Seasonal variation in the glucosinolate content of North American Brassica nigra and Dentaria species

Concentrations of glucosinolates in the leaves of the woodland cruciferous herbs Dentaria laciniata, D. diphylla and D. maxima declined during the 1974 growing season but generally equalled or exceeded the levels found in the foliage of Brassica nigra, a crucifer of neighbouring open habitats. The superior quality of Dentaria foliage for certain crucifer-feeding insects does not seem to result from an unusually low concentration of glucosinolates. The qualitative and quantitative compositions of glucosinolates in the leaves and rhizomes of D. maxima were found to be intermediate between the corresponding values for D. laciniata and D. diphylla. This finding is consistent with a hybrid origin for D. maxima.     

APOMIXIS IN THE POA SECUNDA COMPLEX

Observation of cleared ovules of Poa secunda Presl showed that plants produce aposporous embryo sacs, and emasculation studies showed that they do not set seed without pollen. The ratio of sexual to asexual embryo sacs on a given plant does not correlate with percent stainable pollen, nor does pollen stainability correlate with meiotic regularity or with chromosome number. Percent apomictic ovules, percent pollen stainability, and regularity of meiosis all vary within a given population; they also vary if the same plant is moved from one site to another. Although there is morphological variation in the species, none of the reproductive features correlates with morphology. Pollen from any morph will stimulate seed set in any other morph, and hybrids have been produced in low frequency. Plants will also set seed using their own pollen. In its reproductive biology, P. secunda exhibits many similarities with P. pratensis L.     

POLLEN REMOVAL AND POLLEN DEPOSITION AFFECT THE DURATION OF THE STAMINATE AND PISTILLATE PHASES IN CAMPANULA RAPUNCULOIDES

We examined factors affecting the duration of the staminate and pistillate phases in the protandrous flowers of Campanula rapunculoides L. (Campanulaceae). Under conditions of natural pollinator visitation, flowers experiencing low rates of pollen removal lasted significantly longer than flowers that had faster rates of pollen removal. Experimental manipulations showed that low levels of pollen removal resulted in extension of the staminate phase. Hand-pollinations in which we varied the amount and source of pollen showed that when the number of fertilized ovules within an ovary is low, senescence of the flower is delayed, resulting in extension of the pistillate phase. We also report on pollinator foraging patterns within the vertical inflorescences of C. rapunculoides and the limiting factor for seed set in this population. The results are relevent to recent suggestions that floral characters often serve to reduce interference between the sexual functions in cosexual plants.     

THE BREEDING SYSTEM OF OXALIS SUKSDORFII

The Pacific Coast species Oxalis suksdorfii (section Corniculatae) is morphologically tristylous. Flowers of the 3 kinds of plants present in natural populations differ in style length, stamen length, pollen size, stigma size and orientation, and stylar pubescence. Experimental studies and field observations indicate that plants with long-styled flowers are weakly self-compatible and show little differentiation in size or compatibility relationships of pollen from the 2 sets of stamens. Plants with mid-styled flowers have strongly differentiated pollen, but produce few capsules or seed after any artificial or natural pollinations. Plants with short-styled flowers are self-incompatible and show slight pollen differentiation. The data presented suggest that O. suksdorfii approaches functional dimorphism since the mid-styled form contributes little pollen and few seeds in sexual reproduction. The species may be in the process of losing the mid-styled form completely and provides a model of certain physiological and morphological changes which may have accompanied elimination of the mid-styled form in other Oxalis species. The abundance of plants with mid-styled flowers in natural populations suggests, however, that this form may be retained because it contributes to general population fitness, or because the genetic control of heterostyly in this species prevents its elimination.     

THE GLYCOCALYX AND INITIATION OF EXINE SPINULES ON MICROSPORES OF CANNA

Spinules of Carina generalis pollen are initiated within a tridimensional network during the microspore tetrad period. The network is stained selectively with the hydrazide-silver proteinate method of Thiéry following periodate oxidation and by phosphotungstic acid at low pH, demonstrating the presence of polyanions. Protein is indicated as a component of the network by positive staining with PTA in acetone. These results suggest the presence of polysaccharides and proteins, possibly as mucopolysaccharides or glycoproteins. The network may be considered as a part of the glycocalyx because of its composition and association with the plasma membrane. Sporopollenin accumulates on the tridimensional network or in meshes of the net. The beaded fine structure of spinules resists the acetolysis mixture of Erdtman. Our results imply that the plasma membrane and its glycocalyx are part of the system which mediates genetic expression of exine form. The implication is compatible with formation of specific exines on all pollen grains of a plant and on aborted microspores, diminutive spores with depauperate chromosome complements, and enucleate bodies of cytoplasm resulting from meiotic abnormalities.     

