ASPLENIUM PLENUM: A FERN WHICH SUGGESTS AN UNUSUAL METHOD OF SPECIES FORMATION

Morphological data, chromosome behavior, and occurrence strongly suggest that the Florida fern known as Asplenium plenum arose as a cross (AAB₁B₂) between a sexual 2x species, A. abscissum (AA), and a “sterile” 3x hybrid, A. curtissii (AB₁B₂). A mechanism is described whereby such an origin is possible: A. curtissii, among other so-called “sterile” hybrids, has the ability to produce unreduced spore mother cells capable of becoming spores directly. Such spores are able to form gametophytes with viable sperms and have the potential of producing sporophytes apogamously. It is highly probable that this heretofore overlooked process has operated in the origin of the hybrid fern, A. plenum, and is the first to be reported in the literature.     

CHROMOSOME BEHAVIOR IN HYBRID FERNS: A REINTERPRETATION OF APPALACHIAN DRYOPTERIS

Analyses of chromosome pairing behavior in fern hybrids, as evidenced by the degree of bivalent and/or univalent formation at meiotic prophase, have frequently been employed in studies of evolutionary relationships within polyploid fern complexes. Pairing is often seen to involve genomic numbers of chromosomes. Many examples exist, however, that indicate pairing on subgenomic levels. If this type of behavior is not recognized in analyses of F₁ hybrids and their polyploid derivatives, interpretations of evolutionary relationships based upon pairing behavior may be misleading. Contrary to some views, sterile triploid hybrids possessing less than three distinct genomes may play a significant role in the formation of reticulate polyploid complexes. This possibility must be considered in interpretations of these complexes. With these factors in mind, the Appalachian Dryopteris complex has been reinterpreted. The reinterpretation provides explanations for several unexplained inconsistencies in previous interpretations.     

ARTIFICIAL INTERSPECIFIC HYBRIDIZATION IN RUELLIA (ACANTHACEAE)

Observations were made on the morphology, seed fertility, pollen viability and meiotic configurations of 22 different interspecific hybrids of 12 North American taxa representative of 3 sections of Ruellia. The hybrids were classified as (1) fully vigorous and fertile; (2) vigorous but with reduced fertility; (3) reduced vigor and fertility; (4) weak and sterile. Hybrids showed meiotic irregularities in microsporogenesis: univalents, chains, precocious separation of bivalents, nondisjunction, and micronuclei were commonly observed. Interspecific pollinations within sect. Dipteracanthus resulted in fertile or partially sterile F₁ hybrids. Intersectional crosses produced partially or completely sterile hybrids or failed. Assuming that crossability is correlated with degree of evolutionary relationship, the parental species are interpreted in genecological terms to be grouped in polytypic ecospecies. Hybridization experiments doubtless would be of value in clarifying relationships in tropical and subtropical species of Ruellia.     

INCIPIENT POLYPLOID SPECIATION IN THE MAIDENHAIR FERN (ADIANTUM PEDATUM; ADIANTACEAE)?

Despite the widespread occurrence of polyploids in plant taxa and the many advantages attributed to polyploidy, very little is known about the specific processes that lead to the establishment of polyploids in nature. Classical models suggested that polyploids arise following somatic chromosome doubling in hybrids. However, the production of polyploids from unreduced meiotic products has been receiving greater attention. During an enzyme electrophoretic study of a local population of Adiantum pedatum, a mutant producing viable unreduced spores along with abortive spores was discovered. Studies of sporogenesis showed that a synaptic mutation caused the paired chromosomes to disassociate, with mostly univalents remaining at metaphase I. In such aberrant spore mother cells, one of two pathways was followed in the remaining stages of meiosis. Cells attempting both meiotic divisions formed abortive spores. However, in spore mother cells that bypassed meiosis I and formed a restitution nucleus, meiosis II and subsequent stages of sporogenesis proceeded normally. Unreduced diploid spores resulted from this second pathway. When sown on either agar or sterile soil, these diplospores germinated and produced diploid gametophytes. Tetraploid sporophytes were produced by the gametophytes growing on sterile soil. The discovery of diploid sporophytes producing unreduced spores provided the opportunity to characterize the first step of one possible route to polyploid formation. Continued observations of the natural population may provide insights into the earliest stages of natural polyploid formation.     

