Megaspore Mother Cell Research Articles

Pollen Tube Pathway and Stimulation of Embryo Sac Development inPistacia vera(Anacardiaceae)

Abstract Pistil anatomy of thePistacia veracultivars Kerman and Sirora was studied in relation to the pathway of pollen tube growth using fluorescence, bright field and scanning electron microscopy. The pistil comprised a three-lobed stigma, a short style and a unilocular ovary containing a single ovule with a large funicle. The ovule was crassinucellate and anatropous, with the mid-point of the funicle directly below the base of the style. The transmitting tissue comprised the stigma, the style, the funicle and the ovary wall. The upper style had three transmitting tracts, which in the lower style joined into a single core of tissue, half of which ended at a protuberance of the ovary wall. The ovary wall protuberance lay opposite a protuberance of the funicle which comprised loose papillate cells connecting with the single core of transmitting tissue in the distal funicle. The other half of the stylar transmitting tract entered the ovary wall, lining up with the suture join of the two smaller stigma lobes. At the base of the ovary the transmitting tissue divided into three tracts which entered the funicle. The three tracts of transmitting tissue in the lower part formed one large tract in the upper funicle. Most pollen tubes grew towards the embryo sac via the stigma and the style, and penetrated the funicle via the ovarian and funicular protuberances and the ovarian cavity at the base of the style. They grew through the distal funicular transmitting tissue and penetrated the nucellus via the chalaza. A minority of pollen tubes grew via the ovary wall transmitting tissue before penetrating the proximal funicular transmitting tissue. Some tubes grew in the space between the inner integument and nucellus, and some stopped growing in the nucellus or integument. Successful tubes entered the embryo sac via the egg apparatus. At anthesis unpollinated embryo sacs were immature and comprised a megaspore mother cell, a tetrad or a one cell embryo sac. Following anthesis unpollinated embryo sacs developed slowly and a maximum of 50% reached maturity by 4 d after anthesis. In contrast all embryo sacs of pollinated pistils were mature by 1 or 2 d after pollination. In the cultivar Kerman approx. 25% of embryo sacs were penetrated and fertilized at the second day after pollination compared to 43% fertilization of Sirora at 3 d. Up to 60% of pistils had no pollen tube, and up to 50% of unpollinated and 29% of pollinated pistils had no embryo sac.

Cytological and Cytoembryological Studies on Paeonia tenuifolia L.

The cytology and cytoembryology of Paeonia tenuifolia L. (Paeoniaceae) at Ortakçi Village near Edirne during the years 1989-1991 are considered in this study. Chromosome counts were done on adventive roots, and the results were found to be 2n=10. In the karyotype, 3 median (M), 1 submedian (SM) and 1 subterminal (ST) chromosome pairs were found, in two of which SAT-chromosomes were detected. The anther wall of P. tenuifolia consists of an epidermis , a 1-3 layered endothecium, 1-2 middle layers and a 1-2 layered tapetum. In the secretory tapetum, normal mitosis and the ÒstickyÓ type of division were observed. In most microspore mother cells, the course of meiosis is regular. In some cells irregularities were observed, including chromosome bridges, univalent and fragments. Microspore tetrads are isobilateral and tetrahedral, polen grains are tricalporate and are shed at the 2 celled stage. In rare cases, polyspory and multinucleate pollen grains were observed. The course of meiosis in the megaspore mother cell is regular. The ovules are anatrapous, bitegmic and crassinucleate. The female archesporium is multicelled. The female gametophyte formed by the functioning chalazal megaspore is monosporic, of the Polygonum type, in the mature embryo sac, the egg apparatus, the normal appearence of the secondary nucleus and of the antipodal cells. The occurence of double and triple embryo sacs due to the further development of chalazal megaspores of double and triple tetrads was observed.

AtDMC1, the Arabidopsis homologue of the yeast DMC1 gene: characterization, transposon‐induced allelic variation and meiosis‐associated expression

Based on homologies between the yeast DMC1 and the lily LIM15 meiosis-specific genes, degenerate PCR primers were designed that amplified the Arabidopsis DMC1 gene (AtDMC1). AtDMC1 genomic DNA (8 kb) was sequenced, and the transcript was characterized by reverse transcriptase-polymerase chain reaction (RT-PCR) and by 5' and 3' RACE (rapid amplification of cDNA ends). The AtDMC1 gene contains 15 exons and 14 introns. RNA in situ hybridization analysis showed that expression of the AtDMC1 is restricted to pollen mother cells in anthers and to megaspore mother cells in ovules. The AtDMC1 promoter was fused to the GUS reporter gene, and conferred meiosis-associated expression in both male and female floral lineages. Comparison of AtDMC1 isolated from Landsberg erecta ecotype to its Columbia allele ArLIM15, revealed the presence of a 1874 bp transposon-like element within the promoter region of ArLIM15. RT-PCR analysis showed that the expression levels of AtDMC1 and ArLIM15 are similar. Possible uses for the AtDMC1 promoter are discussed.

