Heterosporous Pteridophytes

Heterospory is a plant phenomenon where a single plant produces two different types of spores—microspores and megaspores. These spores vary in size, structure, and function, with microspores being smaller and megaspores larger. Heterospory evolved during the Devonian period from isospory independently in several plant groups: the club mosses, the ferns including the arborescent horsetails, and progymnosperms. This occurred as part of the process of evolution of the timing of sex differentiation. Four extant groups of plants are heterosporous; Selaginella, Isoetes, Marsilea, Salvinia and seed plants.

1.0Introduction of Heterospory

• Plants producing two different types of spores are termed heterosporous whereas a more  common condition is homosporous where a plant generates only one type of spore. In contrast to heterosporous plants, the majority of pteridophytes are homosporous, producing a single type of spore with similar characteristics, a condition known as homospory. 
• In heterosporous plants, specialized structures called sporangia differentiate into microsporangia and megasporangia. Microsporangia contain numerous microspores, while megasporangia stores a smaller number of megaspores. 
• The evolution of producing two types of spores with distinct sexual characteristics originated in pteridophytes.

2.0Origin of Heterospory

• Emergence of heterosporous plants started with the separation of sporangia, which allowed for the development of two different types of spores; numerous small spores that are easily dispersed, and fewer, larger spores that contain adequate resources to support the developing seedling. During the Devonian period there were many species that utilized vertical growth to capture more sunlight. Heterospory and separate sporangia probably evolved in response to competition for light. Disruptive selection within species resulted in there being two separate sexes of gamete or even the whole plant. This may first have led to an increase in spore size and ultimately resulted in the species producing larger megaspores as well as smaller microspores.
• Heterospory is advantageous in that having two different types of spores increases the likelihood that plants would successfully produce offspring. Heterosporous spores can respond independently to selection by ecological conditions in order to strengthen male and female reproductive function. Heterospory is thought to have emerged in the Devonian era, mostly in wet/damp places based on fossil record evidence. In addition to being an outcome of competition for light, it is thought that heterospory was more successful in wetter areas because the megaspore could move more easily around in an aquatic environment while microspores were more easily dispersed by wind. Differing sized spores have been observed in many fossilized plant species. For example, the species Lepidophloios, also known as the scale tree, has been shown in fossils to have been heterosporous; The scale tree had separate cones containing either male or female spores on the same plant. Modern heterosporous plants such as many ferns exhibit endospory, in which a megagametophyte is fertilized by a microgametophyte all while still inside the spore wall, gaining nutrients from the inside of the spore. Both heterospory and endospory seem to be one of the many precursors to seed plants and the ovary. Heterosporic plants that produce seeds are their most successful and widespread descendants. Seed plants constitute the largest subsection of heterosporic plants.

3.0Microsporophyll and Megasporophyll

• A sporophyll is a leaf that bears sporangia. Both microphylls and megaphylls can be sporophylls. In heterosporous plants, sporophylls (whether they are microphylls or megaphylls) bear either megasporangia and thus are called megasporophylls, or microsporangia and are called microsporophylls.

4.0Sporangia in Heterosporous Pteridophytes

• In heterosporous pteridophytes, sporangia play a crucial role in the production and dispersal of spores. The sporangia are specialized structures that give rise to two distinct types of spores—microspores and megaspores.

5.0Differentiation into Microsporangia and Megasporangia

Heterosporous pteridophytes have sporangia that are specialized into two types: microsporangia and megasporangia. Microsporangia are responsible for producing microspores, while megasporangia produce megaspores.

Microsporangia: Microsporangia are structures within the plant that are specifically dedicated to the production of microspores. These microspores are typically smaller in size compared to megaspores. Microsporangia contain a large number of microspores, which are involved in the male reproductive function of the plant.

Megasporangia: Megasporangia are specialized sporangia that produce megaspores. Megaspores are larger in size and are involved in the female reproductive function of the plant. Megasporangia contain a relatively smaller number of megaspores compared to the abundance of microspores in microsporangia.

Microspores and megaspores:

Microspores: These are haploid spores in the endosporic species that contain the male gametophyte, which is carried to the megaspores by wind, water currents or animal vectors. Microspores are not flagellated, and are therefore not capable of active movement. The morphology of the microspore consists of an outer double walled structure surrounding the dense cytoplasm and central nucleus.

Megaspores: Megaspores contain the female gametophytes in heterosporic plant species. They develop archegonia that produce egg cells that are fertilized by sperm of the male gametophyte originating from the microspore. This results in the formation of a fertilized diploid zygote that develops into the sporophyte embryo. While heterosporous plants produce fewer megaspores, they are significantly larger than their male counterparts.

In exosporic species, the smaller spores germinate into free-living male gametophytes and the larger spores germinate into free-living female gametophytes. In endosporic species, the gametophytes of both sexes are very highly reduced and contained within the spore wall. The microspores of both exosporic and endosporic species are free-sporing, distributed by wind, water or animal vectors, but in endosporic species the megaspores and the megagametophyte contained within are retained and nurtured by the sporophyte phase. 

6.0Significance of Heterospory in Pteridophytes

• Heterospory, a key aspect of sex determination in plants, involves the production of two distinct spore types, microspores and megaspores, which give rise to male and female gametophytes, respectively. This differentiation occurs during sporogenesis at the sporophyte stage. In contrast to homosporous genera, where sex determination takes place during the gametophyte stage, heterospory offers certain advantages. 
• The nutritional dependency of fern gametophytes on soil and environmental conditions is contrasted with Selaginella, where gametophytes draw nutrition from the sporophyte, rendering them more independent of external conditions.
• Heterospory's biological advantage lies in the large megaspore, housing the female gametophyte, which derives nourishment from the sporophyte. This dependence on the sporophyte ensures independence from external factors that might hinder the growth of a free-living gametophyte. Consequently, the megaspore becomes a more favorable starting point for embryo development compared to an independent green prothallus that must produce its own food. Female gametophyte of heterosporous pteridophytes depends on sporophyte for food and nutrient, 
• The significance of heterospory extends to the development of seeds, acting as a prerequisite for the seed habit. This association is underscored by several important prerequisites, including the production of two spore types, the development of a single megaspore in the megasporangium, retention and germination of the megaspore within the megasporangium, and the sustained connection between the megaspore, megasporangium, and female gametophyte throughout fertilization. 

7.0Examples of Heterosporous Pteridophytes 

There are 9 genera in which heterospory conditions are found: 

1. Selaginella: Selaginella is a pteridophyte. It is also called spikemoss or club moss. It is the largest and the only living genus of the family Selaginellaceae. It contains more than 800 species distributed all around the world with the highest diversity found in the tropical regions.

2. Isoetes: Isoetes, commonly known as the quillworts, is a genus of lycopod. It is the only living genus in the family Isoetaceae and order Isoetales.

3. Marsilea: Marsilea is a genus of approximately 65 species of aquatic ferns of the family Marsileaceae. The name honours Italian naturalist Luigi Ferdinando Marsili. These small plants are of unusual appearance and do not resemble common ferns.

4. Salvinia: Salvinia, a genus in the family Salviniaceae, is a floating fern named in honor of Anton Maria Salvini, a 17th-century Italian scientist.

5. Azolla: Azolla is a genus of seven species of aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, looking nothing like other typical ferns but more resembling duckweed or some mosses.

6. Regnellidium
7. Platyzoma
8. Pilularia
9. Stylites