How diverse were ferns in the Baltic amber forest?

Diverse temperate forest types and a high atmospheric humidity have recently been suggested for the Eocene source area of Baltic amber. However, ferns are astonishingly rare as inclusions in this amber, which is in contrast to other seed‐free land plants, fungi, and lichens. Moreover, the identities of some of the few described putative fern taxa are dubious, and some fossils were even assigned to the Paleozoic seed fern genera Alethopteris, Pecopteris and to the form genus Sphenopteris containing Paleozoic and Mesozoic fern‐like leaf fossils. Here, we review previously described fern inclusions from Baltic amber and identify further fern‐like leaf inclusions as belonging to the extant angiosperm genus Comptonia (sweet ferns, Myricaceae). We conclude that only one taxon, Matonia striata (Matoniaceae), can with confidence be identified as a Polypodiopsida representative. Although “Pecopteris” humboldtiana is so far only known as sterile foliage, its leaf morphology strongly suggests that also this taxon belongs to the Polypodiopsida rather than to any other tracheophyte lineage. We propose accommodating “Pecopteris” humboldtiana in the new genus Berendtiopteris. “Alethopteris” serrata and “Sphenopteris” phyllocladoides are not to be regarded as evidence of ferns from Baltic amber. Reinvestigation of the holotypes of these two taxa did not reveal to which tracheophyte lineages these fossils belong. We suggest that the scarcity of fern remains from Baltic amber may reflect both a relatively low fern diversity in the source area of the fossil resin, and an absence or rarity of epiphytic and climbing ferns as observed in modern temperate forest ecosystems.     

THE EFFECTS OF KINETIN AND NAPHTHALENEACETIC ACID ON IN VITRO SHOOT MULTIPLICATION AND ROOTING IN THE FISHTAIL FERN

This study is concerned with the determination of the shoot and root inducing effects of kinetin (K) and naphthaleneacetic acid (NAA) on shoots ofthe fishtail fern (Nephrolepisfalcata formafurcans) in sterile tissue culture. The data shows that K is the major factor involved in maximal shoot production. The NAA is not essential. Specific concentrations ofNAA must be present with specific concentrations of K for maximal root production. At the same time, the data also demonstrate that shoots on media containing no NAA but with K concentrations of 5 × 10⁻⁷ and 10⁻⁶ M produced as many roots as with any other NAA concentration. The data can be used as a guide to rapid commercial propagation of fishtail fern, and demonstrate that media available commercially for Boston fern multiplication will induce maximal shoot production in fishtail fern.     

Spore abortion index (SAI) as a promising tool of evaluation of spore fitness in ferns: an insight into sexual and apomictic species

Ferns reproduce through small and usually haploid spores. The general paradigm states that whereas species produce good shaped spores, hybrids are sterile and form aborted spores. Apomictic fern species represent an unusual case, and it is believed that they produce an unbalanced spore spectrum. Until now, no comprehensive comparison of sexual and apomictic taxa using extensive spore fitness data has been published. Based on a representative data set of 109 plants from 23 fern taxa, we accomplished the first robust analysis of spore fitness using spore abortion index (SAI), the ratio of aborted to all examined spores. One thousand spores were analyzed for each plant. Focusing mainly on two major European fern taxa (Asplenium, Dryopteris), we compared this trait for different fern reproductive types (sexual/apomicts/hybrids) and ploidy levels (diploid versus polyploid). Our results confirmed the general assumption that shows higher SAI for apomictic taxa (18%) when compared to sexual taxa (3%). Furthermore, hybrids are characterized by having almost all spores aborted (99.8%) with the notable exception of pentaploid Dryopteris × critica (93.1%), the hybrid between sexual and apomictic taxa. We found no significant difference in SAI between sexual taxa of various ploidy levels or between sexual taxa of genera Dryopteris and Asplenium. Additionally, we carried out an optimization of the SAI method, outlying important guidelines for the use of this method in the future.     

