Fertilisation of polar nuclei and formation of early endosperms in Dendrobium catenatum: evidence for the second fertilisation in Orchidaceae

Whether the second fertilisation, i.e. fertilisation of polar nuclei, or fusion of the second sperm with polar nuclei occurs in Orchidaceae has long been controversial because of lack of evidence. In the present study, we observed fusion and fertilisation of polar nuclei and formation of early endosperms in the orchid Dendrobium catenatum Lindl., by using a resin-embedded section technique. As the product of the second fertilisation, the primary endosperm nucleus (fertilised polar nuclei) can last until the global embryo stage, indicating that initiation of endosperm development and that of embryo development were fully asynchronous. The present study demonstrated the occurrence of the second fertilisation in D. catenatum by providing lines of new evidence.

Apomixis in flowering plants: an overview

Apomixis is a common feature of perennial plants, which occurs in ca . 60% of the British flora, but has been largely ignored by reproductive theoreticians. Successful individuals may cover huge areas, and live to great ages, favoured by ‘symmetrical’ selection. Apomixis is favoured by colonizing modes, for instance post–glacially. Despite its theoretical advantages, apomixis usually coexists with sexuality, suggesting ‘hidden’ disadvantages. Agamospermy (apomixis by seed) is relatively uncommon, but gains from the attributes of the seed. It pays agamospermy genes, which discourage recombination, to form co–adapted linkage groups, so that they become targets for disadvantageous recessive mutant accumulation. Consequently, agamospermy genes cannot succeed in diploids and agamosperms are hybrid and highly heterotic. Agamospermous endosperm may suffer from genomic imbalance, so that nutritious ovules, which can support embryos without endosperm, may be preadapted for agamospermy. When primary endosperm nucleus fertilization (‘pseudogamy’) continues as a requirement for many aposporous agamosperms, selfing sex becomes preadaptive and archesporial sex remains an option. Apomictic populations can be quite variable although apomictic families are much less variable than sexuals. Only in some diplosporous species does sex disappear completely, and in those species some release of variability may persist through somatic recombination. The search for an agamospermy gene suitable for genetic modification should target fertile sexuals with a single localized agamospermy ( A ) gene, which therefore lack a genetic load. The A gene should coexist alongside sexuality, so that it would be easy to select seedlings of sexual and asexual origins. Plants with sporophytic agamospermy provide all these attributes.

Endosperm mitotic activity and endoreduplication in maize affected by defective kernel mutations

A group of 35 defective kernel (dek) mutants in maize has been studied with regard to their effect on endosperm development. Information is reported on kernel weight, kernel viability, mutant transmission, DNA content per endosperm nucleus, endosperm cell numbers during development, and DNA endoreduplication patterns. All of the dek mutations reduced mitotic activity and resulted in greatly reduced cell numbers. All except one mutation decreased DNA endoreduplication. The exception indicates that the processes of mitotic activity and endoreduplication can be uncoupled. Notable differences in DNA endoreduplication patterns were observed among the dek strains. Defective kernels with homozygous defective embryos did not germinate in any of these strains, although some morphologically defective kernels did germinate and were shown to have normal embryos of +/+ or +/dek genotype. Dek mutants that had a defective endosperm and an embryo that developed normally were not identified. The mutations investigated are recessive, but F2 segregation for many of the mutants revealed significant deviations from expected 3:1 ratios.Key words: defective kernels, endosperm, endoreduplication.

Corrigendum - Cell Development in the Young Seed of Two Inbred Lines of Pearl Millet, Pennisetum Americanum.

Rate of cell development in embryo and endosperm during the 1st 4 days after pollination was similar in 2 lines of P. americanum under field conditions. 1st division of the endosperm nucleus was complete within 6 h after pollination. Synchronous mitoses, the mitotic cycle, divisions within the embryo and the endosperm and embryo volume increases are described.

Cell Development in the Young Seed of Two Inbred Lines of Pearl Millet, Pennisetum Americanum.

Rate of cell development in embryo and endosperm during the 1st 4 days after pollination was similar in 2 lines of P. americanum under field conditions. 1st division of the endosperm nucleus was complete within 6 h after pollination. Synchronous mitoses, the mitotic cycle, divisions within the embryo and the endosperm and embryo volume increases are described.

Capsella Embryogenesis: The Central Cell

The central cell is the binucleate cell of the angiosperm megagametophyte which contains the polar nuclei and participates in double fertilization. The structure of the mature central cell, the fusion of the polar nuclei and the primary endosperm nucleus were studied with the electron microscope. The central cell cytoplasm appears very active and has an extensive ER, many mitochondria, dictyosomes, microbodies, polysomes, chloroplasts with well developed grana and starch and lipid reserves. A single, giant mitochondrion appears in the cytoplasm near the polar nuclei at the time of fertilization, but its origin, fate and function are not known. Cytoplasmic aggregates of dense, granular material are associated with the primary endosperm nucleus and structurally resemble the nucleolus and similar aggregates in the nucleoplasm. It is suggested that these cytoplasmic perinuclear bodies may represent extruded nucleolar material. The central cell cytoplasm does not undergo any notable structural reorganization as a result of fertilization. The relationship of the central cell to the other cells of the mature megagametophyte and its possible role in embryogenesis is discussed.

EMBRYOLOGY OF INTERSPECIFIC CROSSES IN MELILOTUS

Twelve species of Melilotus were intercrossed and the embryology of the hybrids was studied. The species involved in this study are M. alba, M. officinalis, M. suaveolens, M. polonica, M. dentata, M. altissima, M. hirsutus, M. taurica, M. messanensis, M. italica, M. sulcat, and M. speciosa. Among partially compatible crosses, M. officinalis × M. alba produces the most advanced embryo. Growth of the embryo proceeds normally until about eight days, and more slowly thereafter until the 12th or 13th day, when growth is completely inhibited and the embryo aborts. The reciprocal M. alba × M. officinalis embryo does not grow as large or differentiate as much before aborting by the 11th day. Other crosses, including M. officinalis × M. suaveolens and M. alba × M. messanensis form a normal proembryo that grows slowly to about the sixth day. The proembryo then loses polarity, organ development becomes abnormal, and the ovule aborts about the 12th day. Aborted embryos are also produced in the cross, M. alba × M. dentata. Reciprocal crosses of M. suaveolens and M. altissima and M. altissima × M. polonica produce essentially normal embryos up to eight days. These crosses may be sources of economically important germ plasm. Crosses of M. altissima × M. alba and M. italica × M. altissima exhibit early embryo abortion. The suspensor becomes necrotic in four or five days and the proembryo floats into the ovule cavity, which contains abundant noncellular endosperm. In the cross M. officinalis × M. altissima, neither the zygote nor the primary endosperm nucleus divides. When M. altissima is used as the female parent, the zygote does not divide but the endosperm proliferates. In the cross, M. italica × M. officinalis, neither the zygote nor the endosperm divides. Embryos of M. italica × M. sulcata grow for four or five days, but the primary endosperm nucleus does not divide. The hybrid seed of M. alba × M. suaveolens weighs less than seed of either parent. Although developing ovules are smaller than those of M. suaveolens × M. alba, the embryo of the former is much larger and more differentiated, and endosperm is more abundant. This relationship between these two compatible species is of particular theoretical interest. Although many of the crosses do not mature viable seed, some embryos develop normally to a point where they would be worthy subjects for culture on nutrient agar.