Heterospory in Pteridophytes || Origin and Importance

Heterospory in Pteridophytes

Pteridophytes serve as a link between bryophytes and seed plants. In Pteridophytes, heterospory was reported in the late Devonian period. They are vascular plants with roots, stems, and leaves but no flower or seed. Most of the Pteridophytes are homosporous as they produce only one kind of spore. In homospory, the sex may be differentiated only at the gametophytic stage. However, some of the Pteridophytes are heterosporous as they produce spores different in size, structure, and function. The phenomenon of producing different types of spores is called “heterospory”.

What is Heterospory?

“Production of different types of spores”, different in size, shape, and structure.

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In heterospory, Pteridophytes produce microspores (smaller in size which develop into male gametophytes) and megaspores (large and develop into female gametophytes). Only seven genera of Pteridophytes are heterosporous;

1. Azolla
2. Isoetes
3. Marsilea
4. Pilularia
5. Regnellidium
6. Salvinia
7. Selaginella

Origin of Heterospory in Pteridophytes

The history and origin of heterospory in pteridophytes can be better understood based on evidence from different studies.

Evidences of Heterospory

1. Evidence from Paleobotany: Evidence showed that earlier vascular plants were homosporous. The fossil record of the spores showed that in some sporangia, a few spores degenerate, leaving more space and nutrition for the surviving spores. Thus remaining spores grew better and increased in size. Heterosporous genera belonging to Lycopsida, Sphenopsida, and Pteropsida appeared in the late Devonian period.
2. Evidence from Developmental Studies: In Pteridophytes following heterospory, the development of microsporangia and megasporangia follows the same pattern producing identical microspores and megaspores respectively. However, after meiosis, most of the megaspore mother cells degenerate providing space and nutrition to the growing single functional megaspore. In microsporangia, all the microspores are functional.
3. Evidence from Experimental Studies: In Selaginella and Marsilea, nutritional and external environmental factors govern the phenomenon of heterospory. During the period of low light intensity and low photosynthetic rates, Selaginella produces microspores only. Under the condition of low temperature, some microspores degenerate leaving more space for the others thus the size of microspores increased by six times.

Read More: Heterospory and Seed Habit in Pteridophytes

Biological importance of Heterospory

In Selaginella, the nutrition of gametophytes is derived from the sporophyte, and there they are more independent of the external condition than those of fems. The gametophytes of the ferns are, however dependent for their nutrition on soil and environmental condition. Gametophyte dependent on sporophytes has some advantages;

1. The gametophyte is independent of external conditions that might interfere with its growth.
2. It provides a better start for the new embryo than independent green prothallus which has to manufacture its food.

Advantages of Heterospory over Homospory

1. Heterospory produced endospores gametophytes (gametes inside the spore wall). Endosporic gametophyte is independent of harsh external conditions that might interfere with their growth.
2. Endosporic gametophytes have a continuous food supply from diploid sporophytes.
3. Endosporic development provides a better starting point for the young embryo.
4. Heterospory reduces the size of gametophyte to a few cells only.
5. It has also facilitated sex differentiation in Pteridophytes.
6. The most important advantage of heterospory is the formation of seed habit.

Heterospory and Seed Habit (Seed Formation)

Heterospory leads to the formation of seeds. In homosporous plants, there are a few chances of genetic variability as the sperm from the same prothallus fuses and fertilizes the egg. But in heterosporous plants, male and female gametangia, separated in space and time, reduce the chances of self-fertilization. As both gametangia are separated in space and time, there should be a mechanism and medium to bring compatible gametangia together for fertilization. Thus the mechanism of pollination evolved. This mechanism was first evident in Selaginella.

Advantages of Seed over Spores

Seed is an integument megasporangium. The formation of the seed is an evolutionary event of great importance in the phylogeny of seed plants. Seeds have many advantages over spores;

1. Seeds are covered with a seed coat that offers great protection against adverse biotic and abiotic factors.
2. Seeds contain mature embryos ready to grow while single cells of spores undergo a cell division process before germination of plant/fungus.
3. All the seeds do not necessarily require water for germination however, all spores required water before growing.
4. Seeds have endosperm for the nourishment of the young embryo. Spores are devoid of such a built-in system to help plants/fungi during early life stages.
5. Seeds are multicellular, while spores are unicellular.
6. Spores require more favorable conditions for germination while seeds enable them to germinate in harsh conditions.

What are the Heterosporous Condition for the Development of Seed Habit

All seed-producing plants (spermatophytes), developed different conditions in them that led them to evolve seed habits in prehistoric times. Those conditions are;

1. Production of two types of spores (i.e. heterospory).
2. Reduction in the number of functional megaspores.
3. Production of single megaspore per megasporangium.
4. Permanent retention of megaspore within megasporangium.
5. Development and availability of sufficient nutrients.
6. Formation of protective layers (integuments).
7. Development of organic connection between megasporangial wall and megaspores.
8. Development of pollination mechanism.
9. Retention of the young embryo inside the megaspore wall.
10. Development of seed with protective layers.
11. Development of seed dispersal mechanisms.

Seed Habit in Pteridophytes

A seed is a ripened ovule that contains a fertilized embryo, stored food, and a covering (seed coat) Seed habit is the characteristic feature of spermatophytes. The phenomenon of heterospory and retention of megaspores within megasporangia led to the process of seed habit. This phase is much important in the evolution of plants. In early vascular plants, the phenomenon of seed habit was associated with the following conditions;

1. Production of micro and megaspores (Heterospory).
2. Single functional megaspore within megasporangia.
3. Retention of megaspore within megasporangia even after fertilization.
4. Development of fertilized egg into an embryo within fertilized megasporangia.

All the above-mentioned conditions were followed by Selaginella, however, seeds are not formed in it because of the following conditions;

1. Megaspore without integuments.
2. Megaspore did not remain permanently inside the megasporangium.
3. The embryo did not undergo a resting period and immediately gave rise to the sporophyte.

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