Biosystematic studies on the genus Isoetes (Isoetaceae) in Japan. II. Meiotic behavior and REPRODUCTIVE MODE OF EACH CYTOTYPE

Meiotic behaviors and reproductive modes of Japanese Isoetes were studied. The hexaploid (2n = 66) and the octaploid (2n = 88) of I. japonica consistently formed 33 and 44 bivalents, respectively, at diakinesis and/or metaphase I in both micro‐ and megaspore mother cells. The tetraploid (2n = 44) of I. sinensis formed 22 bivalents and its hexaploid made 33 bivalents in both types of spore mother cells. At diakinesis and/or metaphase I of microspore mother cells in I. asiatica with 2n = 22, 11 bivalents were detected. Because behaviors of meiosis in all cytotypes mentioned above were quite regular and plants yielded normal‐appearing spores, they should reproduce sexually. Aneuploids of I. japonica with 2n = 87 formed 43 bivalents and one univalent, and I. sinensis with 2n = 65 formed 32 bivalents and one univalent in microspore mother cells. Meiosis of both cytotypes was almost regular and yielded microspores of normal appearance. In the heptaploid (2n = 77) of I. japonica, a configuration of 22 bivalents and 33 univalents was detected in micro‐ and megaspore mother cells, and various irregularities were observed throughout the meiotic divisions. Therefore, the genomic formula of the heptaploid is symbolized as AABBCDE, and the heptaploid is a sterile F, hybrid between the hexaploid (AABBCC) and the octaploid (AABBDDEE) of I. japonica. Since diploid and even‐numbered polyploids regularly formed bivalents and odd‐numbered ones displayed irregularities, allopolyploidy should act as a significant speciation mechanism in this genus.

Studies on the development and three-dimensional reconstruction of giant mitochondria and their nuclei in egg cells ofPelargonium zonale Ait.

The preferential development of giant mitochondria and their nuclei (nucleoids) in the egg cells ofPelargonium zonale Ait. during megasporogenesis and megagametogenesis was examined by fluorescence microscopy, after Technovit embedding and 4′,6-diamidino-2-phenylindole (DAPI) staining, fluorimetry for DNA content, using a video-intensified microscope photon-counting system (VIMPICS), and by three-dimensional reconstruction of mitochondrial nuclei (mt-nuclei). Reproductive cells during the megaspore mother cell, meiosis, tetrad, and functioning megaspore stages contained many small mitochondria with characteristic, uniformly DAPI-stained mt-nuclei about 0.3 μm in diameter, containing a small amount of DNA (0.3 Mbp). During formation of the 2-, 4-, and 8-nucleate embryo sac, mt-nuclei did not markedly change in shape or DNA content. When the embryo sac formed and differentiation of each cell began, mitochondria and their nuclei in the egg cell took on a small ring or string-like shape. Accompanying the maturation of the embryo sac, they underwent progressive enlargement and gradually altered to long thick strings, or stacks of concentric or half concentric rings. By flower opening, they have developed to an extremely large size. One of these stacks of mt-nuclei was reconstructed in three dimensions; each ring in the stack was cup- or plate-shaped; 5 to 10 rings made up the stack, though each remained discontinuous from the others. From serial sections, we counted 44 mitochondria in one egg cell. Fluorometry using VIMPICS revealed that DNA amount within the stacked mitochondrion increased to 40 times that of the megaspore mother cell stage; a single stack of mitochondria contained 340–1700 Mbp DNA; which means that one egg cell contains at least 15000 Mbp mt-DNA, a value greater than the cell-nuclear DNA content.

Callose localization in cell walls during meiotic and apomeiotic megasporogenesis in diploid alfalfa (Medicagospp.)

