Megaspore Development Research Articles

CHR721, interacting with OsRPA1a, is essential for both male and female reproductive development in rice.

CHR721 functions as a chromatin remodeler and interacts with a known single-stranded binding protein, OsRPA1a, to regulate both male and female reproductive development in rice. Reproductive development and fertility are important for seed production in rice. Here, we identified a sterile rice mutant, chr721, that exhibited defects in both male and female reproductive development. Approximately 5% of the observed defects in chr721, such as asynchronous dyad division, occurred during anaphase II of meiosis. During the mitotic stage, approximately 80% of uninucleate microspores failed to develop into tricellular pollen, leading to abnormal development. In addition, defects in megaspore development were detected after functional megaspore formation. CHR721, which encodes a nuclear protein belonging to the SNF2 subfamily SMARCAL1, was identified by map-based cloning. CHR721 was expressed in various tissues, especially in spikelets. CHR721 was found to interact with replication protein A (OsRPA1a), which is involved in DNA repair. The expressions of genes involved in DNA repair and cell-cycle checkpoints were consistently upregulated in chr721. Although numerous genes involved in male and female development have been identified, the mode of participation of chromatin-remodeling factors in reproductive development is still not well understood. Our results suggest that CHR721, a novel gene cloned from rice, plays a vital role in both male and female reproductive development.

Photoperiod Extension Enhances Sexual Megaspore Formation and Triggers Metabolic Reprogramming in Facultative Apomictic Ranunculus auricomus.

Meiosis, the key step of sexual reproduction, persists in facultative apomictic plants functional to some extent. However, it still remains unclear how and why proportions of reproductive pathways vary under different environmental stress conditions. We hypothesized that oxidative stress mediates alterations of developmental pathways. In apomictic plants we expected that megasporogenesis, the stage directly after meiosis, would be more affected than later stages of seed development. To simulate moderate stress conditions we subjected clone-mates of facultative apomictic Ranunculus auricomus to 10 h photoperiods, reflecting natural conditions, and extended ones (16.5 h). Reproduction mode was screened directly after megasporogenesis (microscope) and at seed stage (flow cytometric seed screening). Targeted metabolite profiles were performed with HPLC–DAD to explore if and which metabolic reprogramming was caused by the extended photoperiod. Prolonged photoperiods resulted in increased frequencies of sexual vs. aposporous initials directly after meiosis, but did not affect frequencies of sexual vs. asexual seed formation. Changes in secondary metabolite profiles under extended photoperiods affected all classes of compounds, and c. 20% of these changes separated the two treatments. Unexpectedly, the renowned antioxidant phenylpropanoids and flavonoids added more to clone-mate variation than to treatment differentiation. Among others, chlorophyll degradation products, non-assigned phenolic compounds and more lipophilic metabolites also contributed to the dissimilarity of the metabolic profiles of plants that had been exposed to the two different photoperiods. The hypothesis of moderate light stress effects was supported by increased proportions of sexual megaspore development at the expense of aposporous initial formation. The lack of effects at the seed stage confirms the basic assumption that only meiosis and sporogenesis would be sensitive to light stress. The concomitant change of secondary metabolite profiles, as a systemic response at this early developmental stage, supports the notion that oxidative stress could have affected megasporogenesis by causing the observed metabolic reprogramming. Hypotheses of genotype-specific responses to prolonged photoperiods are rejected.

Sporogenesis and gametogenesis of Delavaya toxocarpa (Sapindaceae) and their systematic implications

AbstractIn order to investigate the embryological characteristics of Delavaya toxocarpa Franch. and provide a basis for further understanding the phylogeny within Sapindaceae s.l., we studied the sporogenesis and gametogenesis of D. toxocarpa using the conventional paraffin section method. The results were as follows: anthers are tetrasporangium; tapetum is typically secretory type; cytokinesis in the microsporecyte meiosis is of the simultaneous type and microspore tetrads are tetrahedral; mature pollen contains two cells; the ovary is bilocular with two ovules per locule; placentation is axial; the ovule is amphitropous, bitegmic, and crassinucellate; the chalazal megaspore in a linear tetrad becomes functional; and the development of megaspore is of the polygonum type. Most similarities shared by the species observed suggest that the species and genera of Sapindaceae s.l. have phylogenetic consistency. The distinctive trait, lacking hypostase, indicates Delavaya (and Handeliodendron) might be more primitive than other genera in Sapindaceae. Moreover, some characters, such as opposite palmate compound leaf, apical thyrse, rounded seed without wing, 2 hemitropous ovules per locule, and lacking aril, indicate the close relationship between Delavaya, Aceraceae, and Hippocastanaceae. The preliminary data about the embryological and morphological characteristics in Delavaya might justify the basic systematic position of this genus in the family Sapindaceae s.s.

