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.     

MOLECULAR EVIDENCE FOR THE ORIGIN OF THE S-DERIVED GENOMES OF POLYPLOID TRITICUM SPECIES

The genus Triticum includes several polyploid species that arose due to hybridization between two or more diploid species. Section Sitopsis is comprised of five diploid species given the genome designation S. Four polyploid species are recognized that contain an S or S-derived genome. We have used two repetitive DNA sequences found primarily in the S genomes of Triticum to determine the likely diploid progenitors of the polyploid species. Comparison of restriction fragments that hybridize to probes for these sequences suggests that T. speltoides is distinct from other members of section Sitopsis (i.e., T. longissimum, T. bicorne, T. searsii, and T. sharonense). The S-derived genome of T. aestivum is more closely related to T. speltoides than to the other Sitopsis diploids. The restriction fragment pattern of T. timopheevii is 98% identical to that of T. speltoides, while those of T. kotschyi and T. syriacum are identical to the group of diploids represented by T. longissimum, T. bicorne, T. searsii, and T. sharonense. Our results are compatible with previous molecular and biochemical data regarding relationships among Triticum species containing an S or S-derived genome.     

ELECTROPHORETIC AND CYTOGENETIC EVIDENCE FOR ALLOPOLYPLOID ORIGIN OF MARSHALLIA MOHRII (ASTERACEAE)

Marshallia mohrii is a tetraploid species, 2n = 4x = 36, with approximately 17% of its pollen mother cells exhibiting a single quadrivalent at diakinesis of Meiosis I. The species is morphologically most similar to M. grandiflora, a member of the Grandiflora complex along with M. mohrii and M. trinervia. These data led to the preliminary hypothesis that M. mohrii originated by autopolyploidy. However, we rejected the autopolyploid hypothesis because the number of quadrivalents observed in 93 cells is significantly less than predicted by the Jackson-Casey-Hauber model for autotetraploids that have zero to two chiasma per pachytene bivalent. Enzyme electrophoresis was used to test the alternative hypothesis of allotetraploidy and to determine possible diploid progenitor(s). Eleven enzymes encoded by 25 loci were resolved for the three species in this complex. Marshallia mohrii exhibits fixed heterozygosity for the polymorphic loci. The diploid species possess three duplicated loci, one for isocitrate dehydrogenase and two for phosphoglucose isomerase. Of sixteen alleles among the polymorphic loci in M. mohrii, one allele (for Tpi-1) is also found in M. trinervia, and three alleles (one each for Tpi-1, Tpi-2, and Lap) are found in M. grandiflora. Marshallia mohrii also possesses one allele each at Idh-1, Idh-2, Pgi-4, and two each at Me and Mnr that are not shared with either of the two diploid species analyzed in this study. The cytogenetic and electrophoretic evidence suggest an allotetraploid origin of M. mohrii, possibly involving M. grandiflora, M. trinervia, and a third species. Inferences about ancestry are difficult because of the paucity of qualitative allozyme divergence among the diploids and because of the number of high frequency alleles in M. mohrii, not found in either M. grandiflora or M. trinervia.     

CO-REQUIREMENT FOR CALCIUM AND POTASSIUM IN THE GERMINATION OF SPORES OF THE FERN ONOCLEA SENSIBILIS

Spores of Onoclea sensibilis L. do not germinate on distilled H₂O if they are pretreated for sufficient time with dilute NaClO solution. However, spores will germinate, after NaClO pretreatment, on a simple mineral medium containing the major and trace elements. Complete germination after pretreatment also is obtained on a solution containing only Ca²⁺ and K⁺ as the cations, but neither ion by itself is sufficient. Rb⁺, but not Li⁺ or Na⁺, can replace K⁺. Hypochlorite-treated spores do not require the continuous presence of Ca²⁺ and K⁺ to germinate; exposure during the first 4 hr of culture, with the remainder of the time on distilled H₂O, is sufficient. Extraction of spores with ethylene glycol bis(aminoethyl ether) tetraacetic acid [EGTA] makes their germination dependent on Ca²⁺, as reported by other workers, but it does not produce a co-requirement for K⁺. Colorimetric analysis with arsenazo III confirms that Ca²⁺ is extracted from Onoclea spores by NaClO. Extractable Ca²⁺ amounts to about 78 nmol/mg spore dry wt. Of this amount, 31% is contained in the perispore. The perispore comprises 13% of the total spore dry wt.     

BREEDING SYSTEM OF THE FERN DRYOPTERIS EXPANSA: EVIDENCE FOR MIXED MATING

The levels and distribution of genetic variation within and among populations of the homosporous fern Dryopteris expansa were assessed using enzyme electrophoresis. Twelve loci representing six enzymes were examined in 502 sporophytes from nine populations. Values of P, H, and mean number of alleles per locus are much lower in D. expansa compared to values for other fern species. Values of F varied from –0.014 to 0.745 with an average F of 0.335. There was also considerable population to population variation in F. In three populations the observed heterozygosity closely approximated that expected at Hardy-Weinberg equilibrium, whereas in the remaining populations observed heterozygosity was significantly lower than the expected heterozygosity. In D. expansa the major component of F_IT is F_IS; the relatively high F_ST value suggests a high degree of interpopulational differentiation. Electrophoretic data suggest that this species possesses a mixed mating system. This study, in conjunction with other investigations, indicates that just as in angiosperms, a variety of breeding systems operates in homosporous ferns, ranging from extreme inbreeding, through mixed mating to outcrossing.     