SUMMARYThe localization of callose during meiotic and apomeiotic megasporogenesis in diploid species of the genus Medicago was investigated by fluorescence microscopy. Callose deposition and cytoembryological details were studied by treating flowers with a clearing/staining medium. Ovules were classified into different developmental stages and the distance of growing integuments from the micropyle measured. The relationship between the stage of development and the growth of integuments proved to be an effective method for correlating megasporogenesis events with callose deposition and assessing the cytological mechanism of apomeiosis. Within meiotic ovules callose deposition appeared as thin micropylar caps in the megaspore mother cells and as thick cell plates between dyad, triad and tetrad cells. The degradation of callose started at the beginning of embryo sac differentiation. Ovules with diplosporic cells were characterized by complete absence of callose. When callose was present at the megaspore moth...

Floral Biology and Embryo Development in Chestnut (Castanea sativa Mill.)

Floral biology of chestnut, from sporogenesis to mature embryo, is described. Microsporogenesis in flowers of unisexual catkins occurred in the first week of June 1991. Anthesis started in mid-June (≈70 days after budbreak) and lasted 2 weeks. In mid-June, in each pistillate flower, six to eight styles began to emerge, and 4 to 7 days later, they were extended fully (i.e., full bloom). In each flower, 10 to 16 anatropous ovules developed from the ovary axis. The megaspore mother cell had formed by the end of bloom. The mature ovule consisted of two integuments and a long, narrow nucellus with a small embryo sac of the Polygonum type. Zygotes were found 15 to 20 days after pistillate flower full bloom. Embryo development followed the Onagrad type, Trifolium variation. Seeds attained full size in mid-September, and fruit were mature in early November. The embryonal axis averaged 4.5 mm long × 2.1 mm wide. An apical meristem and the radicle were evident at opposite ends of the axis.

Localization of poly(A)+-containing RNA during female gametophyte development in Medicago sativa and the diploid mutant Medicago sativa ssp. falcata using digoxigenin-labelled oligo-dT probes

The detection and distribution pattern of poly(A)+-containing RNA during embryo sac development in the wild type and in a diplosporic mutant of alfalfa was studied using digoxigenin-labelled oligo-dT probes. In the very early stage of development, the megaspore mother cell does not show any poly(A)+RNA accumulation. Subsequently, poly(A) + RNA appears concomitantly in the megaspore mother cell and the nucellus tissue, both in wild and mutant alfalfa. The distribution pattern of poly(A) + RNA reveals some differences. In the mutant, the absence of label in the chalazal part of the nucellus at the megaspore mother cell stage seems to be significant. Compared to the wild type, the hybridization signal at the functional megaspore as well as at the two-nucleate coenocytic stage was reduced. However, the late stages of embryo sac development in the two types were almost comparable, except that the central cell of the mutant accumulated more label. The significance of the results in relation to reduced and unreduced embryo sac development is discussed.

Mise en évidence de callose autour des cellules nucellaires à l'origine des embryons adventifs du Citrange troyer [Citrus sinensis(L.) Osbeck xPoncirus trifoliata(L.) Raf.

Summary Callose has been characterized in the cell walls of initial cells of nucellar embryoids that develop in situ in a polyembryonic hybrid of Citrus (Citrus sinensis x Poncirus trifoliata). Callose is laid after differentiation of the megaspore mother cell, which is also surrounded by a thick callosic wall, and before flower opening. In a monoembryonic species (Citrus limon) the megaspore mother cell is the only nucellar cell surrounded by callose. This could emphasize the role of callose in isolation of cells that engage in a new morphogenetic path. This s (1–3) glucan can be used as a precocious marker of nucellar embryogenesis in situ and of somatic embryogenesis in vitro.

Embryo Sac Morpho-structural Analysis of Developmental Pathways of Fertilization Failure, Seed Abortion and Poor Germination in Cassava (Manihot esculenta Crantz).