STEWARTIOTHECA GEN. N. AND THE NATURE AND ORIGIN OF COMPLEX PERMINERALIZED MEDULLOSAN POLLEN ORGANS

Stewartiotheca gen. n. is a bell-shaped, unisynangiate pollen organ with eccentric radial symmetry and a single series of about 80 pollen sacs. Infoldings that vary in depth occur circumferentially and extend from the periphery to a point off center. This position also marks the location of a sclerenchyma column (proximally) and a sclerenchyma-lined, conical hollow (distally) that opens onto the distal face of the organ. Plates of ground parenchyma extend inward from the outer covering of the organ at locations of infoldings, while similar plates with sclerenchyma strands occur between these locations. Pre-pollen of Monoletes type was released through distal longitudinal slitlike openings of the pollen sac faces toward the sclerenchymatous ground tissue plates. Vascular bundles entering the organ undergo repeated dichotomies, and lead to numerous bundles both in the cover (one per sac for those sacs that abut directly on cover tissue) and internally (one per pair of pollen sacs that lie opposite one another across the location of an infolding). The most complex permineralized medullosan pollen organs Sullitheca, Stewartiotheca, and Dolerotheca are considered to have evolved from a similar type of cup-shaped organ with a single ring of pollen sacs, broadly open distally, and with a central hollow. Circumferential infoldings of one organ of this type were involved in the origin of both Stewartiotheca and Sullitheca, while four similar organs, each showing infoldings non-circumferentially, fused to produce the Dolerotheca type organ (exemplified by D. formosa), a compound synangium.     

NATURAL HYBRIDIZATION BETWEEN THE SYMPATRIC HAWAIIAN SPECIES DROSOPHILA SILVESTRIS AND DROSOPHILA HETERONEURA

Two newly formed, morphologically distinct species of Drosophila from the island of Hawaii have been found to form fertile hybrids in two areas of sympatry. Both F₁ and backcross hybrids have been recognized in nature; in one case, the hybridization events extended over three years. Original hybridizations involved one or more D. silvestris females mating with D. heteroneura males. Female F₁ hybrids from this cross have participated in backcrosses to D. silvestris. In any one locality, less than 2% hybrids have been found in nature. A hybrid swarm was not formed; selection appears to favor a strict maintenence of morphologies characteristic of the separate species. This result is attributed to pervasive sexual selection, which serves to preserve the syndromes of sexual characteristics that arose during past allopatric divergence. Populations of D. silvestris both within and outside the present range of D. heteroneura often display heritable variation in color patterns involving the abdomen, pleurae, legs, and wings. Genes effecting variation in these characters may be derived from genes involved in a past introgression from D. heteroneura. Independent evidence for past hybridization between these species comes from study of mitochondrial DNA. Although the inferred direction of the cross is the opposite of that observed in the recent case described here, both reciprocal crosses have been obtained experimentally in the laboratory. Accordingly, we suggest that these species may have been open to hybridization since their first sympatic encounters following their inception in allopatry. That they remain as strictly recognizable morphological entities is due both to their current partial allopatry and to the action of sexual selection in maintaining two separate major modes of efficient reproduction. There is no reason to invoke specific reinforcing selection that has imposed reproductive isolation.     

REPRODUCTION AND SYSTEMATICS OF THE MARINE ALGA DERBESIA (CHLOROPHYCEAE) IN NEW ENGLAND1,2

The morphology, gross cytology, reproduction, and habitat ecology are described for Derbesia marina based on observations of New England field populations and laboratory cultures of this plant. These data, and additional observations on cultures of several other species of Derbesia from elsewhere and on type and other important collections, have been used to evaluate the systematic relationship of New England Derbesia with other species of this genus. The single Derbesia species in New England is referred to D. marina. The systematic criteria previously used to distinguish species within Derbesia are reviewed and interpreted within the context of the present investigation. In systematic studies, we stress the importance of the use of sporangial and chloroplast morphology, the presence or absence of pyrenoids, and reproductive history. Two types of life history are reported for D. marina in New England: (1) A form of D. marina collected at 20 m reproduces directly with stephanokontous zoospores growing into sporophytic plants of Derbesia morphologically identical to their parent thalli. (2) On the other hand, at least some New England, populations of D. marina have retained the genetic potential for producing a sexual generation (Halicystis ovalis), even though the latter is unknown for the coast of northeastern North America.³ Gametophytes (H. ovalis) were produced directly from enlargement and subsequent differentiation of uncleaved lateral sporangia in 2 cultured populations of New England D. marina. A single female and numerous male vesicles formed in this manner produced gametes, but neither fertilization nor parthenogenesis occurred; thus the entire life history was not completed in culture. The occurrence of the directly reproducing deep water form of D. marina is presented as evidence for speciation of a sporophyte (Derbesia) independent of its alternate gametophyte (Halicystis). An hypothesis is advanced to explain the source and means for expression of genetic variability necessary for speciation in a population of nonsexually reproducing Derbesia.     

