Alternation of Generation and Its Significance

What is the Alternation of Generation and its Significance?

ALTERNATION OF GENERATIONS:

• In the life history of liverworts, mosses and hornworts there are two distinct multicellular phases or generations.

• These generations are haploid gametophyte and diploid sporophyte, which regularly alternate with each other.

• The gametophyte is the dominant generation because it is more conspicuous.

• It produces gametes called spermatozoids and eggs, therefore called gamete producing generation.

• A haploid spermatozoid fuses with a haploid egg to produce a diploid oospore.

• The oospore does not produce the gametophyte directly but produces a totally different plant called the sporophyte.

• The sporophyte is a less conspicuous generation, which is usually differentiated into foot, seta and capsule (also called sporogonium).

• Spores develop within the capsule by reduction division (meiosis) from spore mother cells.

• The sporophyte produces spores and is, therefore, called spore-producing generation.

• The spore on germination does not develop into a sporophyte but gives rise to the gametophyte.

• Thus in the life history of a bryophytic plant, the two generations, the gametophyte and the sporophyte, regularly alternate with each other.

• The phenomenon of alternation of gametophyte and sporophyte in the life history of a plant is called alternation of generations.

• It should be noted that the gametophyte or haploid stage begins within spores and ends at gametes, whereas the sporophyte being with oospore and ends at spore mother cell.

THE SIGNIFICANCE OF ALTERNATION OF GENERATIONS:

• During the formation of spores from a spore mother cells by meiotic division reshuffling of genes occurs.

• As a consequence, a great variety of spores with different genetic make-up are produced.

• These spores, in turn, produce gametophytes with different genetic combinations.

• The gametophytes with better genetic make-up will have a better chance of survival in the environment where they occur.

• On the other hand, the gametophytes with less advantageous characteristics will be eliminated.

• There is no reshuffling of genes during gametogenesis in the gametophyte as gametes are produced after mitosis.

• The oospore developing after fertilization now has a new genetic make-up as compared to the parent.

• This genetic variation passes to the new sporophyte which on maturity once again produces further genetic recombination which is transferred to the gametophyte.

• In this natural process, the sporophyte thus provides a large amount of genetic variability and nature selects the best genetic combinations.

• In the long run, this will allow the populations to become increasingly better adapted to their environment.