Development of megaspores and microspores in Isoetes japonica A. Br. (Lycopodiophyta: Isoetaceae)

Microspore and megaspore development in Isoetes japonica was studied using light, scanning electron and transmission electron microscopy. Differences in cytokinesis following meiosis result in monolete microspores with bilateral symmetry, and trilete megaspores with radial symmetry. Wall development follows essentially the same sequence in both microspores and megaspores, but with the wall of the latter being very much thicker. A significant difference was observed in the tetrad stage, when microspores have an essentially rounded outline with the plasma membrane folded into a crest at the site of the future aperture. Megaspores in the tetrad stage are strongly invaginated over their entire surface, creating numerous crests, and this appears to have a role in determining the morphology of the perispore. The outline shape of the megaspore perispore having been established in the tetrad stage, most of its substance is added later and is derived from the degenerating tapetum.

Biology and life-cycle of the microsporidium Kneallhazia solenopsae Knell Allan Hazard 1977 gen. n., comb. n., from the fire ant Solenopsis invicta

Thelohania solenopsae is a unique microsporidium with a life-cycle finely tuned to parasitizing fire ant colonies. Unlike other microsporidia of social hymenopterans, T. solenopsae infects all castes and stages of the host. Four distinctive spore types are produced: diplokaryotic spores, which develop only in brood (Type 1 DK spores); octets of octospores within sporophorous vesicles, the most prominent spore type in adults but never occurring in brood; Nosema-like diplokaryotic spores (Type 2 DK spores) developing in adults; and megaspores, which occur occasionally in larvae 4, pupae, and adults of all castes but predominantly infect gonads of alates and germinate in inseminated ovaries of queens. Type 2 DK spores function in autoinfection of adipocytes. Proliferation of diplokaryotic meronts in some cells is followed by karyogamy of diplokarya counterparts and meiosis, thereby switching the diplokaryotic sequence to octospore or megaspore development. Megaspores transmit the pathogen transovarially. From the egg to larvae 4, infection is inapparent and can be detected only by PCR. Type 1 DK spore and megaspore sequences are abruptly triggered in larvae 4, the key stage in intra-colony food distribution via trophallaxis, and presumably the central player in horizontal transmission of spores. Molecular, morphological, ultrastructural and life-cycle data indicate that T. solenopsae must be assigned to a new genus. We propose a new combination, Kneallhazia solenopsae.

The Inheritance of Apomixis inPoa pratensisConfirms a Five Locus Model with Differences in Gene Expressivity and Penetrance

The genetic control of apomixis was studied in numerous segregating progenies originated from intercrossing and selfing of obligate sexual and facultative apomictic parents in Poa pratensis by means of the flow cytometric seed screen. The data support a novel model with five major genes required to control asexual seed formation: the Apospory initiator (Ait) gene, the Apospory preventer (Apv) gene, a Megaspore development (Mdv) gene, the Parthenogenesis initiator (Pit) gene, and the Parthenogenesis preventer (Ppv) gene. Differences in expressivity and interactions of these genes are responsible for the wide variation of the mode of reproduction. Apospory and parthenogenesis as well as the initiator and preventer genes of these components segregate independently. The genotypes with the highest expressivity of apospory and parthenogenesis were assigned as Ait-/apvapv/Pit-/ppvppv, those with intermediate expressivity as Ait-/Apv-/Pit-/Ppv-, and those with low expressivity as aitait/apvapv/pitpit/ppvppv. Among the self progenies of obligate sexual individuals, plants with a low capacity for apospory and/or parthenogenesis occurred, indicating that the sexual parents were heterozygous for the preventer genes and homozygous for the recessive initiator alleles (aitait/Apv-/pitpit/Ppv-). The dominant allele Ait exhibits incomplete penetrance. The degree of expressivity of apospory and parthenogenesis was constant among several harvest years of F1 plants.

Reproductive cycles of two allopatric subspecies of Juniperus oxycedrus (Cupressaceae)

Summary Reproductive cycles of Juniperus oxycedrus subsp. oxycedrus and subsp. macrocarpa were studied during three consecutive years in southwestern Spain. The two subspecies showed two-year reproductive cycles in which all the events were similar and synchronous. Pollen was dispersed in late October to early November. Seed cones began to secrete pollination drops in late October; unpollinated ovules secreted until February. Cones in which no ovule was pollinated aborted. Pollen grains germinated in early February. Fertilisation took place at the end of May or beginning of June, and seventeen months later, the embryo was completely mature. Unpollinated ovules failed to mature. Some pollinated ovules aborted in late January, probably as a consequence of a failure in megaspore development. Many others aborted at the mature gametophyte stage, just at fertilisation time. Results are dicussed in relation to deficient pollination and low pollen viability or vigour.

