INTERSPECIFIC HYBRIDIZATION AND TAXON UNIFORMITY IN ARABIS (CRUCIFERAE)

Intermixed populations of Arabis holboellii Hornem. var. pendulocarpa (A. Nels.) Roll., A. williamsii Roll., and their putative hybrids; and of A. holboellii var. retrofracta (Grah.) Rydb., A. drummondii Gray and A. divaricarpa A. Nels., the latter also considered to be a probable hybrid taxon, were studied in the field in Montana and Wyoming. In spite of the “intermediacy” of the putative hybrids in both cases, hybrid swarms were not found. In each instance, the “intermediate” as well as the taxa regarded as parental were remarkably uniform. The existence of three relatively uniform morphotypes in each of the several populations is contrary to the usual situation involving hybridizing taxa. A possible explanation is that seed production, at least in the “intermediate” morphotype individuals, involves apomixis, thus circumventing the usual sexual process that would result in a hybrid swarm. Amphiploidy as a possible mechanism for producing A. divaricarpa as a stable hybrid was ruled out in one population where the chromosome number 2n = 14 was found in all three taxa present in the population. Arabis divaricarpa, widely distributed in northern North America, often occurs as a uniform morphotype where no other species of Arabis grows. If it is of hybrid origin, as the evidence suggests, a mechanism other than sexual reproduction must be present to keep up uniformity and at the same time perpetuate the taxon. It is suggested that facultative apomixis is involved in the reproductive process and that it controls the genetic variability that would otherwise be present by insuring maternal inheritance in each successive seed crop.     

Meiotic restitution in wheat-barley hybrids

Meiotic restitution occurs in pollen mother cells (PMCs) of reciprocal F₁ hybrids between wheat and barley. In occasional PMCs, all or most of the 28 chromosomes assemble at the equatorial plate at metaphase I, but instead of undergoing anaphase I separation they reform into a mass of chromatin to form a restitution nucleus. Some of these restituted nuclei undergo a regular second division and dyads are produced among other non-restituted cells which have reached the tetrad stage of division. Other restituted nuclei fail to undergo a second division and then the PMCs appear as monads among neighbouring tetrads. Both the monads and dyads are expected to produce microsporocytes with the diploid complement of chromosomes. Chromosomes which fail to become included in the restituted nucleus form separate micronuclei and, depending on whether they undergo a regular second division or not, the PMCs containing them eventually appear as tetrads, triads or dyads. These partially restituted nuclei are expected to produce unreduced gametes, deficient for one or more chromosomes. It is postulated from these observations that restitution in wheatbarley F₁ hybrids depends on a high frequency of univalent accumulation at the equatorial plate at metaphase I and the subsequent failure of the chromosomes to undergo anaphase I separation.     

Two cryptospore-bearing land plants from the Lower Devonian (Lochkovian) of the Welsh Borderland

Two examples of fragmentary, coalified plant fossils with cellular preservation and in situ spores are described using scanning and transmission electron microscopy (SEM and TEM), from a Lower Devonian (Lochkovian) locality in the Welsh Borderland. A sporophyte which produced dyads of ?Cymbohilates of. horridus is unique in that stomata are numerous on the supporting axis, and are comparable with those described from contemporary vascular plant remains. The in situ dyads possess a bilayered exospore wall, with an outer exosporal envelope present over the distal faces. A fusiform sporangium, with externally smooth epidermis, contains specimens of the tetrad Velatitetras sp. Each tetrad is encompassed by a laevigate, folded, exosporal envelope of uniform thickness, which contains a layer of regular voids. Spores within the tetrads are ultrastructurally bilayered, with a complex, digitate outer margin presumably representing spore wall ornamentation. Neither the tetrads nor dyads reported in this paper are lamellate in ultrastructure. The combination of stomata, branching and dyads in the same sporophyte holds significance for the understanding of cryptospore affinities.     

