A megaspore is a type of spore in certain plants that gives rise to a female gametophyte, the structure that contains the female gamete, or egg. It is the larger of two distinct spore types produced by these organisms. The primary function of the megaspore is to initiate the process that leads to the formation of a seed after fertilization.
The Formation of a Megaspore
The creation of a megaspore, a process called megasporogenesis, begins within a plant’s ovule. Inside a specialized tissue called the nucellus, a single large, diploid cell known as the megaspore mother cell (MMC) is differentiated. This cell is destined to undergo a specific type of cell division.
The diploid MMC undergoes meiosis, a reductional division that halves the chromosome number, resulting in four haploid cells. These four megaspores are typically arranged in a straight line, referred to as a linear tetrad. In the most common pattern of development, three of the four megaspores in the tetrad will disintegrate, leaving only one functional megaspore and ensuring that developmental resources are channeled into a single, viable cell.
Development into the Female Gametophyte
Following its formation, the single functional megaspore develops into the female gametophyte. This process, known as megagametogenesis, involves a series of mitotic divisions without the formation of new cell walls initially. The megaspore begins to enlarge, drawing nutrients from the surrounding nucellus tissue.
In a common pattern among flowering plants, the nucleus of the functional megaspore divides by mitosis three consecutive times. This produces a single large cell containing eight haploid nuclei. These nuclei then arrange themselves into a specific configuration, and cell walls form, resulting in a seven-celled structure known as the embryo sac. This mature female gametophyte contains the egg cell, which is the cell that will be fertilized by the male gamete.
Megaspores and Heterospory
The production of megaspores is a defining characteristic of a reproductive strategy called heterospory. Plants that are heterosporous produce two distinct types of spores that differ in size and function. The megaspore is the larger of the two and develops into the female gametophyte.
In contrast, these plants also produce a much smaller spore, the microspore. The microspore develops into the male gametophyte, which produces the sperm cells contained within a pollen grain. This system of producing two different kinds of spores is a feature of all seed-producing plants, including gymnosperms, angiosperms, some water ferns, and clubmosses.