Bisporic Embryo Sac Research Articles

Poles Apart: Monosporic, Bisporic, and Tetrasporic Embryo Sacs Revisited

Bisporic embryo sacs contain nuclei derived from two members of a megaspore tetrad whereas tetrasporic embryo sacs contain nuclei derived from all four members of a tetrad. Megaspores from the same tetrad are less genetically similar than are megaspores from different tetrads. Therefore, cells derived from different megaspores in bisporic and tetrasporic embryo sacs are expected to compete with each other rather than cooperate. The tacit assumption of the plant embryological literature that bisporic and tetrasporic embryo sacs function as integrated organisms is demonstrably false, both theoretically and empirically. This competition within embryo sacs can be expressed as the formation of eggs by descendants of more than one megaspore or by the suppression of the descendants of all but one megaspore. Both phenomena have evolved multiple times. In contrast, all nuclei of monosporic female gametophytes are genetically identical. Consequently, monosporic development is predicted to be more evolutionarily stable than bisporic or tetrasporic development. The triploid endosperm produced by most monosporic gametophytes was probably derived from an ancestrally diploid endosperm and has been considered a key adaptation of the most successful lineages of flowering plants. The greater weight given to the doubled maternal genome in triploid endosperm may have facilitated a more efficient distribution of resources among a mother’s seeds because it reduced the influence of competition for maternal resources among unrelated paternal genomes of endosperm.

Embryological and floral studies in Weddellina squamulosa Tul. (Podostemaceae, Tristichoideae)

The embryological developmental pattern of the aquatic Podostemaceae is marked by characteristic deviations from the usual pattern of angiosperms. Reduced types of embryo sac development and a nucellar plasmodium are of particular interest. The present study of Weddellina squamulosa Tul. deals with the development of the male and female gametophyte, the nucellar plasmodium, embryogenesis and seed structure. A reinvestigation of embryo sac development reveals a five-nucleate ‘Apinagia type’ (form B1 after Battaglia, 1987) of embryo sac. The embryo sac is monosporic in origin. The description of a bisporic embryo sac development (‘Podostemum’ type) in Weddellina squamulosa (Chiarugi, 1933) is refuted.The developmental pattern of the nucellar plasmodium represents a third mode not otherwise found in Podostemaceae. Its generation begins from the central row of the nucellus with the outer layer kept intact. The cells of the outer layer disintegrate later. Thus, the nucellar plasmodium seems to be established only by the outer nucellar layer.Embryogenesis conforms to those of other Podostemaceae. Microspore and pollen development show no deviations from the usual angiospermous pattern. Pollen is a monad and shed in the two-celled stage. Seed set of 100% is possibly due to autogamy during anthesis.The structure of the gynoecium in Weddellina squamulosa differs from other Podostemaceae. The bicarpellate ovary consists of four sections. The main part, above an ascidiate region, is the syncarpous zone with an axile placenta, superimposed on this is a paracarpous region that lacks a septum. The uppermost section develops an additional ‘apical septum’, a mode structurally connected with anacrostyly. This brings about a slender (compound) style (instead of stylodes as in other Podostemaceae) and a globular stigma. New data favour a re-establishment of the former subfamily Weddellinoideae.

Megasporocyte Callose in Apomictic Buffelgrass, Kentucky Bluegrass, Pennisetum squamulatum Fresen, Tripsacum L., and Weeping Lovegrass

Callose, a β‐l,3‐glucan, is synthesized and deposited in the walls of megaspore mother cells (MMCs) of angiosperms that produce either normal monosporic (>90% of all angiosperm species) or bisporic embryo sacs. Such deposits are absent in angiosperms that produce tetrasporic embryo sacs and were recently shown to be absent in diplosporous Elymus rectisetus (Nees in Lehm.) A. Löve & Connor, Tripsacum L. spp., and Poa nemoralis L. We determined the extent of callose deposits in MMCs of diplosporous weeping lovegrass (Eragrostis curvula Schrad.), several additional diplosporous Tripsacum accessions and aposporous buffelgrass (Pennisetum ciliare (L.) Link), Kentucky bluegrass (Poa pratensis L.), and Pennisetum squamulatum Fresen. Callose deposition was studied in pistils cleared in an aqueous aniline blue clearing medium and observed by means of UV microscopy. Pistils cleared with conventional clearing media were studied by means of interference contrast microscopy. Some pistils were cleared and analyzed in one clearing medium and recleared and reanalyzed in the other. A callosic MMC was observed in only one of 376 appropriately staged pistils taken from diplosporous species. This pistil, one of 89 weeping lovegrass pistils studied, was probably facultatively sexual. An absence of callose during megasporogenesis may be characteristic of most types of diplospory. Though present, the distributions and quantities of callose observed in MMC walls of aposporous apomicts were erratically reduced, possibly as a result of early MMC abortion or early activation of nucellar initials. Callose deficiencies and vacuolization patterns support the following hypotheses: (i) apospory is caused by a moderately precocious gametophytization of nucellar cells, and (ii) bispory, tetraspory, and diplospory are caused by a moderately to extremely precocious gametophytization of the MMC that is nearly complete in the latter case and variably incomplete in the former two cases. Understanding how these precocious gametophytizations evolved and are regulated may be crucial to the eventual transfer of apomixis to major crop species and to its successful manipulation by breeders.