Embryo sac developmental examination in four cultivated Asian cassava clones: ‘Rayong 3’ (MMex 57×Mven 307), ‘Rayong 60’ (MCol 1684×Rayong 1), ‘Rayong 90’ (CMC 76×V 43, Sarakarn 1993), and ‘CMR 25-105-112’ (27-77-10×Rayong 3, Kawano et al. 1990) was conducted to elucidate the reproductive and conventional breeding barriers (fertilization failure, seed abortion and poor germination) in the genus Manihot. Late pre- and post-pollination embryology revealed absence of functional megaspore mother cell and megagametophytes respectively. Malformed megagametophytes were characterized with reduction in embryo sac volume (8-12.5% of normal), devoid of nucleate cells especially, polar and the antipodal cells, massively vacuolated with no recognizable zygotes or embryos. Regular embryo sacs commenced degeneration 8 days after anthesis in unfertilized ovules of all zero pollinated pistils. Degeneration signal initiated from its thin cell wall and transducted radially inwards and outwards to the central cell and the surrounding nucellus respectively. Both integuments (inner and outer) as well as the nucellus of functional (possessing nucleate cells) and non-functional (deprived of nucleate cells) megagametophytes appeared healthy and developed normally. Physical seed analysis showed that phenotypically normal but functionally deprived ‘pseudo-seed’ (without endosperm and embryo) and ‘true seed’ are produced at a ratio of approximately 1:2. Two distinct but complementary developmental pathways of fertilization failure, seed abortion as well as poor germination in cassava have been identified. They are: 1. Ovules devoid of functional megaspore due to lack of megasporocyte during megasporogenesis and 2. Embryo sacs devoid of nucleate cells due to mitotic inactivity at megagametogenesis resulting in structurally and functionally deprived megagametophyte but morphologically normal ovules. These pathways, either separately or collectively are implicated as largely responsible for the reproductive and conventional breeding barriers in the genus Manihot.

439 PB 218 CHROMOSOMAL VARIATION AMONG PROGENY OF FACULTATIVE APOMICTIC GUAYULE

Progress toward the domestication of guayule (Parrhenium argentatum Gray) as a domestic source of latex, has been slowed because of its complicated reproductive biology. This work was performed to help elucidate the facultative nature of apomictic reproduction in guayule. Ploidy level and six morphological characters were measured in one-hundred and eighty open-pollinated, half-sib progeny derived from twelve different apomictic tetraploid (4n=4x=72) female parents. The number of chromosomes observed among the half-sib progeny varied from 36 to 81, with the majority (77.3%) of the progeny with 72 chromosomes. All of the observed chromosome numbers could be explained either by meiotic reduction in the megaspore mother cells or fertilization of egg cells or both. The frequencies of meiotic reduction and fertilization resulting in these progeny were estimated to be 27.2 and 22.7%, respectively. Among the six measured morphological characters, significant correlations were found only between chromosome numbers and plant height (r=0.37) and leaf area (r=0.31).

An Embryological Study of Kava, Piper methysticum

An embryological study of Piper methysticum Forster f. (Piperaceae), undertaken to identify the probable cause for the absence of fruits in cultivated varieties, has revealed that the majority of plants in all the cultivars examined bore only male flowers. The anthers are tetrasporangiate and the anther wall comprises a well-developed endothecium with fibrous thickenings, two middle layers and a glandular tapetum. Meiosis is normal and simultaneous cytokinesis leads to tetrahedral microspore tetrads. Pollen degeneration was observed but was of very rare occurrence. The pollen grains are released when they are 2-celled and appear to be healthy and capable of germination. The unilocular ovary contains a single basal ovule which is orthotropous, bitegmic, and crassinucellar. Both integuments contribute to the formation of the micropyle. The development of the female gametophyte conforms to the tetrasporic Fritillaria-type. No abnormalities were noticed either in meiosis of the megaspore mother cell or in subsequent development leading to the formation of the mature embryo sac. Occasional parthenocarpy was observed but without viable seeds being present inside. It is suggested that, although the lack of fruit formation in this species is not because of any deficiencies in its embryological development, it may be due to a simple self-incompatibility mechanism.

A contribution to the embryology of Desmodium motorium (Houtt.) Merr. (= D. gyrans (L.f.) DC.) (Fabaceae)

Abstract The anthers are tetrasporangiate. The anther wall comprises epidermis, fibrous endothecium, middle layer and tapetal layer. The tapetum is of the Glandular type and its cells remain uninucleate. Meiosis in pollen mother cells is normal and simultaneous cytokinesis leads to the formation of tetrahedral and decussate microspore tetrads. The pollen grains are shed at 2-celled stage. The ovule is campylotropous, bitegmic and crassinucellate. Meiosis in megaspore mother cell results in the formation of linear or occasionally T-shaped megaspore tetrad. The chalazal megaspore develops into Monosporic Polygonum type of embryo sac. Endosperm development is of the Nuclear type.

