Question

List and discuss the major adaptations in the evolution of alternation of generations starting with algae -> bryophytes -> ferns -> pines.

Answer 1

Alternation of generation or metagenesis is the alteration of sexual phase and asexual phase in the life cycle of the organism. The two phases are morphological and genetically distinct. The sexual phase called gametophyte and asexual phase called sporophyte.

Alternation of generations in algae:

The lifecycle of primitive algae consists of one celled haploid phase and one celled diploid phase. As in higher plants algae donot have dintinct alternation of generation. In the algae the gametophyte is filamentous showing vegetative cell divison and alternates with one celled diploid phase ie sporophyte. The haploid filamentous plants reproduce asexually by zoospores. In some algae there is an alternation of generation from one celled haploid to double celled diploid phase. Eg. Bryopsis. In some other cases, there is an alternate of many celled haploid to many celled diploid generation. In such cases two alternating generation are identical and are called isomorphic. Eg. Ulvaceae. In cladophora the reduction division doesnt take place and the zoospores remain diploid. In brown algae the two generations remain morphologically similar but physiologically different. In the lifecycle of Ectocarpus the diploid and haploid generations remain alike and they are called isomorphic.

Alternation of generation in Bryophytes

In the lifecycle of bryophytes it shows regular alternation of generation between diploid and haploid phases. Here the haploid phase is called the gametophytes which produce gametes. The sporophyte formed from haploid phases is 2n and germinate to form gametophytes.In bryophytes when two generations are morphologically different it is called heteromorphic.The gametophytic generation is long lived phase compared to sporophytic phase. Moreover sporophytic phase depends on gametophytic phase for its nutrient supply.

Alternation of generation in Ferns

The life cycle of plants is different from ferns as the diploid and haploid phases are free living . Here in ferns the sporophytic generation is diploid.. Many sporangia are mounted on the stalks. Each sporangia contains many spore mother cells which divide by meiosis to form haploid spores. On suitable conditions the spores produces gametes. In the gametophyte generation , the prothallus is the fern gametophyte. It grows into filament of cells containing antheridia or archegonia or both. The anthredium and archegonium produces fertilised egg.

Alternation of generation in Pines

In pines Mature sporphyte produces spores by meiosis. Spores germinate and grow into gametophytes. The spores fuse to form diploid sporophyte. All gymnosperm are heterosporous. In gametophytic generation, the male cone produces microspores producing haploid number of cells. Further mitosis produces male gametophyte.The female cone produces megaspores and produces haploid cells, Further mitosis produces female gametophyte. Male and female gametes produces zygote.