Floral morphology and embryo sac development in Burretiodendron kydiifolium Y. C. Hsu et R. Zhuge (Tiliaceae) and their systematic significance

Floral morphology and development of megaspore and megagametophyte in Burretiodendron kydiifolium are described. The ovule is anatropous, crassinucellate and bitegmic. The archesporium is multicellular and usually two cells develop to megasporocytes. The tetrad is linear or very rarely T-shaped. The chalazal megaspore functions and develops into a Polygonum-type of embryo sac. The antipodals degenerate very early and the fertilizable embryo sac consists of only one egg cell, two synergids and two polar nuclei. In a fertilizable embryo sac the micropyle is formed by both outer and inner integuments, which are three to four and five cells thick, respectively. The epidermal cells of the nucellus divide to form a nucellar cap. A group of special cells with large nuclei and less staining are observed below the sporogeneous cells. Comparison of the embryological features of Bumtiodendmn S.S. with those of Tiliaceae indicates that this genus differs greatly from other members of Tiliaceae and no close relationship is established among Burretiodendmn and other tiliaceous taxa. Most embryological features of Bunetiodendmn are shared by the genus Ptmspennum of Sterculiaceae although they differ considerably in gross morphology.

Floral morphology and embryo sac development inBurretiodendron kydiifoliumY. C. Hsu et R. Zhuge (Tiliaceae) and their systematic significance

Floral morphology and development of megaspore and megagametophyte inBurretiodendron kydiifoliumare described. The ovule is anatropous, crassinucellate and bitegmic. The archesporium is multicellular and usually two cells develop to megasporocytes. The tetrad is linear or very rarely T-shaped. The chalazal megaspore functions and develops into a Polygonum-type of embryo sac. The antipodals degenerate very early and the fertilizable embryo sac consists of only one egg cell, two synergids and two polar nuclei. In a fertilizable embryo sac the micropyle is formed by both outer and inner integuments, which are three to four and five cells thick, respectively. The epidermal cells of the nucellus divide to form a nucellar cap. A group of special cells with large nuclei and less staining are observed below the sporogeneous cells. Comparison of the embryological features ofBurretiodendron s.s.with those of Tiliaceae indicates that this genus differs greatly from other members of Tiliaceae and no close relationship is established amongBurretiodendronand other tiliaceous taxa. Most embryological features ofBurretiodendronare shared by the genusPterospermumof Sterculiaceae although they differ considerably in gross morphology.

Dissimilar Behaviour of the Two Kinds of Duplication-Deficiency Female Gametophytes in the T3-6 Interchange Heterozygote of Pearl Millet (Pennisetum glaucum (L.) R. Br.).

In structural heterozygotes for the T3-6 chromosomal interchange in pearl millet, the percent pollen fertility correlated with percent seed set. The duplicate-deficient (dp/df) microspores degenerated at binucleate stage. The effect of the dp/df condition on the structure and function of the female gametophyte was investigated in this interchange heterozygote. Inflorescences from the interchange heterozygote plants were fixed at five developmental stages : -at pre pollination (i.e. stage 1) and post pollination (i.e. stages 2 to 5) at 24 hr intervals up to 96 hr after pollination (hap). The embryo sac (female gametophyte) was dissected from the ovule and stained with 2% acetocarmine. In the post pollination samples the rate of early seed development was estimated by counting the number of cells in the embryo and endosperm. A normal looking embryo sac was found in all the ovules at pre pollination stage. Post pollination, two kinds of abnormalities were found in 50% of the ovules : some had unfertilized embryo sacs; some showed a lag in development of the embryo and endosperm. These abnormalities could be due to the dp/df condition of the embryo sacs-the two kinds representing the two types of dp/df products formed in the interchange heterozygote. The difference in tolerance of the dp/df condition on the male and female sides is attributed to a) the (programmed) difference in the control of gametophyte development in the two sexes, and b) the existence of competition among the (three) microspore categories (normal, dp/df of one kind and dp/df of another kind) as opposed to the non-competitive aspect of megaspore development (which ever is the category)