Quantitative analysis of aposporous parthenogenesis inPoa pratensisgenotypes

SUMMARY Several embryological and cytological techniques were developed to determine the degree of aposporous parthenogenesis vs. sexuality in different genotypes of the facultative apomictic grass Poa pratensis L. Fixed embryo sacs were mechanically isolated and individually examined with phase-contrast and interference-contrast light microscopes. The degree of apospory was calculated as the ratio of embryo sacs with an embryo but without endosperm and the total number of embryo sacs studied per plant. The degree of apospory varied between 9 and 76% in nine plants. Photocytometry and flow cytometry were used to determine ploidy levels in embryo and endosperm of individual embryo sacs in order to determine their origin, asposporous vs. sexual. Results based on the calculated degree of apospory agreed with the degree of apomixis deduced from homogeneity scores in field progeny tests. Callose deposition on the megaspore mother cell during megasporogenesis was studied in the same set of genotypes and the percentage of abnormal, micropylar callose deposition closely agreed with results both of the embryo sac analysis and the progeny tests. Combined use of these techniques yields quantitative data on the aposporous tendency of individual genotypes of Poa pratensis.

Enzymatic isolation of viable nucelli at the megaspore mother cell stage and in developing embryo sacs in Nicotiana tabacum

Nucelli and developing embryo sacs were enzymatically isolated from ovules of Nicotiana tabacum. Megaspore mother cells, tetrads, uninucleate, binucleate, four-nucleate, eight-nucleate and mature embryo sacs were obtained. The isolated embryo sacs were intact and living, and maintained their original shape and organization. Cytoplasmic streaming was clearly observed. Prolonged incubation of ovules or reincubation of isolated embryo sacs in the maceration mixture resulted in the liberation of the gametophytic cells as individual, living protoplasts.

Isolation and characterisation of somatic hybrids of diploid Solanum tuberosum and Solanum brevidens and the use of amylose-free starch mutation for detection of introgression

Somatic hybrids of diploid amylose-free (amf) Solanum tuberosum and diploid S. brevidens were made by Poly-Ethylene-Glycol (PEG) or electrofusion methods. For the isolation of interspecific hybrids the use of selection markers (kanamycin and hygromycin resistance) was useful but not essential. In this 2x+2x interspecific combination 4x and 6x somatic hybrids were obtained. Seed set was the best in 4x×4x (S. tuberosum) backcrosses, but seed germination was the best in 6x×4x combinations, using in vitro germination of unripe seeds harvested 25 days after pollination. A high degree of pollen stainability (30–40%) was observed in 7 tetraploid hybrids and very low in all hexaploids. After iodine staining, the recessive amf marker was expressed by a red colour instead of blue, visible in starch-containing cells like columella cells of root tips, (micro)tubers or microspores. As expected, complementation was observed in starch-containing cells of the fusion hybrids. Segregation of the amf marker was clearly observed in microspores of 4x and 6x hybrids. Segregation ratios in the 4x hybrids showed variable recombination frequencies. In the backcross progeny of hexaploid F12-5 with a tetraploid amf mutant one amylose-free recombinant among 67 plants was found, indicating the occurrence of meiotic recombination in the megaspore mother cells.

Effects of several meiotic mutations on female meiosis in maize

AbstractA modified enzyme digestion technique of ovary isolation followed by staining and squash preparation has allowed us to observe female meiosis in normal maize meiotically dividing megaspore mother cells (MMCs). The first meiotic division in megasporogenesis of maize is not distinguishable from that in mi‐crosporogenesis. The second female meiotic division is characterized as follows: (1) the two products of the first meiotic division do not simultaneously enter into the second meiotic division; as a rule, the chalazal‐most cell enters division earlier than the micropylar one, (2) often the second of the two products does not proceed with meiosis, but degenerates, and (3) only a single haploid meiotic product of the tetrad remains alive, and this cell proceeds with three rounds of mitoses without any intervening cell wall formation to produce the eight‐nucleate embryo sac. This technique has allowed us to study the effects of five meiotic mutations (aml, aml‐pral, afdl, dsy *‐9101, and dvl) on female meiosis in maize. The effects of the two alleles of the aml gene (aml and aml‐pral) and of the afdl and dsy *‐9101mutations are the same in both male and female meiosis. The aml allele prevents the entrance of MMCs into meiosis and meiosis is replaced by mitosis; the aml‐pral permits MMCs to enter into meiosis, but their progress is stopped at early prophase I stages. The afdl gene is responsible for substitution of the first meiotic (reductional) division by an equational division including the segregation of sister chromatid centromeres at anaphase I. The dsy * ‐9101 gene exhibits abnormal chromosome pairing; paired homologous chromosomes are visible at pachytene, but only univalents are observed at diakinesis and metaphase I stages. These mutation specific patterns of abnormal meiosis are responsible for the bisexual sterility of these meiotic mutants.The abnormal divergent shape of the spindle apparatus and the resulting abnormal segregation of homologous chromosomes observed in micro‐sporogenesis in plants homozygous for the dv1 mutation have not been found in meiosis of megasporogenesis. Only male sterility is induced by the dv1 gene in the homozygous condition. © 1993 Wiley‐Liss, Inc.

