Desarrollo del gametofito femenino de Tagetes patula (Asteraceae)

The genus Tagetes reproduces sexually by seed, but recent morphological and hybridization studies in Tagetes patula suggest an apomictic type of reproduction (seed development without fertilization). In order to determine the sexual or apomictic origen of the embryo, we have studied megasporogenesis, megagametogenesis and the early development of the embryo. Tagetes patula L. has a typical ovule for the family Asteraceae: anatropous, unitegmic, tenuinucellate and with basal placentation. A single hypodermal archesporial cell develops directly as the megaspore mother cell. Megaspogenesis is normal and embryo sac develops from the chalazal megaspore. The embryo sac is of the Polygonum type. Female ray flowers show irregularities in megagametophyte development such as formation of more than eight nuclei, inverted polarity and incomplete differentiation of the megagametophyte cells in mature flowers. These irregularities do not necessarily prove apomictic reproduction in Tagetes patula.

Ovule and megagametophyte development in selected species of Apeibeae and Grewieae (Malvaceae–Grewioideae) from South America and its systematic implications

This is the first embryological report of the Grewioideae subfamily, which is meant to contribute to the characterisation of the genera Corchorus, Luehea and Triumfetta. Ovule and female gametophyte development in C. argutus, L. divaricata and T. semitriloba was analysed. The ovules of all species are anatropous, bitegmic and of crassinucellate mixed type. The micropyle of the studied species is formed by the outer integument (exostome). The ovule of L. divaricata differs from those of the other two species because the chalazal tissue expands forming a cap, which gives rise to a wing in the seed. All species present one hypostase. The megaspore mother cell gives rise to a linear megaspore triad in C. argutus and L. divaricata, whereas in T. semitriloba, triads and diads can be observed in the same ovule. The chalazal megaspore develops a seven-celled and eight-nucleate female gametophyte corresponding to the Polygonum type. The synergids of L. divaricata have hooks and a conspicuous filiform apparatus. The antipodal cells in C. argutus are persistent, whereas in the other species, they are small and ephemeral. The embryological characters are compared with those of other taxa within the family and the megagametophyte formation in these species is discussed.

Megaspore competition in F1 and F2 hybrids between Oenothera hookeri and Oe. suaveolens

Megasporogenesis and development of the embryo sac were investigated in F₁ and F₂ hybrids from crosses of <em>Oe. hookeri</em> and <em>Oe. suaveolens</em>. All hybrids form heteropolar and homopolar magaspore tetrads; the embryo sac, however, usually develops from the micropylar megaspore. Its development may occur immediately after degeneration of three other megaspores or after a period of competition between both apical megaspores. They develop simultaneously for a relatively short time, after which the growth of the chalazal megaspore is inhibited, although the latter does not degenerate. The micropylar megaspore as a rule develops without disturbances into the embryo sac, but in some ovules it is formed from the chalazal megaspore or double ones arise from both apical megaspores of the tetrad. The frequency of the micropylar embryo sac formation seems to be dependent above all on the hybrid plant genome and not on the haploid genome of the megaspore.

Embryology of Macroptilium arenarium (Leguminosae)

Macroptilium arenarium (Bacigalupo) S.I.Drewes & R.A.Palacios produces two floral morphs, aerial chasmogamous flowers and cleistogamous flowers in geophyte racemes. A comparative study of the sporogenesis, gametogenesis and the development of the related sporophytic structures in both floral morphs is reported. The anther is tetrasporangiate, its wall consists of epidermis, endothecium, one or two middle layers and an uninucleate secretory tapetum. The mature endothecium presents fibrilar thickenings that are more developed in cleistogamous flowers. Pollen grains are tricolporate, angulaperturate, and are shed at bicellular stage. The ovule is crassinucelate, bitegmic and anacampylotropous. Megaspore tetrads with linear arrangement have been observed in chasmogamous flowers, whereas only megaspore dyads have been found in cleistogamous flowers. In both floral morphs the chalazal megaspore develops into an embryo sac of Polygonum type. Apomixis is considered as a possible replacement for sexual reproduction in cleistogamous flowers.

Embryo sac development in soybean: ultrastructure of megasporogenesis and early megagametogenesis

The soybean ovule is bitegmic with the megasporocyte three to four cell layers beneath the nucellar epidermis. The megasporocyte is much larger than the surrounding nucellar cells, is connected to the nucellus by plasmodesmata, and at this stage exhibits a cytoplasmic density comparable with cells of the nucellus. After meiosis, the chalazal megaspore becomes functional in megagametogenesis. It alone retains plasmodesmatal connections to the nucellus. Chalazal megaspore expansion is accompanied by development of many small vacuoles having a uniform distribution. The first megaspore mitosis results in two nuclei lying on an axis parallel to the longitudinal axis of the embryo sac. Ultimately, these two nuclei are separated by a large vacuole. Numerous Golgi vesicles and proteinlike bodies are observed along the periphery of vacuoles in the 1-, 2-, and 4-nucleate embryo sacs. As the contents of vesicles and proteinlike bodies are observed deposited in vacuoles, it is probable that they both add osmotica to the vacuoles, thus promoting a water flux. We believe that the production of Golgi vesicles and putative protein bodies may be important in the formation and expansion of the large vacuole that appears to drive embryo sac expansion during early megagametogenesis in soybean. It is also believed that the timing to this vacuole's development has important developmental consequences.

