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.     

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.     

GENETIC EVIDENCE FOR ZYGOTIC MEIOSIS IN PHYTOPHTHORA CAPSICI

Variation of several traits among single-oospore isolates of the heterothallic oomycete, Phytophthora capsici, was studied to determine genetically the place of meiosis in the life cycle. Low percentages of met^– offspring were recovered from two crosses of met^– X prototrophic isolates. Arg^– cultures were also recovered from the crosses indicating the possible presence of a suppressed arg^– gene in one of the parental cultures. Three arg^– X met^– crosses yielded all four of the possible classes of offspring. Thus meiosis was either zygotic or, if gametangial, the original parental cultures must have been heterozygous at both the arg and met loci. However, when these parental cultures were crossed, only prototrophic offspring were recovered. A ratio of 3 prototrophs: 1 met^– was obtained from cross R in which two prototrophic offspring from a previous arg^– X met^– cross were mated. This indicated the presence of a suppressor of met^– and may explain the low recovery of met^– offspring from earlier crosses. From backcrosses of met^– offspring from this cross to the A² prototrophic parent, ratios close to the expected 1:1 were obtained. When the A¹ prototrophic parent of cross R (K-10) was crossed to an A² prototrophic isolate, met^– offspring were again recovered, which indicated that K-10 carried a suppressed met deficiency. In two crosses of streptomycin-resistant (S^r) X sensitive (S^s) isolates, ratio of 108 S^s: 14 S^r and 84 S^s:14 S^r were obtained. Although 1:1 ratios would have been expected if meiosis were zygotic, segregation of S^s and S^r in the F₁ generation indicated that gametangial meiosis was unlikely. Segregation of the A¹ and A² mating types in all crosses also indicated that meiosis was zygotic in P. capsici. Possible control of mating type by a single pair of alleles, by plasmids, and by more complex mechanisms is discussed in relation to the mating type ratios obtained.     

SAFETY FACTORS IN VERTICAL STEMS: EVIDENCE FROM EQUISETUM HYEMALE

Predictions from a mechanical model for hollow vertical stems are tested against morphometric and mechanical studies of the vertical stems of Equisetum hyemale. The model predicts 1) that the wall thickness of hollow internodes must be at least 15% of the external radius of shoots, 2) that the elastic modulus of stems is quantitatively related to the ratio of apoplast (cell walls) to symplast (cytoplasm) areas in transverse sections through stems, and that (3) hollow stems are designed to sustain an additional and significant proportion of their own weight. The “safety factors” predicted for a hollow vertical stem are used to examine two adaptationist explanations for hollow stems: 1) “economy in design,” which argues that natural selection will favor a reduction in the metabolic cost in constructing an organ, and 2) “mechanical design,” which argues that stems are designed to maximize their mechanical stability during vertical growth. Evidence from E. hyemale indicates that 1) there is a developmental limit to the maximum allotment of biomass invested in the construction of stems, 2) as stem height increases, morphometric adjustments in internodal wall thickness occur which converge on predicted safety limits, and 3) the elastic modulus of stems changes as a function of the ratio of apoplast to symplast areas seen in transverse sections through shoots. Biomechanical and developmental evidence and the allometry of E. hyemale stems are consistent with the view that stems are designed for safety and are inconsistent with some predictions based on the economy in design.     

