filix-femina are not the products of duplicated loci resulting from polyploidy, but are distinct and subcellularly compartmentalized as demonstrated in heterosporous plants. Thus A. filix-femina is functionally diploid in spite of its high chromosome number of 2n = 80.     

AN ANALYSIS OF OUTCROSSING IN CERTAIN COMPLEX-HETEROZYGOUS EUOENOTHERAS. I. FREQUENCY OF OUTCROSSING

Hoff , Victor J. (U. Arkansas, Fayetteville.) An analysis of outcrossing in certain complex-heterozygous Euoenotheras. I. Frequency of outcrossing. Amer. Jour. Bot. 49(7): 715–721. 1962.—Eleven complex-heterozygous races of Euoenothera were used for study. The races were grown in the midst of the experimental garden in which plants from approximately 150 races and hybrid combinations were growing together in close proximity so as to afford the maximum opportunity for outcrossing. Many capsules (126) resulting from unguarded flowers were collected from the chosen races and the seed grown to determine the number of hybrid plants resulting from outcrossing within each fruit. Among the various capsules, the number of outcrosses ranged from 0% to over 50% of the plants grown, while the total amount of outcrossing ranged from 0.7 to 7.8% among 5 different collections of capsules. A relationship between the amount of pollen produced by a flower or normally deposited on its stigma and the degree of outcrossing was noted. It is concluded that outcrossing probably occurs with sufficient frequency to account for the large number of complex-combinations found in nature.     

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.     

TEMPERATURE DETERMINATION OF MALE OUTCROSSING ABILITY IN A SIMULTANEOUS HERMAPHRODITE

Phally, a genital dimorphism found in some species of self-compatible simultaneous hermaphrodites, presents an opportunity to examine factors maintaining outcrossing within an animal species in the presence of recombination. Both aphallics and euphallics can self-fertilize but only euphallics develop a functional penis and prostate allowing them to donate sperm. Previous studies of phally in the gastropod Bulinus truncatus (Mollusca: Pulmonata) suggest that phally may be under direct genetic control in some populations and strongly influenced by environmental factors in others. Experiments reported here identify temperature as a cue affecting phally determination in two populations of B. truncatus. In both populations, a higher proportion of euphallics was produced at low temperature (22 ± 1°C) than at high temperature (30 ± 1°C). Temperatures experienced by parents did not affect the proportion of euphallics they produced. Instead, phally was sensitive to temperature during the egg stage postoviposition and during the hatchling stage; the relative influence of temperature before and after hatching varied between populations. The total number of hatchlings reaching maturity at high and low temperature did not differ, but at low temperature, snails took longer to hatch and mature, and had lower survivorship. Just as studies of environmental sex determination have shed light on selective pressures influencing sex ratio evolution, we suggest that temperature-sensitive phally determination may shed light on the selective pressures maintaining outcrossing in B. truncatus.     

EVOLUTION IN A CONSTANT ENVIRONMENT: THERMAL FLUCTUATIONS AND THERMAL SENSITIVITY OF LABORATORY AND FIELD POPULATIONS OF MANDUCA SEXTA

Adaptation to temporal variation in environmental conditions is widespread. Whether evolution in a constant environment alters adaptation to temporal variation is relatively unexplored. We examine how constant and diurnally fluctuating temperature conditions affect life-history traits in two populations of the tobacco hornworm, Manduca sexta : a field population that routinely experiences fluctuating temperatures; and a laboratory population (derived from this field population in the 1960s) maintained at a constant temperature for more than 250 generations. Our experiments demonstrate that diurnal fluctuations significantly alter body size and development time in both populations, and confirm that these populations differ in their responses to a mean temperature. However, we found no evidence for population divergence in responses to diurnal temperature fluctuations. We suggest that mean and extreme temperatures may act as more potent selective forces on thermal reaction norms than temperature variation per se.     

PHYCOLOGY IN THE FIELD AND IN THE LABORATORY1

Field and laboratory phycologists are still following different paths. This is regrettable because their labors supplement one another, and only by co-operation can progress be achieved.     