Postmeiotic cytokinesis and pollen aperture number determination in eudicots: effect of the cleavage wall number

Summary.  In eudicot postmeiotic tetrads, apertures are usually joined in pairs in highly conserved areas. These appear to be located at the last points of contact persisting at the end of cytokinesis between the cytoplasm of the future microspores. In order to investigate the relationship between cytokinesis and aperture formation, aperture distribution within postmeiotic tetrads and the progression of meiosis were studied in Nicotiana tabacum cv. Ambalema. This variety (inbred line) produces about 85% tricolporate pollen and 15% tetracolporate pollen grains. In addition, about 7% of tetrads are composed of four equal-sized microspores and a supernumerary pseudomicrospore of small size and an equal proportion of tetrads exhibit unpaired apertures (these apertures are not joined in pairs within tetrads). Observation of cytokinesis indicates that both unpaired apertures and pseudomicrospores could result from the persistence of late communications between microsporocytes. Observations of tetrads indicate that an increase in the number of elements that are separated during cytokinesis is correlated with an increase in microspore aperture number. All data converge to support the hypothesis that aperture site determination is partly controlled by the number of walls formed to separate the different elements of the tetrad. Received May 22, 2002; accepted October 29, 2002; published online April 2, 2003 RID="*" ID="*" Correspondence and reprints: Laboratoire de Ecologie, Systematique et Evolution, Batiment 362, Université Paris Sud, 91405 Orsay cedex, France.     

Permanent spore dyads are not ‘a thing of the past’: on their occurrence in the liverwort Haplomitrium (Haplomitriopsida)

The liverwort Haplomitrium gibbsiae is shown to regularly produce spores released in the form of permanent dyad pairs. Developmental studies indicate that the dyads are produced via a unique half‐lobed configuration of the developing sporocyte. Many fossil cryptophytes of Siluro‐Devonian age, which are clearly embryophytes based on their morphology, contain permanent spore dyads in their sporangia, but this is the first demonstration of their occurrence in a living plant, a species belonging to Haplomitriopsida, which resolves in a clade that is considered to be sister to all remaining liverworts. Dispersed spore‐like dyads are found in the rock record as far back as the mid‐Cambrian, but most researchers still regard the first occurrence of isomorphic, tetrahedral tetrads in the mid‐Ordovician as the benchmark age for the origin of land plants. Regardless of the geological antiquity of the embryophytes, it appears that H. gibbsiae has retained a non‐simultaneous form of sporogenesis that may ultimately be traced to a charophytic origin. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179 , 658–669.     

Evolution of drought tolerance and defense: dependence of tradeoffs on mechanism, environment and defense switching

Plants evolve defenses against herbivores and pathogens in stressful environments; however, plants that evolve tolerances to other environmental stressors may have compromised defenses. Such tradeoffs involving defenses may depend on limited resources or otherwise stressful environments; however, the effect of stressful environments on defense expression might be different for different genotypes (G×E). To test these predictions, we studied genetic variation and co‐variation of drought stress tolerance and defenses at two levels of genetic variation: between and within closely related species. We did this across an experimental drought stress gradient in a growth room for species for which genetic variation in drought tolerance was likely. In apparent contrast to predictions, the species Boechera holboellii (Brassicaceae) from lower and dryer elevations had slower inherent growth rates and correspondingly higher total defensive glucosinolate concentrations than the closely related species B. stricta from higher elevations. Thus, B. holboellii was both drought tolerant and defended; however, optimality theory does predict tradeoffs between defense and growth. Differences between species in the direct effect of water deficiency on glucosinolate production did not obscure the grow‐or‐defend tradeoff. B. holboellii may also have been more resistant to the specialist herbivore Plutella xylostella; a trend that was less clear because it depended on plant development and water deficient conditions. At finer scales of genetic variation, there was significant variation among families and naturally occurring inbred lines of B. stricta in drought tolerance measured as inherent growth, the reaction norm of growth across drought treatments, shoot water potential, and transpiration rates. Evidence for tradeoffs was also found within B. stricta in genetic correlations between resistance and transpiration rates, or glucosinolates and growth rates. No G×E was detected at these finer scales of genetic variation, although sometimes the tradeoff was dependent on drought conditions. Direct effects of drought stress resulted in an apparent plastic switch between resistance and tolerance to damage, which might be a cost avoidance mechanism because tradeoffs never involved tolerance to damage. Thus, when drought tolerance is manifest as slow inherent growth rates, plants may also have relatively high defense levels, especially in stressful environments. Otherwise, defenses may be compromised by drought‐coping mechanisms, although plastic switches to less costly defenses may alleviate constraints in stressful environments.     