SEXUAL REPRODUCTION AND MEIOSIS IN PERIDINIUM INCONSPICUUM LEMMERMANN (DINOPHYCEAE)

In Peridinium inconspicuum Lemmermann, sexual reproduction occurs in both nitrogen-enriched and nitrogen-deficient media. In this homothallic strain, protoplasmic fusion begins between two thecate gametes; but zygote formation is completed in a space outside the fusing pair. This diploid cell can form a plated theca which is shed as the cell enlarges. This spherical zygote then forms a new non-plated theca. The process of ecdysis and the formation of a new non-plated theca is repeated several times. During this process the zygote gradually elongates and by cytoplasmic infurrowing becomes peanut-shaped. Eventually two cells are formed. The first and second meiotic divisions are greatly separated in time. The first meiotic division occurs in the spherical non-thecate zygote. The second meiotic division can occur in the peanut-shaped zygote before it completes cytokinesis. This meiotic division may not be synchronous, occasionally resulting in a trinucleate stage. Eventually four flagellated, haploid products are produced.     

A NOMARSKI INTERFERENCE STUDY OF MEGASPOROGENESIS AND MEGAGAMETOGENESIS IN SMELOWSKIA CALYCINA (CRUCIFERAE)

Ovule development, megasporogenesis and megagametogenesis in an aneuploid population of the arctic-alpine crucifer, Smelowskia calycina, were examined to test for the possibility of apomictic seed production. Whole mounts of ovules cleared in Herr's “4½” clearing fluid were examined using Nomarski differential interference microscopy. The campylotropous ovule was bitegmic, with a micropyle formed by both integuments. The single archesporial cell of a crassinucellar nucellus functioned directly as a megasporocyte, dividing to form a linear tetrad of megaspores. The chalazal megaspore divided to form an 8-nucleate, 7-celled gametophyte of the Polygonum type, having hooked synergids with a well-developed filiform apparatus and polar nuclei that fused prior to fertilization. In the absence of any anomalous development indicative of agamospermy, seed production was assumed to be sexual.     

MORPHOLOGICAL AND EMBRYOLOGICAL STUDIES ON THE LEMNACEAE. II. THE ENDOSPERM AND EMBRYO OF LEMNA PAUCICOSTATA

Maheshwari, Satish C., and R. N. Kapil. (U. Delhi, Delhi, India.) Morphological and embryological studies on the Lemnaceae. II. The endosperm and embryo of Lemna paucicostata. Amer. Jour. Bot. 50(9): 907–914. Illus. 1963.—The first division of the primary endosperm nucleus is followed by wall formation. The second division is also transverse so that a longitudinal row of 4 cells is formed. The next 2 divisions are vertical and result in a 16-celled endosperm arranged in 4 tiers of 4 cells each. The development is, therefore, Cellular (even in L. minor) from the beginning and not Helobial, as reported earlier. The embryogeny conforms to the Asterad type. The radicle is absent in the mature embryo. Comparative studies of the structure of the endosperm and embryo furnish strong evidence in favor of a relationship of the Lemnaceae with the Araceae rather than with the Helobiales.     

ENVIRONMENTAL EFFECTS ON THE PERCENTAGE OF STAINABLE AND PRESUMED NORMAL POLLEN IN ASTER (COMPOSITAE)

Pollen stainability is often equated with pollen viability or fertility and is used by botanists to evaluate degree of hybridity among individuals of a plant population. Under certain conditions, however, the effects of meiotic irregularities resulting from hybridization may be masked by those of the environment. In the genus Aster high percentages of pollen stainability were obtained in most samples from plants flowering in the greenhouse, including hybrids of distantly related species. When grown outdoors, plants of the same clones frequently suffered significant reduction in the amounts of normal pollen produced. During the period of flower development, two climatic parameters were assessed, relative humidity and maximum/minimum temperatures. Fluctuations in relative humidity were considerable but of nearly equal amplitude both in the greenhouse and in the open field. Diurnal temperature fluctuations, however, were much less drastic in the greenhouse than outdoors. The differential percentages of stainable pollen observed are probably attributable to the sum of environmental factors, including hard rains and strong winds, rather than any single parameter. In the genus Aster high amounts of aborted pollen grains appear to be largely a reflection of sensitivity to unfavorable microclimatic conditions during the post-meiotic period of flower development, and data on pollen stainability cannot be employed as a measure of fertility or degree of hybridity.     