PATTERNS OF MICROFIBRILLAR ORDER IN A DORMANT FERN APEX

An excised fern apex and young leaf primordium when seen in polarized light show distinct multicellular domains of parallel cellulose alignment which reflect the successive segmentations of the apical cell. The superficial walls of the most recent apical cell derivatives show the effects of highly ordered cellulose deposition in which the microfibrils lie parallel to the newly formed cell plate.     

REGIONS OF CELL WALL EXPANSION IN THE PROTONEMA OF A FERN

Protonemata of Onoclea sensibilis were grown for 7 days in darkness and were then transferred into light on new media, either liquid or agar-solidified, which contained 0.15% colchicine. The growth of individual plants was observed on solid media in microchambers. Unequivocal evidence was obtained that cell wall expansion and an increase in cell diameter occurred in regions well behind the apex of the protonema. This finding is related to an hypothesis which proposes light-induced changes in microtubule orientation and cell wall structure as an explanation for certain changes in cell form in fern gametophytes.     

EVOLUTION OF A POLYMORPHISM FOR OUTCROSSING RATE IN SENECIO VULGARIS: INFLUENCE OF GERMINATION BEHAVIOR

A difference in germination behavior between the highly selling, nonradiate variant of Senecio vulgaris and the more outcrossing, radiate form had a large effect on the relative female fitness of these two morphs raised in mixed stands under conditions similar to those experienced in the wild. Of particular significance was the finding that female fitness differences between morphs were reversed in early- and late-sown plots. This was because a greater proportion of nonradiate than radiate seed germinated directly after sowing, and in early-sown plots a relatively large proportion of early-germinated nonradiate seedlings survived winter to produce large, highly fecund plants the following summer, that contributed greatly to the total female fitness of the nonradiate morph. In contrast, in late-sown plots (i.e., sown two weeks later), survivorship of early germinated seedlings was much reduced, and the radiate morph had an advantage because most radiate seed delayed germination until spring, therefore avoiding seedling mortality during winter. The effect of the association between morph type and germination behavior on morph female fitness in S. vulgaris is clearly important in regard to the evolution of the polymorphism for outcrossing rate in the species. This, in turn, emphasizes the point that an understanding of factors responsible for the evolution and maintenance of polymorphisms for outcrossing rate in the wild should be based on a detailed examination of the ecological genetics of such polymorphisms that extend beyond traditional studies of pollen discounting and inbreeding depression.     

桫椤生态系统生物量与生产力

 活化石植物是最接近于化石物种的现存相似种, 并且具有一定的生态学保守性。利用活化石植物生态系统来估算陆地生态系统碳固存能力的进化趋势是一种有效的手段。该研究中利用标准地-标准木法调查了活化石物种树蕨桫椤(Alsophila spinulosa)生态系统的生物量和生产力。与现存的裸子植物为优势种的生态系统和被子植物为优势种的生态系统相比, 桫椤的生物量(36.151±8.159 Mg C•hm^–2)和生产力(2.535±0.174 Mg C•hm^–2•a^–1)都比较小。与植物化石调查方法相比, 活化石生态系统生物量和生产力数据在描述古生态系统碳固存能力进化趋势方面提供了一种有意义的手段, 并且有助于进一步的理解全球碳平衡演变的过程。     

THE INITIATION OF SPOROPHYTES BY OBLIGATE APOGAMY IN CHEILANTHES CASTANEA

The initiation of apogamous sporophytes in Cheilanthes castanea was recorded by daily photography of individual gametophytes. Whereas an ordinary embryo arises from a zygote, apogamous embryos of C. castanea originate from one to three initial cells which occur just behind the apical region of the prothallus. The initial (or initials) produce cells with small chloroplasts behind the sinus of the gametophyte. The appearance of cells with smaller chloroplasts than those normally found in gametophytes is the first indication that apogamy is occurring. The cells with small plastids produce a group of densely-cytoplasmic meristematic cells. The size of the meristematic mass increases until shoot and root apices of the apogamous embryo are organized.