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.

Relationships among taxa of Elymus (Poaceae: Triticeae) in Australia: reproductive biology

Nomenclatural and taxonomic problems are common among members of the tribe Triticeae and in particular the genus Elymus L. For the Australian representatives of this genus, confusion surrounds the number of taxa present, and which literature belongs to which 'taxon'. The literature indicates at least four major taxa: (1) long-awned forms of E. scaber var. scaber equated to E. rectisetus, (2) shorter-awned forms of E. scaber var. scaber, (3) the very short-awned E. multiflorus and (4) E. scaber var. plurinervis. In addition, a form intermediate between the long- and short-awned forms of E. scaber, as well as populations containing polymorphic forms have been reported. A recent taxonomic treatment of the species complex identified a fifth taxon, E. sp. A. This taxon has previously been identified as E. scaber or E. multiflorus. The current study examined 500 pistils from megaspore mother cell to mature embryo sac, somatic and haploid chromosome numbers, and seed set of nine populations of E. scaber var. scaber sensu lato (common wheatgrass). They included forms equating to E. rectisetus, E. scaber and E. sp. A, a population with intermediate characteristics, and three polymorphic populations. Taxon (1) above is apomictic; taxon (2) contains two entities, one a facultative apomict encompassing the intermediate and polymorphic populations, and the other, the sexual E. sp. A. Exclusive sexuality was also confirmed for material of (3) and (4).

The dyad gene is required for progression through female meiosis in Arabidopsis

In higher plants the gametophyte consists of a gamete in association with a small number of haploid cells, specialized for sexual reproduction. The female gametophyte or embryo sac, is contained within the ovule and develops from a single cell, the megaspore which is formed by meiosis of the megaspore mother cell. The dyad mutant of Arabidopsis, described herein, represents a novel class among female sterile mutants in plants. dyad ovules contain two large cells in place of an embryo sac. The two cells represent the products of a single division of the megaspore mother cell followed by an arrest in further development of the megaspore. We addressed the question of whether the division of the megaspore mother cell in the mutant was meiotic or mitotic by examining the expression of two markers that are normally expressed in the megaspore mother cell during meiosis. Our observations indicate that in dyad, the megaspore mother cell enters but fails to complete meiosis, arresting at the end of meiosis 1 in the majority of ovules. This was corroborated by a direct observation of chromosome segregation during division of the megaspore mother cell, showing that the division is a reductional and not an equational one. In a minority of dyad ovules, the megaspore mother cell does not divide. Pollen development and male fertility in the mutant is normal, as is the rest of the ovule that surrounds the female gametophyte. The embryo sac is also shown to have an influence on the nucellus in wild type. The dyad mutation therefore specifically affects a function that is required in the female germ cell precursor for meiosis. The identification and analysis of mutants specifically affecting female meiosis is an initial step in understanding the molecular mechanisms underlying early events in the pathway of female reproductive development.