Question

What is the event that marks the transition from haploid to the diploid stage of the life cycle and what event marks the transition back to the haploid stage again. Describe the timing of these two cellular even in life cycles that are dominated by haploid cells, dominated by diploid cells or that have alternation of generations.

Answer 1

Zygote formation is a process in which two haploid gametes get fused to form a diploid embryo. This process is also known as fertilisation.

When a diploid cell goes under process of spore formation or gamete formation, by meiosis, it forms haploid cells.

• Life cycles that are dominated by haploid cells are called haplontic cycle. In this, majority of the life is in haploid form that is diplod cell only consists of single zygote. The diploid form arises only when zygotes are formed which further gets tanstioned into haploid cells when spores are formed from them.
• Life cycles that are dominated by diploid cells are called diplontic cycles. Most of the life is in diploid form. Haploid cells are only produced during gametogenesis which upon fertilisation, gives rise to diplod organisms.
• Alternation of generation is described as the formation of gametophyte and sporophyte independently. This form of life cycle is called haplo-diplontic/diplo-haplontic. The haploid spores undergoes mitosis to form a fully functional plant body known as gametophyte and survive in this haploid condition. Alternately, the diploid zygote can also form diplod plant body by mitosis and survive. Thus two different stages of life can co-exist independently. As in case of bryophtes.