Embryology ofMalaxis saprophyta, with comments on the systematic position ofMalaxis (Orchidaceae)

InMalaxis saprophyta, anther wall development corresponds to the Monocotyledonous type. The uninucleate tapetum is of secretory type and the endothecium develops U- and V-shaped thickenings on the inner tangential and radial walls. Cytokinesis is simultaneous; tetrahedral, isobilateral and T-shaped tetrads are formed which are compactly aggregated in pollinia. At anthesis the microspore tetrads are 2-celled. The ovule is anatropous, bitegmic and both integuments are dermal in origin. A single hypodermal cell develops directly into a megaspore mother cell. Embryo sac development is predominantly monosporic and less often bisporic. Irrespective of the type of development, the mature embryo sac is 6-nucleate. Although double fertilization occurs, the primary endosperm nucleus degenerates. Embryogeny is of the Onagrad type. The mature embryo lacks differentiation into cotyledon, plumule and radicle. The reticulate seed coat is formed entirely by the outer layer of outer integument. There are three sterile and three fertile valves in the ovary. Although initially parenchymatous, the entire three sterile valves in the ovary and the upper half of the three fertile valves become sclerified after fertilization. The embryological characters support the disputed systematic position ofMalaxis within subtribeMalaxidinae ofEpidendreae.

Anther and Ovule Development in Tasmannia (Winteraceae)

Three species of Tasmannia R.Br. ex DC., T. glaucifolia, T. insipida and T. stipitata are studied. The anther is tetrasporangiate and its waU development conforms to the Basic type. The tapetum follows the secretory type of development. Cytokinesis in the microspore mother cells is simultaneous but an evanescent cell plate is present at telophase I and anaphase I1 during meiosis. Pollen tetrads are permanent and tetrahedral. The mature pollen is anaulcerate, reticulate and 2-celled. The ovule. is anatropous, bitegmic and crassinucellate. The micropyle in T. stipitata and T. Glaucifolia is formed by the inner integument only whereas in T. insipida it is formed by both the integuments and is zigzag in outline. Meiosis in the single megaspore mother cell produces a linear or T-shaped megaspore tetrad in T. stipitata and T. glaucifolia but only a linear tetrad in T. insipida. Female gametophyte development is of the monosporic Polygonum type. Fertilisation is porogamous; triple fusion and syngamy occur simultaneously.

Comparative Histology of Cell Walls during Meiotic and Apomeiotic Megasporogenesis in Two Hexaploid Australasian Elymus Species

Diplospory in Elymus rectisetus (Nees in Lehm.) Löve & Connor has been compared with megasporogenesis in the close sexual relative Elymus scabrus (R. Br.) Löve. Distinct differences in cell wall morphology were reported. The present study considered the cause of these cell wall differences by comparing cell wall behavior and composition during megasporogenesis in two E. rectisetus and two E. scabrus accessions (Australian wheatgrasses). Micropylar walls of meiotic megaspore mother cells (MMCs) during prophase and metaphase I (MI) were significantly thicker than those of apomeiotic MMCs in corresponding stages. Thickness of chalazal walls during these early stages did not differ. Micropylar, chalazal, and lateral walls of meiotic dyads and tetrads were significantly thicker than those of apomeiotic late stage MMCs, dyads, or binucleate monads. The walls of nucellar cells in E. rectisetus were generally thinner than those in E. scabrus, particularly during the latter stages of megasporogenesis. Callose, which normally envelops MMCs of angiosperms that form monosporic or bisporic female gametophytes, accumulated in and around cell walls of E. scabrus MMCs and was subsequently degraded as in other sexual species. Absence of callose in and around cell walls of MMCs of E. rectisetus appeared to be the major reason for the differences in MMC shape and cell wall thickness. In contrast, callose was detected at abnormal locations in both the nucellus and ovary wall of E. rectisetus. Meiosis with the normal accumulation of callose in and around the cell walls of the MMC and its dyad and tetrad derivatives was infrequently observed in E. rectisetus. Thus, callose accumulation appears to inherently accompany meiosis in these taxa.