SYMMETRY IN EQUISETUM

Bierhorst , David W. (Cornell U., Ithaca, N. Y.) Symmetry in Equisetum. Amer. Jour. Bot. 46(3) : 170-179. Illus. 1959.—A total of 118 leaf whorls and corresponding nodes from a total of 9 species of Equisetum were studied in serial cross-section. The number of whorls having the same leaf number as the 2 adjacent ones was 67. The size of the arc measured in degrees of circumference occupied by each leaf, as well as its position relative to leaves of adjacent whorls were measured. The disposition of the internodal and trans-nodal vascular strands was determined for each of the nodes. The average per cent deviations from theoretical leaf size within whorls were: whorls not involved in change in leaf number, 3.9; those with fewer leaves than the one below, 9.5; those with more leaves than one above, 7.1; those with more leaves than one below, 9.4; those with different leaf number from the 2 adjacent ones, 12.5. In Equisetum, it is a general rule that (1) a leaf which falls directly above a leaf in the next whorl where the younger whorl possesses fewer leaves than the older one is usually the largest leaf within its whorl, (2) a leaf which falls directly below a leaf in the next younger whorl where the younger whorl possesses fewer leaves than the older one is usually the smallest leaf within its whorl, (3) a leaf which falls directly above a leaf in the next older whorl where the younger whorl possesses a greater number of leaves than the older one is usually the smallest leaf in its whorl, and (4) a leaf which falls directly below a leaf in the next younger whorl where the younger whorl possesses a greater number of leaves than the older one is usually the largest leaf within its whorl. The numerous variations in the disposition of vascular tissue associated with changes in leaf number are described. Double leaf traces which originate in various ways are common, as well as vertical strands traversing the nodes. The leaf trace system is considered to be determined by the number, position, and relative sizes of leaf primordia. The disposition of the trans-nodal protoxylem seems to be determined to a large extent by the proximity of the leaf traces above and below the node.     

VARIATION IN SEED CHARACTERS IN NEMOPHILA MENZIESII: EVIDENCE OF A GENETIC BASIS FOR MATERNAL EFFECT

A growing body of evidence indicates that phenotypic selection on juvenile traits of both plants and animals may be considerable. Because juvenile traits are typically subject to maternal effects and often have low heritabilities, adaptive responses to natural selection on these traits may seem unlikely. To determine the potential for evolutionary response to selection on juvenile traits of Nemophila menziesii (Hydrophyllaceae), we conducted two quantitative genetic studies. A reciprocal factorial cross, involving 16 parents and 1960 progeny, demonstrated a significant maternal component of variance in seed mass and additive genetic component of variance in germination time. This experiment also suggested that interaction between parents, though small, provides highly significant contributions to the variance of both traits. Such a parental interaction could arise by diverse mechanisms, including dependence of nuclear gene expression on cytoplasmic genotype, but the design of this experiment could not distinguish this from other possible causes, such as effects on progeny phenotype of interaction between the environmental conditions of both parents. The second experiment, spanning three generations with over 11,000 observations, was designed for investigation of the additive genetic variance in maternal effect, assessment of paternal effects, as well as further partitioning of the parental interaction identified in the reciprocal factorial experiment. It yielded no consistent evidence of paternal effects on seed mass, nor of parental interactions. Our inference of such interaction effects from the first experiment was evidently an artifact of failing to account for the substantial variance among fruits within crosses. The maternal effect was found to have a large additive genetic component, accounting for at least 20% of the variation in individual seed mass. This result suggests that there is appreciable potential for response to selection on seed mass through evolution of the maternal effect. We discuss aspects that may nevertheless limit response to individual selection on seed mass, including trade-offs between the size of individual seeds and germination time and between the number of seeds a maternal plant can mature and their mean size.     

GENETIC EVIDENCE FOR TRACE PATERNAL TRANSMISSION OF PLASTIDS IN LIRIODENDRON AND MAGNOLIA (MAGNOLIACEAE)

Transmission of plastids in Liriodendron and Magnolia was studied by examining inheritance of plastid DNA markers in interspecific crosses. In congeneric hybrids of Liriodendron and Magnolia, 2.9% and 11.1% of progeny, respectively, exhibited uniparental paternal transmission of plastids. This departure from strict uniparental maternal transmission of plastids indicates that mechanisms for the elimination of paternal plastids are not 100% efficient and that any proposed model for plastid transmission must account for a continuum of variation. Our results, and other results reviewed here, show a discrepancy with data obtained by cytological observations and point to the need of confirming the mode of plastid inheritance by genetic analysis. In addition, if paternal plastids are occasionally inherited, intensive sampling of plastid DNA in a putative hybrid population could identify both parents of hybrids and thereby aid in the phylogenetic reconstruction of hybrid zones.     