The influence of tetrad shape and intersporal callose wall formation on pollen aperture pattern ontogeny in two eudicot species

Background and Aims

In flowering plants, microsporogenesis is accompanied by various types of cytoplasmic partitioning (cytokinesis). Patterns of male cytokinesis are suspected to play a role in the diversity of aperture patterns found in pollen grains of angiosperms. The relationships between intersporal wall formation, tetrad shape and pollen aperture pattern ontogeny are studied.

Methods

A comparative analysis of meiosis and aperture distribution was performed within tetrads in two triporate eudicot species with contrasting aperture arrangements within their tetrads [Epilobium roseum (Onagraceae) and Paranomus reflexus (Proteaceae)].

Key Results and Conclusions

Intersporal wall formation is a two-step process in both species. Cytokinesis is first achieved by the formation of naked centripetal cell plates. These naked cell plates are then covered by additional thick, localized callose deposits that differ in location between the two species. Apertures are finally formed in areas in which additional callose is deposited on the cell plates. The recorded variation in tetrad shape is correlated with variations in aperture pattern, demonstrating the role of cell partitioning in aperture pattern ontogeny.     

Production and viability of unreduced gametes in triploid interspecific blueberry hybrids

Summary Three triploid (2n=3x=36) blueberry hybrids were obtained by hand-pollinating approximately 7,000 flowers of tetraploid highbush blueberry cultivars (based on Vaccinium corymbosum L.) with pollen from the diploid species V. elliottii Chapm. Meiotic analysis of these triploids revealed trivalents, bivalents and univalents in all metaphase I cells, with lagging chromosomes evident at anaphase I. Pollen of the three triploids was mostly aborted and did not stain with acetocarmine. However, the three triploids did produce from 0.9%–1.3% giant pollen grains that stained with acetocarmine and were present as monads, dyads or triads, rather than the normal tetrads. Pollination of 10,853 flowers of hexaploid V. ashei Reade cultivars with pollen from the triploids produced 266 berries, which averaged fewer than two fully-developed seeds per berry. One triploid clone showed partial female fertility when crossed to hexaploids, self-pollinated, or intercrossed with other triploids. Ploidy levels of the resulting hybrids were determined.Florida Agricultural Experiment Station Journal Series No. 8672     

传统傣药竹叶兰的花粉团发育及分类学意义

该研究利用石蜡切片技术观察并描述了兰科单型属竹叶兰属的花粉团发育过程,包括花形态解剖特征、8个花粉团的形成机制、花药壁发育模式、小孢子发生及雄配子体发育等特征,为该属复杂的系统亲缘关系提供胚胎学证据。结果表明:(1)成熟花药有两个药室,每个药室有4个一簇金色的花粉团,被白色花药帽;早期花药原基分化出的一对并列侧生药室,每个药室中央的小孢子囊在极面观方向分化出两条十字交叉的纵向不育隔膜组织,将其沿花药室纵轴方向深切为4个不等大的棒状次生孢子囊,最后发育为4个花粉团。(2)花药成熟时,靠花药开裂处的隔膜组织比近药隔膜组织的降解速度快且彻底,因此每个药室内的4个花粉团在花药开裂处粘合成一簇。(3)发育完好的花药壁共有6～7层,由外到内为表皮、3～4层药室内壁、中层和双核绒毡层,符合多层型花药壁的发育模式;花药成熟时,表皮退化,纤维性加厚发生在3～4层药室内壁,中层和绒毡层彻底降解。(4)小孢子母细胞通过同时型胞质分裂产生了正四面体型、左右对称、十字交叉型排列的小孢子四分体;小孢子四分体继续保持在同一胼胝质内完成了雄配子体发育,形成了2-细胞型的四合花粉;四合花粉两两或松散或紧密排列,构成了粉质花粉团。在前人的研究基础上,本文证实、补充并分析了竹叶兰属的花粉团发育特征,为该属的亲缘关系提供了胚胎学证据。     