“MULTIPLE SPINDLE”—A MEIOTIC IRREGULARITY IN CLARKIA EXILIS

Vasek , F. C. (U. California, Riverside.) “Multiple spindle”—a meiotic irregularity in Clarkia exilis. Amer. Jour. Bot. 49(5): 536–539. Illus. 1962.—“Multiple spindle” is a meiotic irregularity in which more than one spindle occurs in a high proportion of first-metaphase cells. Abnormal telophase segregations, the occurrence of micronuclei with the microspore nuclei, and reduced pollen fertility are further manifestations of this irregularity. Reciprocal hybrids between normal and multiple spindle are normal. Segregation data from F₂ and F₃ progenies indicate that multiple spindle is inherited as a single recessive gene, m. The mm genotype has persisted at a high frequency (about 15%) for at least 7 years in the natural population from which cultures were derived.     

SPERM PRODUCTION AND STERILITY IN HYBRIDS BETWEEN TWO SUBSPECIES OF DROSOPHILA PSEUDOOBSCURA

Subspecies of Drosophila pseudoobscura, one occurring in the United States and the other in Bogota, Columbia, exhibit Haldane's Rule in one direction of the cross. Additionally, D. pseudoobscura produces two sperm types: short, sterile sperm and long, fertile, sperm. Here I examine the relationship between the production of short and long sperm and hybrid sterility. Fertile and sterile hybrid males produce a greater proportion of short sperm compared to parental males with sterile hybrids producing mainly short, immotile sperm. Sperm transfer and storage patterns were similar between fertile hybrid and parental strains; and unexpectedly, short, immotile sperm from sterile hybrids were stored. These findings raise the question of whether different genetic mechanisms disrupt both sperm heteromorphic production and sperm motility and whether this indicates that females exert some control over sperm storage.     

THE CYTOLOGY AND DERIVATION OF A TEMPERATURE-SENSITIVE MEIOTIC MUTANT IN THE FERN CERATOPTERIS

Meiotic mutants were obtained from an inbreeding program following a hybridization between two diploid species of the homosporous fern Ceratopteris. The mutants are characterized by high levels of asynapsis at high temperatures (27 C, 33 C) and by aberrant spindle function at low temperatures (18 C). Spore viabilities vary with temperature and are highest at 23 C. Occasional restitution during the first division results in the production of dyads of diploid spores, some of which are viable. The inbreeding program was characterized by the expression of high levels of apparent heterozygosity in spite of intragametophytic selfing. It appears that segregation within the inbreeding program was responsible for the production of the mutants. The apparent heterozgosity and segregation can be explained by a previously documented genetic system within the polyploid homosporous ferns.     

SUPPRESSION OF SEX-RATIO MEIOTIC DRIVE AND THE MAINTENANCE OF Y-CHROMOSOME POLYMORPHISM IN DROSOPHILA

Like several other species of Drosophila, D. quinaria is polymorphic for X-chromosome meiotic drive; matings involving males that carry a “sex-ratio” X chromosome (X^SR) result in the production of strongly female-biased offspring sex ratios (Jaenike 1996). A survey of isofemale lines of D. quinaria from several populations reveals that there is genetic variation for partial suppression of this meiotic drive. Crossing experiments show that there is Y-linked, and probably autosomal, variation for suppression of drive. Y-linked suppressors of X-chromosome drive have now been described in several species of Diptera. I develop a simple model for the maintenance of Y-chromosome polymorphism in species polymorphic for X-linked meiotic drive. One interesting feature of this model is that, if there is a stable Y-chromosome polymorphism, then the equilibrium frequency of the standard and sex-ratio X chromosomes is determined solely by Y-chromosome parameters, not by the fitness effects of the different X chromosomes on their carriers. This model suggests that Y-chromosome polymorphism may be easier to maintain than previously thought, and I hypothesize that karyotypic variation in Y chromosomes will be found to be associated with suppression of sex-ratio meiotic drive in other species of Drosophila.     