THE ULTRASTRUCTURE OF PALEOZOIC FERN SPORES: I. BOTRYOPTERIS

The spores of four species of the Paleozoic filicalean fern Botryopteris are examined at the ultrastructural level. Spores of B. cratis, B. forensis, B. globosa, and an unnamed species from the Lower Pennsylvanian, are compared on the basis of sporoderm stratification and the presence or absence of a sculptine layer. The species examined differ widely as to the type of reproductive unit in which they are borne and include forms that range throughout the Pennsylvanian. In all species the exine is homogeneous, lacking cavities and lamellae. A thin nexine is present in the Middle and Upper Pennsylvanian taxa, but is absent in the Lower Pennsylvanian spores. Only one spore type (B. cratis) possesses a clearly defined sculptine layer. Features of the sporoderm are compared with those of extant, homosporous pteridophyte spores.     

WEIGHING THE EVIDENCE FOR A SINGLE ORIGIN OF PLASTIDS1

The Journal of Phycology recently published a research paper entitled “A single origin of plastids revisited: convergent evolution in organellar genome content” ( Stiller et al. 2003 ). Also appearing in that issue was a minireview by Jeffrey D. Palmer ( Palmer 2003 ), “The symbiotic birth and spread of plastids: how many times and whodunit?” In his review, Palmer discussed evidence in support of a single endosymbiotic origin, in light of our analyses showing that similarities in plastid genome content are explained better by convergent evolution than by shared evolutionary history. Palmer raised a number of important issues that were not addressed in our paper, including the point that, in his view, no real evidence has been cited against a single plastid origin. After carefully considering Palmer's arguments, and this key point in particular, I am prompted to offer a few additional comments in the spirit of furthering a useful discussion begun in the February issue.     

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.     

ORIGIN AND EARLY MORPHOGENESIS OF LATERAL BUDS IN THE FERN DAVALLIA

The general organography, vascular organization, and leaf and bud development in Davallia solida and D. trichomanoides are described. These epiphytic species have creeping shoots with dorsally-borne leaves in a distichous phyllotaxis and the buds occur near each leaf base. Roots are borne on the ventral and flanking surfaces of the rhizome, but only at bud positions. The vascular pattern of these species is a perforated solenostele. Leaf and bud traces have distinctly different origins. While the proximity of buds to leaves has suggested that bud origin is axillary, observations show that the origin of buds is cauline and that their position is extra-axillary from inception. The stages of structural morphogenesis in Davallia buds differ significantly from the scheme proposed by Wardlaw. The principal difference is the absence of a resting period occurring between the origin and continued development of buds in Davallia. The elongated internodes which separate leaf-bud pairs from one another, the topographically distinct and predictable positions of leaves and buds, the structural equivalence of unexpanded buds, and vascular differences in leaves and buds make Davallia an useful species for physiological studies of differential bud expansion.     

沟鞭藻的起源:分子地球化学证据

沟鞭藻为单细胞的原始的海洋生物。古生物学家根据形态特征,超微结构和生物化学信息认为沟鞭藻可能在前寒武纪时已存在,并推测古生代疑源类可能是现代沟鞭藻的先祖,但确凿的沟鞭藻囊孢的化石记录始于晚三叠世。生物标志物以分子化石的形式记录了沟鞭藻的起源及演化历史。在古生代及前寒武纪地层中甲藻甾烷及三芳沟鞭藻的检出,建立了沟鞭藻与其古代先祖（疑源类）的联系,提供了沟鞭藻先祖前寒武纪存在的证据。     

ELECTROPHORETIC EVIDENCE FOR GENETIC DIPLOIDY IN PSILOTUM NUDUM

Psilotum nudum (2n = 104) has been considered an ancient polyploid, having resulted from repeated cycles of hybridization and allopolyploidy. However, electrophoretic analysis indicates that this species is genetically diploid despite its high chromosome number. Sixteen enzymes, encoded by 28 loci, revealed in P. nudum the number of isozymes typical of diploid seed plants. There is, therefore, no evidence of polyploid gene expression for the enzymes analyzed. These results for Psilotophyta are similar to those obtained for other lineages of homosporous pteridophytes, i.e., Arthrophyta and homosporous Microphyllophyta and Pteridophyta, all of which should be considered genetically diploid. Several hypotheses have been proposed to explain these results, most notably 1) cycles of allopolyploidy followed by massive gene silencing, and 2) initiation of these lineages with high chromosome numbers, possibly via chromosomal fission. Discrimination between these hypotheses awaits testing with molecular genetic techniques.     

PHYLOGENETIC EVIDENCE FOR THE CRYPTOPHYTE ORIGIN OF THE PLASTID OF DINOPHYSIS (DINOPHYSIALES,DINOPHYCEAE)1

Photosynthetic members of the genus Dinophysis Ehrenberg contain a plastid of uncertain origin. Ultrastructure and pigment analyses suggest that the two‐membrane‐bound plastid of Dinophysis spp. has been acquired through endosymbiosis from a cryptophyte. However, these organisms do not survive in culture, raising the possibility that Dinophysis spp. have a transient kleptoplast. To test the origin and permanence of the plastid of Dinophysis, we sequenced plastid‐encoded psbA and small subunit rDNA from single‐cell isolates of D. acuminata Claparède et Lachman, D. acuta Ehrenberg, and D. norvegica Claparède et Lachman. Phylogenetic analyses confirm the cryptophyte origin of the plastid. Plastid sequences from different populations isolated at different times are monophyletic with robust support and show limited polymorphism. DNA sequencing also revealed plastid sequences of florideophyte origin, indicating that Dinophysis may be feeding on red algae.