Megaspore wall in Lycophyta-ultrastructure and function

New contributions on megaspore walls in the genus Selaginella are presented. The study was carried out with electron microscopy (SEM and TEM), light microscopy including phase and differential interference contrast, fluorescence and confocal microscopy. The observations were based on mature material from herbarium specimens and on fresh material in different stages of development. The results refer to: 1. (1) Units: They are coiled rods. Two types have been recognized, one with a central axis and the other without an axis. 2. (2) Patterns of ultrastructure: Two patterns have been distinguished, ordered and slack, according to the type of unit with which they are built. 3. (3) Gaps: They are located within the wall between units, in places where some features change and in the middle and inner parts of the exospore. 4. (4) Exospore ultrastructure in young stages: Two layers have been recognized. Both of them are built by coiled rods. 5. (5) Connections between the tapetum and the plasma membrane of the growing megaspore: During megaspore development there are highly organized structures called wicks. They run across the exospore between units. 6. (6) Mineral deposits within the wall: Deposits of sulphur and potassium in addition to silicon have been detected by X-ray microanalysis.

Megaspore development in Selaginella. I. ?Wicks?, their presence, ultrastructure, and presumed function

Structures have been found in the locular space between the tapetal cells and megaspores in Selaginella argentea and S. kraussiana that enter the megaspore wall and extend to the plasma membrane of the megaspore cytoplasm. We have called these structures “wicks”. Unless special fixation procedures are used wicks are either very poorly preserved or not apparent. Wicks appear to be routes for the transport of materials from the tapetum to developing megaspores. The entry of the wicks into the megaspore wall and their passage throughout the wall implies that the megaspore wall of Selaginella is a three-dimensional mesh-work of inter-connecting spaces. Wicks have several macromolecular-sized subunits, and the results of our histochemical reactions indicated the presence of glycoprotein and/or mucopolysaccharide. X-ray microanalysis of the S. convoluta exospore showed that silicon is present in rod-shaped structures between units of the exospore in mature megaspores. Because of the size and form of the structures between the exospore units we consider that they are remnants of wicks stabilized by silicon.

Haploid Plant Induction from Unpollinated Ovaries in Onion

Abstract Parthenogenesis was induced in vitro in unpollinated ovaries of onion (Allium cepa L.). Sucrose concentration and stage of megaspore development affected parthenogenesis, while high- or low-temperature shocks had no stimulatory effect. Maximum embryo induction was on B5 medium supplemented with 2 mg 2,4-D/liter + 2 mg BA/liter and 10% sucrose. More than 250 plants were regenerated, of which 70% were determined cytologically to be haploid.

Megasporogenesis and Development of Embryo Sac inCrocus SativusL.

SUMMARYCytological observations during megasporogenesis and embryo sac development of Crocus sativus L. are reported. During meiotic divisions irregular assortment of chromosomes was observed; consequently, megaspores resulted numerically variable (4–6) and they were genetically unbalanced. The megaspore tetrads-hexads showed chalazal heteropolarity, however, the microphylar pole resulted more active than the chalazal one in promoting megaspore development. This was attributed to the nutritional factors liberated from degenerated micropylar spores. Independently on the number of produced megaspores the micropylar ones (M1, M2, M3), generally, degenerated, the chalazal spores (M4–M6) persisting; therefore, besides single embryo sacs, megaspores close to embryo sac or development of two embryo sacs could be observed. Frequently, the megasporogenesis or development of the persistent megaspore were not completed; consequently, ovules lacking embryo sac or showing immature embryo sac occurred. The mature embry...

Heterospory and the Angiosperms

FOR many years it has been generally assumed by botanists that in Angiosperms, as in the heterosporous Pteridophytes, the megaspores are larger than the microspores. In a study of megaspore development in Œnothera rubricalyx (Gates and Sheffield),1 it was incidentally discovered that this is not the case. Series of measurements showed that the corresponding absolute values were about 4930μ3 for the ‘mega-spore’ mother-cell and 12,630μ3 for the ‘microspore’ mother-cell at the end of meiosis. The need for comparative measurements of the size of the spores in the pollen and ovules of various plants was pointed out, and brief reference was made to the possible significance of the above condition.

Megaspore development in Oenothera rubricalyx, with a note on chromosome linkage in Oenothera angustissima