APOGAMY AND SOMATIC RESTITUTION IN THE FERN CERATOPTERIS

A mutant stock of the fern Ceratopteris has been derived from an inbreeding study following an interspecific hybridization between two diploid species. The mutant is characterized by gametophytes that produce non-functional spermatozoids and are incapable of selfing. Sporophytes develop apogamously from the mutant gametophytes and, although they are initially haploid and sterile, portions of the fronds later become doubled somatically and behave like tissues of sexually derived homozygous sporophytes. The mutant segregates from sporophytes in a 1:1 ratio when crosses are made with wild type gametophytes. Certain aspects of the behavior are similar to those seen in some naturally occurring apomictic ferns.     

POPULATION GENETIC STRUCTURE IN CHEILANTHES GRACILLIMA

Population genetic structure was examined in five populations of the xerically adapted homosporous fern Cheilanthes gracillima. F statistics using allozymic data indicated substantial genetic structure in all populations. To determine the factors responsible for genetic structure, we calculated levels of intragametophytic selfing and the fixation index for each subpopulation of each population and estimated levels of intrapopulational gene flow in each population. These analyses indicated that each subpopulation was a panmictic unit; thus, population genetic structure is not due to family structure, arising via matings between relatives. Intrapopulational gene flow was surprisingly low, given the typically high dispersibility of fern spores. However, it seems unlikely that spore dispersal in C. gracillima is significantly reduced relative to other homosporous ferns. Instead, we propose that the low rates of intrapopulational gene flow reflect limited availability of safesites for spore germination and gametophyte establishment. This ecological factor may play a primary role in generating and/or maintaining population genetic structure in C. gracillima.     

EXPERIMENTAL HYBRIDIZATION OF XANTHIUM STRUMARIUM (COMPOSITAE) FROM ASIA AND AMERICA. II. SESQUITERPENE LACTONES OF F1 HYBRIDS

Effects of hybridization on sesquiterpene lactones in Xanthium strumarium were studied for clues to the relationship of Old and New World populations. In crosses between indigenous Asiatic plants in the “strumarium” morphological complex and various American complexes that produce xanthinin as a major sesquiterpene lactone, the F₁ hybrids contained xanthinin and the related compounds, xanthanol, xanthatin, and xanthinosin. In other crosses with various American complexes that produce xanthumin, the stereoisomer of xanthinin, the F₁ hybrids contained xanthinin and xanthumin as well as xanthinin-related compounds and their stereoisomers, xanthumanol, deacetoxylxanthumin, and tomentosin. The Asiatic plants of “strumarium” from Hong Kong involved in the crosses produce approximately equal percentages of xanthinin, xanthatin and xanthinosin, but those from India contain only xanthinosin. The putative introduction of the American morphological complex, “chinense,” contains xanthumin as the major component but shows chemical diversity that indicates genes derived from the indigenous “strumarium” complex.     

ARCHAEOPTERIS MACILENTA,ANATOMY AND MORPHOLOGY OF ITS FROND

The stem, rachides, and pinnae of Archaeopteris macilenta, formerly considered to be a fern of Devonian age, comprise a branch system in which the ultimate divisions heretofore referred to as pinnules are the leaves. The primary vascular system of the “frond” is a lobed siphonostele from which leaf traces arise in a spiral sequence. The anatomy of the “rachis” and of the “pinnae” is shown to be similar to that of the stem, Callixylon, which bore these “fronds.” Branching, epidermal pattern and stomates are described for the spirally arranged leaves (fertile pinnules). Attachment and dehiscence of sporangia as well as their stomates are reported. Archaeopteris is retained in the Class Progymnospermopsida which includes plants with gymnospermous anatomy and pteridophytic reproduction. It is suggested that Actinopodium, Svalbardia and Siderella are related closely to Archaeopteris and that this group of genera shows evolutionary stages in webbing of leaves and planation of branch systems. The opportunities for ontogenetic studies of the arborescent genus Archaeopteris are pointed out.     

Apogamous sporophyte formation in a fernPteris multifida and its characteristics

When spores of the fern,Pteris multifida, were aseptically cultured in the dark, sporophytic plants were apogamously induced. The plants have been subsequently grown in pots until the development of leaves with many sporangia for observations of meiotic characteristics in their sporocytes. The sporophytic plants originated from spores were estimated to be haploid, and the estimation was supported by abnormal meiosis in sporocytes and the absence of mature spores, but some chromosomes (n=58) formed bivalents in the meiotic process.     