FINE STRUCTURE OF SYNAPSED CHOMOSOMES IN F1 LYCOPERSICON ESCULENTUM-SOLANUM LYCOPERSICOIDES AND ITS PARENTS

Typical synaptinemal complexes consisting of electron-dense central and lateral elements and much less dense outer fibrillar material are formed at meiotic prophase in Lycopersicon esculentum (tomato), Solanum lycopersicoides, their diploid hybrid, and occasionally in tomato haploids (in which an average of about one chromosome region per cell is synapsed nonhomologously). Complexes in the hybrid (in which the chromosomes synapse completely but often fail to form chiasmata) are similar to those in the parents. Complexes in the haploid are similar to those of diploid tomato. The data suggest that synaptinemal complexes form whenever chromosomes undergo meiotic synapsis, regardless of whether synapsis leads to chiasmata.     

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.     

FERTILIZATION IN PLUMBAGO ZEYLANICA: GAMETIC FUSION AND FATE OF THE MALE CYTOPLASM

Developmental phases surrounding the processes of gametic delivery and fusion were examined ultrastructurally in the reduced megagametophyte of Plumbago zeylanica, which lacks synergids. Gametic delivery occurs at the end of pollen tube growth and results in deposition of two male gametes, a vegetative nucleus, and a limited amount of pollen cytoplasm between the egg and central cell. Discharge of these materials from the tube is accompanied by loss of inner and outer pollen tube plasma membranes, loss of sperm-associated cell wall components, and disruption of the formerly continuous cell wall between the egg and central cell. The dispersion of egg cell wall components directly exposes female reproductive cell membranes to the unfused male gametes and pollen tube without disrupting gametic cell plasma membranes. Presence of unfused sperms within the female gametophyte appears to be a transitory phenomenon, lasting less than 5 min at the end of over 8½ hr of pollen tube growth. At the time of gametic deposition, plasma membranes of unfused sperm cells become directly appressed to plasma membranes of both the egg and central cell. Gametic fusion is initiated by a single fusion event between membranes of participating male and female cells, which is rapidly followed by subsequent, secondary fusion events between the same two cells at different locations along their surface. Gametic fusion results in the transmission of male gamete nuclei with co-transmission of nearly the entire sperm cytoplasmic volume and organellar complement, and it is possible to identify heritable male cytoplasmic organelles within both the incipient zygote and endosperm. Paternally originating plastids may be distinguished from maternal plastids by differences in morphology and staining characteristics, whereas paternal mitochondria may be distinguished from maternal mitochondria by populational differences in mitochondrial size which are statistically significant. Such observations further indicate that transmitted paternal mitochondria seem to remain viable, as judged by their ultrastructural appearance, and are transmitted exclusively by sperm cytoplasm rather than discharged pollen cytoplasm. The presence of anucleate, membrane-bounded cytoplasmic bodies between the egg and central cell are identifiable on the basis of their enclosed organelles and indicate that fragmentation of a small amount of the sperm cytoplasm associated with the vegetative nucleus commonly occurs. The presence and identification of sperm cytoplasmic organelles and associated membranes within female reproductive cells following gametic transmission represents strong evidence in support of the cellular basis of nuclear and cytoplasmic transmission during sexual reproduction in Plumbago.     

FOUNDER EFFECTS AND THE RATE OF MITOCHONDRIAL DNA EVOLUTION IN HAWAIIAN DROSOPHILA

The nearly-neutral-mutation theory predicts that populations with small effective sizes will undergo more rapid molecular evolution than populations with very large effective sizes. In particular, Ohta (1976) predicted that populations of Hawaiian Drosophila that are known to have small population sizes and to experience repeated population bottlenecks due to founder events should show a more rapid rate of molecular evolution relative to the rate of molecular evolution of species with large population sizes such as the continental Drosophila. In this paper we test this prediction by comparing the rate of molecular evolution in two closely related lineages of Hawaiian Drosophila that have experienced very different evolutionary histories. Both lineages belong to the planitibia subgroup of Hawaiian Drosophila. The beta lineage (which includes D. silvestris, D. planitibia, D. differens, and D. hemipeza) has undergone repeated founder events, as evidenced by their geographic distribution and behavioral biology. On the other hand, evidence on geographic distribution and behavior indicates that the alpha lineage (which includes D. melanocephala, D. cyrtaloma, and D. neoperkinsi) has arisen from large ancestral populations without founder effects. The mitochondrial DNA data reveal that, within a lineage, the rate of molecular evolution is rather uniform, while all comparisons between the two lineages show that the rate of molecular evolution in the beta lineage is three times that of the alpha lineage. This analysis strongly supports the predictions made by Ohta.     