A new species of Boechera (Brassicaceae) from Utah and Colorado

A new species,Boechera glareosa, is described and a key is provided to separate it from its apparent closest relative,B. gunnisoniana (Rollins) W. A. Weber. Twelve transfers are made fromArabis toBoechera to bring the nomenclature of Rocky Mountain region taxa in line with current DNA data.     

Gametogenesis and fertilization in Rhabdias ranae Walton 1929: I. The parasitic hermaphrodite

The somatic, diploid, and haploid chromosome numbers were determined in the hermaphrodite (24, 12, eggs 6, spermatozoa 5 or 6). Oocytes in the ovary synapsis zone contain 6 tetrads. The tetrads seem to disappear in the growth zone and reappear in the oocyte just prior to fertilization. After sperm penetration, the tetrads move to the periphery and form 2 polocytes by rapid meiotic divisions. Spermatogenesis occurs in the testis zone. Primary spermatocytes show 5 large tetrads and 2 small heterochromosomal dyads which lag on the spindle during division. Each resultant secondary spermatocyte has 1 small and 5 large dyads. In the equational division, one of the lagging heterochromosomes (X) is lost in a small cytophore as cytokinesis occurs. Two spermatids are produced, one with 5 chromosomes and the other with 6 chromosomes. Fertilization occurs in the seminal receptacle and results in either free-living males (2N = 11) or females (2N = 12).     

A new type of microtubular cytoskeleton in microsporogenesis of Lavatera thuringiaca L.

Summary. In Lavatera thuringiaca, kariokinesis and simultaneous cytokinesis during the meiotic division of microsporogenesis follow a procedure similar to that which takes place in the majority of members of the class Angiospermae. However, chondriokinesis occurs in a unique way found only in species from the family Malvaceae. Chondriokinesis in such species is well documented, but the relationship between the tubulin cytoskeleton and rearrangement of cell organelles during meiosis in L. thuringiaca has not been precisely defined so far. In this study, the microtubular cytoskeleton was investigated in dividing microsporocytes of L. thuringiaca by immunofluorescence. The meiotic stages and positions of cell organelles were identified by staining with 4′,6-diamidino-2-phenylindole. We observed that, during prophase I and II, changes in microtubular cytoskeleton configurations have unique features, which have not been described for other plant species. At the end of prophase I, organelles (mostly plastids and mitochondria) form a compact envelope around the nucleus, and the subsequent phases of kariokinesis take place within this arrangement. At this point of cell division, microtubules surround the organelle envelope and separate it from the peripheral cytoplasm, which is devoid of plastids and mitochondria. In telophase I, two newly formed nuclei are tightly surrounded by the cell organelle envelopes, and these are separated by the phragmoplast. Later, when the phragmoplast disappears, cell organelles still surround the nuclei but also move a little, starting to occupy the place of the disappearing phragmoplast. After the breakup of tetrads, the radial microtubule system is well developed, and cell organelles can still be observed as a dense envelope around the nuclei. At a very late stage of sporoderm development, the radial microtubule system disappears, and cell organelles become gradually scattered in the cytoplasm of the microspores. Using colchicines, specific inhibitors of microtubule formation, we investigated the relationship between the tubulin cytoskeleton and the distribution of cell organelles. Our analysis demonstrates that impairment of microtubule organization, which constitutes only a single component of the cytoskeleton, is enough to disturb typical chondriokinesis in L. thuringiaca. This indicates that microtubules (independent of microfilaments) are responsible for the reorganization of cell organelles during meiotic division. Correspondence: D. Tchórzewska, Department of Plant Anatomy and Cytology, Maria Curie-Skłodowska University, Akademicka 19, 20-033 Lublin, Poland.     

