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 fe male gametophyte or embryo sac, is contained within the ovule and develops from a single cell, the megaspore which is formed by meiosis of the megaspo re mother cell. The dyad mutant of Arabidopsis, described herein, represent s 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 pro ducts of a single division of the megaspore mother cell followed by an arre st in further development of the megaspore. We addressed the question of wh ether the division of the megaspore mother cell in the mutant was meiotic o r mitotic by examining the expression of two markers that are normally expr essed in the megaspore mother cell during meiosis. Our observations indicat e that in dyad the megaspore mother cell enters but fails to complete meios is, arresting at the end of meiosis 1 in the majority of ovules, This was c orroborated by a direct observation of chromosome segregation during divisi on of the megaspore mother cell, showing that the division is a reductional and not an equational one. In a minority of dqad ovules, the megaspore mot her cell does not divide. Pollen development and male fertility in the muta nt is normal, as is the rest of the ovule that surrounds the female gametop hyte. 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 identificat ion and analysis of mutants specifically affecting female meiosis is an ini tial step in understanding the molecular mechanisms underlying early events in the pathway of female reproductive development.