SATHEE: Heterosporous Pteridophytes

Heterosporous Pteridophytes

Heterosporous plants produce two types of spores which differ in structure, size, and function; namely, smaller microspores and larger megaspores.

Most pteridophytes are homosporous, producing all spores of a similar kind. However, some genera, such as Selaginella and Salvinia, are *heterosporous.

Heterospory is an important phenomenon in terms of evolutionary perspective, as it is found in some pteridophytes, as well as all seed plants, i.e. gymnosperms and angiosperms.

Salient Features of Heterospory in Pteridophytes

Heterospory is the production of two types of spores, the megaspore and the microspore.
The megaspore is a large, female spore that develops into the female gametophyte.
The microspore is a small, male spore that develops into the male gametophyte.
The female gametophyte produces egg cells, while the male gametophyte produces sperm cells.
Heterospory enables the pteridophytes to produce large amounts of reproductive material without having to invest a lot of energy in each individual spore.
This allows them to produce more offspring, increasing their chances of survival.

In heterosporous pteridophytes, two different kinds of spores with different sexes are produced, which evolved first.

The microspores are the smaller spores, while the megaspores are the larger ones.

Microspores are present in microsporangia, whereas megaspores are present in megasporangia.

Microspores are smaller in size but greater in number, whereas megaspores are larger in size but fewer in number.

In Selaginella, all the microsporocytes or microspore mother cells undergo meiosis to produce a large number of microspores, while only one megasporocyte is functional and undergoes meiosis to produce four large megaspores.

Heterospory is evident either before meiosis in plants such as Selaginella, or after meiosis in plants such as Marsilea.

The origin of heterospory is thought to be linked to nutritional factors.

The functional megaspores absorb the nutrients while the rest disintegrate.

The dominant phase in the life cycle of pteridophytes is the diploid sporophyte, which alternates with the haploid gametophyte.

In the homosporous species, sex differentiation occurs during the gametophyte stage, while in the heterosporous species, sexes are separated at the sporophytic stage.

The germination of microspores and megaspores initiates within the sporangium, and the resulting male gametophyte is derived from the microspore, while the female gametophyte is derived from the megaspore.

Heterospory is a major evolutionary advancement which resulted in the development of seeds.

The megaspores germinate within the sporangium to produce a female gametophyte.

Female gametophytes are retained inside the sporophytes. Fertilisation of the egg and development of the zygote into a young embryo occur within the female gametophyte.

The sporophyte provides nutrients to the female gametophyte.

Examples of Heterosporous Pteridophytes

Selaginella
Ophioglossum
Marsilea

Heterospory is seen in 9 genera of pteridophytes:

Lycopodium
Selaginella
Isoetes
Marsilea
Ophioglossum
Psilotum
Osmunda
Equisetum
Azolla

Selaginella

Marsilea

Salvinia

Isoetes

Azolla

Stylites

Pilularia

Platyzoma

Regnellidium

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