The embryology ofStegnospermataceae,with a discussion on its status,affinities and systematic position

The embryology ofStegnosperma halimifolium andS. watsonii has been studied in detail. The tapetum is of the secretory type and its cells become multinucleate. Simultaneous cytokinesis in the pollen mother cells follows meiosis. The ripe pollen grains are 3-celled. The ovule is crassinucellate, bitegmic and amphitropous, with the micropyle formed by the inner integument alone. The female archesporium is one celled, and the parietal tissue 3–5 layered. The embryo sac development conforms to thePolygonum type. A central strand, 6 or 7 cells thick, differentiates inside the nucellus and extends from the base of the embryo sac to the chalazal region. The endosperm is nuclear. The embryogeny conforms to the Caryophyllad type. The seed coat is formed by the outer epidermis of the outer integument and the inner epidermis of the inner integument. Based on this evidence and other data, the status of the genus as an independent family,Stegnospermataceae (Stegnospermaceae) is confirmed. Apparently, it forms a connecting link betweenPhytolaccaceae andCaryophyllaceae.     

EVOLUTION OF ASEXUALITY IN THE COSMOPOLITAN MARINE CLAM LASAEA

The marine clam genus Lasaea is unique among marine bivalves in that it contains both sexual and asexual lineages. We employed molecular tools to infer intrageneric relationships of geographically restricted sexual versus cosmopolitan asexual forms. Polymerase chain reaction primers were used to amplify and sequence homologous 624 nucleotide fragments of COIII from polyploid, asexual, direct-developing individuals representing northeastern Pacific, northeastern Atlantic, Mediterranean, southern Indian Ocean, and Australian populations. DNA sequences also were obtained from the two known diploid congeners, the Australian sexual, indirect developer, Lasaea australis, and an undescribed meiotic Australian direct developer. Estimated tree topologies did not support monophyly for polyploid asexual Lasaea lineages. A robust dichotomy was evident in all phylogenetic trees and each of the two main branches included one of the diploid meitoic Australian congeners. Lasaea australis clustered with two of the direct-developing, polyploid asexual haplotypes, one from Australia, the other from the northeastern Atlantic. Monophyly is supported for the diploid Australian direct-developing lineage together with the remaining polyploid asexual lineages from the northeastern Pacific, northeastern Atlantic, Mediterranean, and southern Indian Ocean. These results indicate that asexual Lasaea lineages are polyphyletic and may have resulted from multiple hybridization events. The high degree of genetic divergence of asexual lineages from co-clustering meiotic congeners (16%–22%) and among geographically restricted monophyletic clones (9%–11%) suggests that asexual Lasaea lineages may be exceptionally long lived.     

Developmental and cytogenetic analyses of pollen sterility in Valeriana scandens L.

Valeriana scandens presents perfect and pistillate flowers, the latter with sterile anthers. The species is composed of two varieties with different ploidy; V. scandens var. scandens (2n = 28) and V. scandens var. candolleana (2n = 56), both of which occur in RS, Brazil. Crosses between these varieties may give rise to hybrids with pollen sterility. In this study, we analyzed the microsporogenesis and microgametogenesis of sterile and fertile anthers, and also investigate whether pollen sterility is caused by an irregular meiotic process. Developmental analysis using light microscopy and scanning electron microscopy showed that sterile anthers develop similarly to fertile anthers until the end of meiosis. After this stage, sterile tetrads do not separate as a consequence of exine fusion between adjacent microspores, which is similar to sterile pollen of Brassica ms-cdl1 mutants. In addition, vacuolated immature pollen grains degenerate after separation. The cytogenetic analysis of the microspore mother cell (MMC) showed that the diploid population of V. scandens var. scandens (2n = 28) has pollen sterility that is not caused by a cytogenetic disturbance. The MMCs analyzed from prophase I to tetrad stage showed a regular meiotic process, indicating the phenotype of V. scandens sterile pollen is a postmeiotic process formed by fusion of exine between opposite microspores.