Are Angiosperms Heterosporous? A Look at Their Spores

Angiosperms, or flowering plants, represent the vast majority of plant life on Earth. They are found in nearly every habitat and include everything from tiny herbs to massive trees. Their defining characteristic is the flower, which serves as the reproductive organ for the plant. This diverse group of plants raises a fundamental question about their reproductive strategy: are they heterosporous?

Homospory Versus Heterospory in Plants

Plant life cycles involve an alternation of generations, cycling between a diploid sporophyte stage and a haploid gametophyte stage. Spores are the reproductive cells that grow into the gametophyte. Plant groups can be categorized based on the types of spores they produce.

Homospory is a condition where a plant produces only one type of spore. This single spore develops into a gametophyte that can be bisexual or produce separate male and female gametophytes that are morphologically similar. Many ferns and mosses exhibit homospory.

In contrast, heterospory is the production of two different types of spores: smaller microspores and larger megaspores. This separation leads to the development of separate male and female gametophytes. All seed-producing plants, including gymnosperms and angiosperms, are heterosporous.

The Heterosporous Nature of Angiosperms

Angiosperms are heterosporous, producing two different kinds of spores within their flowers.

Microspores are the smaller spores that give rise to the male gametophyte. They are produced in vast quantities within structures called microsporangia, located in the anthers of the flower’s stamens. Each anther typically contains four microsporangia, where diploid cells undergo meiosis to produce haploid microspores.

Megaspores are the larger spores that develop into the female gametophyte. These are produced in the ovules, which are contained within the ovary of a flower’s carpel. Inside each ovule is a megasporangium, where a single diploid cell divides through meiosis to form four haploid megaspores. Typically, only one of these four megaspores survives.

Development from Angiosperm Spores

The development of angiosperm spores leads to the formation of the male and female gametophytes. The microspore undergoes mitotic divisions to develop into a pollen grain, the mature male gametophyte. It contains a tube cell and a generative cell, which will later divide to form the two sperm cells required for fertilization.

The single surviving megaspore develops into the female gametophyte, which is known as the embryo sac. The megaspore divides by mitosis multiple times, typically resulting in a seven-celled structure with eight nuclei. This mature embryo sac contains the egg cell, two helper cells called synergids, three antipodal cells, and a central cell with two polar nuclei.

Evolutionary Advantages of Heterospory in Angiosperms

The evolution of heterospory in angiosperms provided several advantages. A major benefit is the protection and nourishment of the female gametophyte. By developing within the ovule, the embryo sac is shielded from environmental stressors and receives a steady supply of nutrients from the parent sporophyte.

This reproductive strategy is a direct precursor to the evolution of the seed. The retention of the megaspore and the subsequent development of the female gametophyte within the parent plant tissue set the stage for seed formation. After fertilization, the ovule develops into a seed, which contains the embryo and a food supply, all enclosed in a protective coat.

Heterospory also promotes genetic diversity. The separation of male and female gametophytes encourages outcrossing, the fertilization between different individual plants. This system also allows for a differential allocation of resources, enabling the plant to produce numerous small, mobile male gametophytes (pollen) and a few large, stationary, and well-provisioned female gametophytes.