GENETIC EVIDENCE FOR MULTIPLE ORIGINS OF DIOECY IN THE HAWAIIAN SHRUB WIKSTROEMIA (THYMELAEACEAE)

Dioecy is unusually common in the Hawaiian Islands, yet little is known about the evolutionary biology of this breeding system. A native shrub, Wikstroemia, has an unusually diverse array of breeding systems: two forms of dioecy, cryptic and morphological dioecy, as well as hermaphroditism (perfect flowers). The existence of two forms of dioecy is significant for three reasons: 1) the presence of cryptic unisexuals that are functionally unisexual, but retain the appearance of hermaphroditism in both sexes, is strong evidence for the ancestral status of hermaphroditism; 2) the production of nonfunctional pollen, by female cryptic unisexuals, is a new instance of a phenomenon which has previously been reported for a few other species; 3) the two forms of dioecy are morphological markers which are useful in hybridization studies for tracing the genetic basis of their inheritance. Crosses were made between cryptically unisexual individuals (C), between morphologically unisexual individuals (M), and between the two types of unisexuality. The offspring of crosses between individuals with the same sex type usually resulted in offspring with that sex type, but most of the progeny of between-sex type crosses were, unexpectedly, perfect-flowered hermaphrodites. These results show that genetic control of sex determination is not homologous in all populations, suggesting that dioecy has evolved at least twice in Hawaiian Wikstroemia. The genetic data further suggest that males are the heterozygous sex.     

EXPERIMENTAL EVIDENCE FOR GENETIC VARIATION IN COMPATIBILITY BETWEEN THE FUNGUS ATKINSONELLA HYPOXYLON AND ITS THREE HOST GRASSES

Variation in compatibility has been documented within and among several natural plant populations infected by fungal pathogens. In this study, seeds and isolates of the fungus Atkinsonella hypoxylon (Ascomycetes, Clavicipitaceae) were collected from three populations of the grass Danthonia spicata, two populations of D. compressa, and four populations of Stipa leucotricha. Each fungal strain was reciprocally inoculated into seedlings grown in aseptic culture from its original host population, into seedlings from other conspecific host populations, and into seedlings from the other two host species. There were three distinct patterns of compatibility, as evidenced by the ability of the fungus to grow on the seedling and to colonize new tillers. Fungal strains from one host genus were incompatible with seedlings from the other host genus. Strains from the two Danthonia species were broadly compatible among Danthonia populations and had very high rates of infection, while strains from Stipa also were broadly compatible among Stipa populations but had relatively low rates of infection. Literature surveys indicate that, in contrast to pathogenic microorganisms, mutualistic microorganisms typically exhibit broad patterns of compatibility among hosts, which lack resistance to infection. The effect of A. hypoxylon on host fitness is most detrimental in Stipa, where the fewest seedlings became infected, and most beneficial in Danthonia, where most seedlings became infected.     

中华鲟天然群体蛋白质水平遗传多样性贫乏的初步证据

为阐明我国Ⅰ级珍稀水生保护动物中华鲟天然群体的遗传结构和遗传多样性特征,为其资源的监测量民保护提供科学依据,采用聚丙烯酰胺梯度凝胶电泳技术对中华鲟天然群体进行了蛋白质遗传多态性研究。共研究了１５种蛋白质,有４种蛋白无活呈活性很低,在有活性的１１种蛋白中人测得２６个座位;在２６个座位中,只有１个座位（ＭＤＨ－１）为多态座位。中华鲟多态座位比例（Ｐ）为３．９０％,遗传杂合度（Ｈ）为０．０４,均远远低于