Embryo sac development in some South African Lantana species (Verbenaceae)

Evidence that the South African Lantana camara L. complex only produces sexual embryo sacs is provided. It is shown that the archesporium occasionally divides mitotically and that both archesporia form tetrads. The chalazal megaspore of one tetrad and the micropylar megaspore of the second tetrad develop into Polygonum type embryo sacs. L. rugosa Thunb. also forms Polygonum type embryo sacs. The L. rugosa embryo sac has a much more densely packed cytoplasm, smaller vacuole and the position of the polar nuclei differs from that of the L. camara embryo sac. It is possible to distinguish between L. camara and L. rugosa on their embryo sac morphology alone.

The development of the ovule and megagametophyte in Rhexia mariana

The ovules of Rhexia mariana are bitegmic and anatropous or rarely atropous. Periclinal divisions in the nucellar epidermis at the micropylar end produces a nucellar cap. A terminal pore in the nucellus is formed by the suppression of anticlinal divisions in the epidermis and subsequent separation of the cells during nucellar enlargement. A single hypodermal archesporium divides, producing a primary parietal cell and primary sporogenous cell. The primary parietal cell establishes a prominent parietal tissue as the primary sporogenous cell differentiates into the megasporocyte. Two prominent nucleoli that consistently appear in the archesporium persist in the primary sporogenous cell and the megasporocyte. Meiosis produces a tetrad of megaspores in either linear or approximately T-shaped arrangement. Cells of the nucellus adjacent to the sporogenous cell, megasporocyte, and tetrad rarely give rise to megagametophytes aposporically. The chalazal megaspore functions in megagametogenesis. Shortly after the first nuclear division, vacuoles migrate and coalesce between the nuclei. Two more nuclear divisions establish the four- and eight-nucleate megagametophytes. The chalazal nuclei are situated laterally and are noticeably smaller than the micropylar nuclei in both four- and eight-nucleate stages. The two polar nuclei remain in close contact, usually near the egg apparatus, but sometimes near the center of the megagametophyte. The three antipodal nuclei are ephemeral with degeneration completed by the time the egg apparatus is established. Orientation of the egg apparatus places the synergids on the side of the megagametophyte adjacent to the funiculus and the egg cell on the opposite side. The similarities and differences in ovule and megagametophyte development between Rhexia mariana and the tropical representatives of Melastomataceae previously investigated are discussed.

Sporogenesis, Gametogenesis, and embryogeny of Wahlenbergia bicolor N. Lothian

In Wahlenbergia bicolor, the anther wall is composed of four layers: epidermis, endothecium, middle layer, and tapetum. Wall formation and microsporogenesis are described, and the pollen grains are shed at the two-celled condition. The ovules are tenuinucellate, with a hypodermal archesporial cell which develops directly as the megaspore mother cell. Megasporogenesis is normal, and a monosporic eight-nucleate embryo sac of the most common Polygonum type develops from the chalazal megaspore. The antipodals degenerate before fertilization. The development of the embryo is of the solanad type. A suspected case of polyembryony was observed. The endosperm is cellular from its inception, and so conforms to the Codonopsis type. A micropylar and a chalazal haustoriurn, both consisting of two uninucleate cells, are formed from the endosperm. Comparative studies were made with a known but as yet undescribed coastal species of Wahlenbergia, and no differences were found.

Embryological studies in the compositae, I. Sporogenesis, Gametogenesis, and Embryogeny in Cotula australis (Less.) Hook. F

Cotula australis has a discoid heterogamous capitulum in which the outermost three whorls of florets are female and naked. The bisexual disk florets are fully fertile and have a four-lobed corolla with four shortly epipetalous stamens. The anthers contain only two microsporangia. Wall formation and microsporogenesis are described and the pollen grains are shed at the three-celled condition. The ovule is teguinucellate and the hypodermal archesporial cell develops directly as the megaspore mother cell. Megasporogenesis is normal and the monosporio embryo sac develops from the chalazal megaspore. Breakdown of the nucellar epidermis takes place when the embryo sac is binucleate and its subsequent development follows the Polygonum type. The synergids extend deeply into the micropyle and one persists until late in embryogeny as a haustorium. The development of the embryo is of the Asterad type, and the endosperm is cellular. C. coronopifolia agrees with C. australis in the presence of only two microsporangia in each anther and the development of a synergid haustorium.