Changes in the pattern of organization of the microtubular cytoskeleton during megasporogenesis inCymbidium sinense

InCymbidium sinense embryo sac development follows the bisporic pattern. This pattern of development is rare in orchids. Changes in the structure and organization of the microtubular cytoskeleton during megasporogenesis in this orchid were followed using an immunofluorescence technique and confocal microscopy. At the initial stage of development the microtubules in the archespore are randomly oriented throughout the cortex and cytoplasm. Later, microtubules undergo reorganization with more microtubules distributed in the cortex and around the nucleus. Megasporocyte formation is marked by the elongation of the archesporial cell. The cytoskeleton in the elongated megasporocyte is radially organized with microtubules extending from the nucleus to the peripheral region of the cell. In the megasporocyte some of the microtubules also show a polarized pattern of distribution with more microtubules at the chalazal end than the micropylar end. After meiosis I, a dyad is formed. Each dyad member possesses a complement of randomly oriented microtubules. But some microtubules in the dyad also show radial orientation. Soon afterwards microtubules in the degenerating micropylar dyad cell disappear. The functional (chalazal) dyad cell proceeds to meiosis II giving rise to a two-nucleate embryo sac. The pattern of microtubule organization in the two-nucleate embryo sac is quite complex. Each nucleus is surrounded by a set of randomly distributed microtubules. But the vacuolar region between the two nuclei is surrounded by a parallel array of cytoplasmic microtubules. No preprophase bands were observed during the development of the megasporocyte. This is the first description of microtubules in bisporic embryo sac development.

Embryology of Calenduleae (Compositae)

Microsporangial conditions, macrosporogenesis, embryo sac development and in some species endosperm formation have been studied in 5 species of Dimorphotheca Vaill. ex Mnch, 2 species of Castalis Cass., 16 species of Osteospermum L., 6 species of Calendula L., 1 species of Gibbaria Cass., and 1 species of Chrysanthemoides Tourn. ex Fabr.The microsporangial wall consists of four different layers of cells. Miocrospore formation is simultaneous. The tetrads are mostly arranged in tetrahedrals. A tapetal periplasmodium develops.The archespore of the macrosporangium is in most cases 1‐celled, sometimes 2‐or many‐celled. The meiosis gives rise to a linear tetrad. Usually the chalazal macro‐spore develops. The embryo sac development is monosporic of the Polygonum type. In a few cases in Dimorphotheca venusta T. Norl. a tetrasporic embryo sac development was observed besides the Polygonum type. Castalis tragus (Ait.) T. Norl. may have a bisporic embryo sac development in addition to the Polygonum type. All the Calendula species develop synergid haustoria. Antipode haustoria occur in Osteospermum sinuatum (DC.) T. Norl. and Chrysanthemoides monilifera (L.) T. Norl. Chalazally situated extra embryo sacs below the ordinary one in Dimorphotheca sinuata DC, D. venusta, and Castalis nudicaulis var. graminifolia (L.) T. Norl. are regarded to be aposporic. Six species have more than three antipodes per embryo sac. The antipodes occasionally become 2‐or 4‐nucleate.

Half tetrad analysis in angiosperms

A biochemical technique involving analysis of endosperm is proposed for detecting meiotic crossovers and for gene mapping in angiosperms with bisporic embryo sacs. In bisporic embryo sac development two spores resulting from meiosis-II division of the same meiosis-I daughter cell contribute two thirds of the total genetic information to the triploid endosperm nucleus, the other third coming from a sperm in the fertilizing pollen grain. In controlled crosses where the marker gene codes for allozymes with phenotypes sensitive to gene dosage, the maternal meiotic contribution to the endosperm nucleus may be determined, thereby allowing crossovers between the marker locus and the centromere to be detected.

Development of the Anther and Ovule in Capsicum L

The structure of the anther and ovule and the development of the male and female gametophytes have been studied in seven cultivated varieties of Capsicum-C. annuum L. var. acuminatum Fingerh. (long red cayenne), C. annuum L. cv. Floral Gem, C. annuum L, var. grossum (L.) Sendt. (giant bell), C. annuum var. longum (DC.) Sendt. (long sweet yellow), C. baccatum L. var. pendulum (Willd.) Eshbaugh, C. frutescens L, var. baccatum (L.) Irish and C. frutescens L, cv. Tabasco. Marked differences in the development were observed between flower buds formed in summer and in winter. In the anthers of winter flower buds of C. annuum var, acuminatum the pollen grains show nuclear multiplication and pollens with all nuclear numbers between two and eight are represented. However, 'pollen embryo sacs' as such are not formed. The normal viable pollen grains are binucleate. The pollen fertility in summer is 93.6% while in winter, on the same plants, it is only 31.3 %. In the development of the ovule there are no marked differences amongst the varieties except in size. Coexistence of mono-, bi- and tetrasporic embryo sac developments has been found in C. annuum var. acuminatum in which monospory is dominant in summer but bispory in winter. Monosporic and bisporic embryo sacs coexist in C. baccatum var. pendulum and C. frutescens var. baccatum. The other varieties show only monosporic development.

AN ADDITION TO THE BISPORIC EMBRYO SACS—THE DICRAEA TYPE

SummaryThe embryo sac in Dicraea is tetra‐nucleate and follows a bisporic development. It comprises a single synergid, an egg cell and two juxtaposed antipodal cells. This justifies the establishment of a new type under the bisporic embryo sacs which may be designated as the Dicraea type.