It has long keen known (Gates, 1908) that at meiosis, in the pollen mother-cells of Oenothera, the chromosomes do not of necessity become paired, but may remain joined together in chains. Of recent years it has transpired that the amount of pairing and chromosome linkage is constant within each species and each mutant. Morphologically different hybrids resulting from a single cross may differ in the amount of linkage exhibited, but within each hybrid type the linkage is constant. Reciprocal hybrids may differ from one another morphologically and cytologically, but each form may breed true if a sufficient number of chromosomes are linked; for, once determined, the amount of linkage in any form is unvarying. In certain other genera of plants various linkages of chromosomes have been found, but Oenothera is at present unique in the degree and constancy of linkage observed. The view has therefore been expressed (Gates, 1928) that the small-dowered species, nearly all of which show complete chromosome linkage, breed true on account of this linkage and are really permanent hybrids, the formation of a ring of 14 chromosomes resulting from the bringing together of non-homologous chromosomes in the fertilization nucleus. Before any very definite conclusions could be drawn from these results it was necessary to find out whether the linkage, which is evident in the nucleus of the pollen mother-cells, also prevails in the nucleus of the embryo-sac mother-cell at reduction division. From an early paper of Davis on Oe. biennis (1911) it was obvious that some suck linkage does occur on the female side. The present work was undertaken primarily to determine whether similar linkages occur in the two types of mother-cells of a single plant. Before the work was completed Håkansson (1928) showed that the process of reduction is similar on the male and female side in Oe. Lamarckiana and several of its derivatives.

The Cytological Aspects of The Determination of Sex in the Diœcious Forms of Lychnis

ABSTRACT The present research is part of a more general inquiry into the conditions of sex determination in the genera Lychnis and Silene. This paper consists of an account of the cytology of the diœcious forms of Lychnis with special reference to sex chromosomes. Lychnis Flos-cuculi is referred to at times for the sake of comparison. The somatic number of chromosomes in L. dioica, (agg.), as well as in L. Flos-cuculi L. F los-Jouis, and Silene péndula, is twenty-four. The hybrid L. alba × L. dioica behaves in a perfectly regular manner and is practically indistinguishable cytologically from its parents. In Lychnis dioica (agg.) the pollen mother cells show a particularly close synizetic knot from which the spireme never opens out fully. Just before the second contraction the spireme becomes differentiated into thicker and thinner portions of which the thicker clearly represent the chromosomes. The thread becomes much shorter and stouter and, as the contraction figure, opens out, the chromosomes separate off in pairs forming typical bivalents lying either crossed or parallel. The reduction is thus typically telosynaptic and there is no trace of a split thread at any stage. As the bivalents contract, one pair is seen to be larger than the rest and to consist of two unequal portions, both of which are larger than the other chromosomes. At diakinesis, the bivalents become completely dissociated, once more giving twenty-four separate chromosomes. L. Flos-cuculi differs in haying ring-shaped chromosomes, both at diakinesis and on the equator of the heterotype spindle. In L. dioica (agg.), the large pair of chromosomes lies at the periphery of the metaphase plate and is seen to consist of one very large hooked member and one smaller, more pear-shaped, chromosome. Anaphase figures show the larger chromosome in the form of a cross, as seen from above, suggesting a double structure. There is no difficulty in distinguishing which member of the unequal pair is present in any given daughter plate. In the interkinesis there are two points worthy of note, (1) Several nucleoli are present, in place of the usual single one typical of all other stages. (2) The twelve chromosomes separate completely into halves, thus giving twenty-four bodies lying within the membrane, just as in diakinesis. At the ensuing metaphase the X and Y chromosomes are again most distinct, especially when the two spindles lie parallel. Megaspore development is briefly described. Meiosis is similar to that found in the male plant, except in relation to the large chromosomes. The large chromosomes are equal in size, and by careful comparison of size and shape, are shown to correspond to the small one of the unequal pair in the male. The Y chromosome is thus larger than the X, quite contrary to expectations based on the condition in animals. It is also probably double in nature though it does not separate into two parts, as far as has been observed. A brief review of the literature shows that in Rumex there is a pair of Y chromosomes ; in Humulus and Vallisneria a double structure is presumed to be the X element, but on the basis of Lychnis may possibly turn out to be the Y instead. The cytological results above are shown to corroborate the results of the experimental work of Shull and Correns on the same forms.

THE INTENSITY OF LINKAGE BETWEEN THE FACTORS FOR SUGARY ENDOSPERM AND FOR TUNICATE EARS AND THE RELATIVE FREQUENCY OF THEIR CROSSING OVER IN MICROSPORE AND MEGASPORE DEVELOPMENT

THE INTENSITY OF LINKAGE BETWEEN THE FACTORS FOR SUGARY ENDOSPERM AND FOR TUNICATE EARS AND THE RELATIVE FREQUENCY OF THEIR CROSSING OVER IN MICROSPORE AND MEGASPORE DEVELOPMENT

THE RELATIVE FREQUENCY OF CROSSING OVER IN MICROSPORE AND IN MEGASPORE DEVELOPMENT IN MAIZE

THE RELATIVE FREQUENCY OF CROSSING OVER IN MICROSPORE AND IN MEGASPORE DEVELOPMENT IN MAIZE