HYBRIDIZATION AND CRYPTIC SPECIES IN DICAMPTODON (CAUDATA: DICAMPTODONTIDAE)

The salamander genus Dicamptodon consists of at least four genetically divergent groups of populations with 10–19 fixed allelic differences (out of 31 loci scored) between populations in alternative groups. One of these groups corresponds to D. copei; the other three until now usually have been considered to belong to the single morphologically homogeneous species D. ensatus. Two instances of geographic contact between genetic units occur in the genus, one between D. copei and “D. ensatus” in northern Oregon and the other between two highly divergent types of “D. ensatus” in coastal northern Calfornia. In the former case no hybridization occurs, while in the latter a narrow hybrid zone has been observed. There appears to be selection against hybrids within this hybrid zone, and introgression beyond the zone is apparently nonexistent. The population groups therefore appear to be genetically independent units, and Dicamptodon is best considered to consist of four species: D. copei, D. ensatus, D. tenebrosus, and D. aterrimus.     

HYBRIDIZATION,POLYPLOIDY AND ADVENTITIOUS GROWTH IN THE GENUS ASPLENIUM

Gametophytes of Asplenium platyneuron, A. rhizophyllum and A. monlanum were grown from spores under controlled conditions. Factors inhibiting and promoting germination and growth were determined. Leaves cut from sexually produced sporophytes of A. platyneuron, A. rhizophyllum and A. montanum were cultured under controlled conditions on agar slants. The leaves produced neoplastic growths of two kinds: (1) two-dimensional growths, (2) three-dimensional growths. The former developed into normal-appearing, rhizoid- and gametangia-producing, heart-shaped gametophytes having a diploid chromosome complement. The formation of two-dimensional growths was favored by conditions drastically reducing the energy supply of the initiating sporophytic cells and by destroying the correlation of the leaf. Three-dimensional growths most often developed into sporophytes unless conditions were limiting; in the latter case they transformed into two-dimensional growths. Both two- and three-dimensional growths developed from fragmented leaves excised from buds growing near the tips of acuminate fronds of A. ebenoides. The high frequency of two-dimensional adventitious growth leads to the conclusion that apospory may provide a mechanism for the production of fertile allotetraploids, by the fusion of diploid gametes of adventitiously produced diploid gametophytes, in the genus Asplenium and perhaps other fern genera, in contrast to that which has been previously suggested in the literature.     

MEIOTIC IRREGULARITY IN SPECIES AND INTERSPECIFIC HYBRIDS OF PINUS

This paper describes a comparative analysis of meiotic conditions in 61 individual trees representing 21 species and 22 interspecific hybrid combinations of the genus Pinus. Material was collected during three successive growing seasons at the Eddy Arboretum of the Institute of Forest Genetics at Placerville, California. Meiotic irregularity occurred in all species and hybrids examined; mean irregularity frequencies of individual trees ranged from 0 to 47.2 percent. Abnormalities in chromosome movement and their consequences, (1) precocious disjunction associated with the occurrence of univalents and (2) the failure of chiasma terminalization leading to lagging chromosomes and to chromosome breakage and fragments, account for most of the observed irregularity. The same kinds of irregularity occur both in the species and in the hybrids, but they were considerably more frequent in certain of the hybrids than in the related species. These abnormalities in chromosome movement seem to be characteristic of Pinus and are attributed primarily to rrechanical difficulties which attend the large pine chromosomes in meiosis. The most spectacular meiotic irregularities were the characteristic bridge-fragment configurations considered to be the result of crossing-over in heterozygous paracentric inversions. Inversion bridges were observed in 59 of the 61 trees and were as frequent in the species as in the hybrids. They apparently do not result from interspecific differentiation in chromosome structure but from spontaneous intra-specific rearrangements. The literature and work now in progress provide increasing evidence that introgression has been an important factor in the evolution of pine populations. The cytological study of pine chromosomes has failed to produce qualitative evidence of introgression, but the quantitative measurement of meiotic irregularity may serve as a useful criterion for recognizing the results of past hybridization.