MICROTUBULES ASSOCIATED WITH SIMULTANEOUS CYTOKINESIS OF COENOCYTIC MICROSPOROCYTES

Microtubule arrays associated with simultaneous cytokinesis in the coenocytic microsporocytes of Lonicera japonica and Impatiens sultani were studied by indirect immunofluorescence. The future division planes are not predicted prior to meiosis by either a preprophase band of microtubules or cytoplasmic lobing. Cleavage planes appear to be determined by position of the four haploid nuclei and the development of postmeiotic microtubule systems. Perpendicular second division spindles in Lonicera result in tetrahedrally arranged tetrads while parallel spindles in Impatiens result in tetragonal arrangement. Immediately following meiosis bands of microtubules, the secondary spindles, develop between both sister and nonsister nuclei. These arrays give way to systems of microtubules that radiate equally from each of the four nuclei in the coenocytic sporocyte. Simultaneous cytokinesis is initiated by centripetal wall deposition at the periphery of the sporocyte and proceeds along planes marked by interaction of the opposing arrays of nuclear-based microtubules.     

POLLINATION DYNAMICS IN EPILOBIUM CANUM (ONAGRACEAE): CONSEQUENCES FOR GAMETOPHYTIC SELECTION

Several hand-pollination experiments have shown that pollen tube competition leads to gametophytic selection, and thus affects genotypic and phenotypic characteristics of the next generation. This study is one of the first to quantify natural levels of pollen tube competition. In a population of Epilobium canum, I measured both the amount of pollen deposited on stigmas and the timing of deposition. Approximately 20 tetrads were required for full seed set within fruits. Hummingbirds deposited >20 tetrads at 50–70% of the flowers examined, often in a single load. When pollen arrived in 2 loads, a portion of the ovules within each ovary was probably sired by competing pollen from the second load. Competition may be relatively weak unless at least 30 tetrads compete for ovules. About 20% of the flowers received >30 tetrads in the first load, and 13% acquired > 30 tetrads in 2 loads. The frequency and intensity of pollen tube competition varied among plants. In some styles, 80% of the pollen tubes were excluded from access to ovules, but in others no competition occurred. Further studies of pollination rates and progeny fitness are needed before we can assess the role of pollen tube competition in natural populations. Potential effects of gametophytic selection are discussed.     

Early meiosis in Rhynchospora pubera L. (Cyperaceae) is marked by uncommon ultrastructural features

The family Cyperaceae has an unusual microsporogenesis in which tetrad formation does not occur. In addition, other cytological features are important, such as the occurrence of holokinetic chromosomes and post-reductional meiosis. We have examined the ultrastructural features of the pollen mother cell (PMC) of Rhynchospora pubera. Anthers of several sizes were analyzed using light and transmission electron microscopy. The PMC before meiosis presented a central nucleus and a regular profile of the nuclear envelope. During prophase I, the nucleus was in the abaxial region of the cell. This cellular polarization was accompanied by other marked ultrastructural features in the nuclear envelope. Morphological changes involved dilations of perinuclear cisterns and polarization of the nuclear pore complexes. The results show that polarization occurs in the initial phases of microsporogenesis in R. pubera, unlike other plant species.     

