Prepare a detailed account on heterospory under the headings – definition, two
examples, incipient heterospory, biological significance, and evolution of seed
habit.
Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from isospory independently in several plant groups: the clubmosses, the arborescent horsetails, and progymnosperms. This occurred as part of the process of evolution of the timing of sex differentiation.
Heterospory developed due to natural selection pressures that encouraged an increase in propagule size. 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 evolved from homospory many times, but the species in which it first appeared are now extinct. Heterosporic plants that produce seeds are their most successful and widespread descendants. Seed plants constitute the largest subsection of heterosporic plants.
Heterospory in Pteridophytes:
Most of the Pteridophytes produce one kind of similar spore. Such Peridophytes are known as homosporous and this phenomenon is known as homospory. However, there are some Pteridophytes which produce two different types of spores (differing in size, structure and function).
Such Pteridophytes are known as heterosporous and the phenomenon is known as heterospory. The two types of spores are microspores and megaspores. Microspores are smaller in size and develop into the male gametophyte while the megaspores are large and develop into female gametophyte.
According to Rashid (1976) only 9 genera of Pteridophytes are heterosporous. These are Selaginella, Isoetes, Stylites, Marsilea, Pilularia, Regnellidium, Salvinia, Azoll and Platyzoma.
Origin of Heterospory:
The origin of heterospory can be better discussed on the basis of evidences from paleobotany, developmental and experimental studies.
1. Palaeobotanical evidences:
It has been suggested that heterospory arose due to degeneration of some spores in a few sporangia. As more nutrition becomes available to less number of spores, the surviving spore grow better, hence increase in their size.
Palaeobotanical evidences show that the
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