Microtubule organization during successive microsporogenesis in Allium cepa and simultaneous cytokinesis in Nicotiana tabacum

Microtubule cytoskeleton organization during microspore mother cell (MMC) meiosis in Allium cepa L. and microsporogenesis in Nicotiana tabacum L. was examined. The MMC microtubules (MTs) were short and well dispersed in the cytoplasm of both taxa. As the MMCs of both species entered metaphase of meiosis I, the MTs constructed a spindle that facilitated the chromosomes to orient in the meridian plane. At anaphase of meiosis I, the spindle MTs differentiated into two types: one MT type became short, pulled the chromosomes toward the two poles, and was designated as centromere MTs; the second type of MT connected the two poles, and was designated as pole MTs. In A. cepa, where successive cytokinesis was observed, pole MTs assumed a tubbish shape. Some new short MTs aggregated in the meridian plane and constricted to form a phragmoplast, which developed into a cell plate, divided the cytoplasm into two parts and produced a dyad. However, in tobacco, a phragmoplast was not generated in anaphase of meiosis I and II and cytokinesis did not occur. The spindle MTs depolymerized and reorganized the radial arrangement of MTs from the nucleate surface to the periplasm during anaphase. Following telophase of meiosis II, the cytoplasm produced centripetal furrows, which met in the center of the cell and divided it into four parts, serving as a form of cytokinesis. In this process, MTs appeared to bear no relationship to cytokinesis.     

GENOME RELATIONSHIPS IN INTERSPECIFIC AND INTERGENERIC HYBRIDS OF RENANTHERA

Kamemoto , H., and K. Shindo . (U. Hawaii, Honolulu.) Genome relationships in interspecific and intergeneric hybrids of Renanthera. Amer. Jour. Bot. 49(7): 737–748. Illus. 1962.—Chromosome numbers and meiotic behavior of species and hybrids of Renanthera (Orchidaceae) were investigated. Renanthera elongata, R. histrionica, R. matutina, R. monachica, and R. storiei were diploid (2n = 38). The race of R. coccinea commonly grown in Hawaii was found to be hexaploid (2n = 114), while a recently introduced clone from Thailand was diploid (2n = 38). Diploid interspecific hybrids formed about 18 bivalents at M-I, indicating a relatively strong homology of parental chromosomes. The tetraploid hybrid of diploid R. monachica and hexaploid R. coccinea showed predominantly 37 or 38 bivalents, indicating good homology of genomes of the 2 parental species, and autosyndetic pairing in the 2 additional genomes of R. coccinea. Intergeneric hybrids involving Renanthera spp. could be classified into 2 distinct groups: those with 14–11 bivalents that are generally oriented at the metaphase plate and eventually lead to a preponderance of tetrads or tetrads with microcytes, and those with fewer bivalents (9–5) many of which are unoriented pseudobivalents and which ultimately give rise to dyads and dyads with microcytes. Relationships of parental species on a taxonomic basis are closer in the former than the latter group.     

The microtubule‐associated kinase‐like protein RUNKEL functions in somatic and syncytial cytokinesis

The microtubule (MT)‐associated putative kinase RUNKEL (RUK) is an important component of the phragmoplast machinery involved in cell plate formation in Arabidopsis somatic cytokinesis. Since loss‐of‐function ruk mutants display seedling lethality, it was previously not known whether RUK functions in mature sporophytes or during gametophyte development. In this study we utilized RUK proteins that lack the N‐terminal kinase domain to further examine biological processes related to RUK function. Truncated RUK proteins when expressed in wild‐type Arabidopsis plants cause cellularization defects not only in seedlings and adult tissues but also during male meiocyte development, resulting in abnormal pollen and reduced fertility. Ultrastructural analysis of male tetrads revealed irregular and incomplete or absent intersporal cell walls, caused by disorganized radial MT arrays. Moreover, in ruk mutants endosperm cellularization defects were also caused by disorganized radial MT arrays. Intriguingly, in seedlings expressing truncated RUK proteins, the kinesin HINKEL, which is required for the activation of a mitogen‐activated protein kinase signaling pathway regulating phragmoplast expansion, was mislocalized. Together, these observations support a common role for RUK in both phragmoplast‐based cytokinesis in somatic cells and syncytial cytokinesis in reproductive cells.