Definitions [7]
A mode of reproduction where a new organism is formed from a part of the parent body without involving gametes.
or
The process of forming a new organism from an organism of the same species without the involvement of gametes is called asexual reproduction.
or
When offspring is produced by a single parent by the formation of specialised asexual reproductive structures by the parent body, the reproduction is asexual.
Union of the second male gamete with two polar nuclei to form endosperm, along with fertilization, is called double fertilization.
Apomixis is the production of seeds without fertilisation and is a form of asexual reproduction that mimics sexual reproduction.
Define Apomixis.
Apomixis is a type of asexual reproduction that mimics sexual reproduction, i.e., the production of seeds without pollination and fertilization.
Presence of more than one embryo within a single seed is called polyembryony.
The process of fruit formation without fertilisation is called parthenocarpy.
Define Parthenocarpy.
It is the condition in which fruit is developed without the process of fertilization. It occurs naturally in some varieties of Pineapple, Banana, Papaya, etc.
Key Points
- Meaning - Asexual reproduction involves only one parent, produces no fusion of gametes, and results in offspring that are genetically identical to the parent, called clones.
- Modes of Asexual Reproduction - Common methods include Binary Fission (Amoeba, Paramoecium), Fragmentation (Spirogyra), Budding (Yeast, Protosiphon), Spore Formation/Zoospores (Chlamydomonas), Conidia (Penicillium), and Gemmules (Marchantia).
- In Plants - Asexual reproduction in plants is called Vegetative Propagation. It can be Natural or Artificial (e.g., cutting, grafting, tissue culture).
- In Animals — Common in lower animals only. Does not involve meiosis or gamete formation. Lower animals reproduce asexually by budding and gemmule formation.
- Key Feature — Since only one parent is involved and no fusion occurs, all offspring are morphologically and genetically identical (clones) to the parent.
- Double fertilisation is a unique feature of angiosperms where two fusion events occur inside the embryo sac.
- The pollen tube enters the ovule usually through the micropyle (porogamy), but sometimes through the chalaza (chalazogamy) or integuments (mesogamy).
- The pollen tube releases two male gametes into the embryo sac after entering a synergid.
- Syngamy: One male gamete fuses with the egg to form a diploid zygote.
- Triple fusion: The second male gamete fuses with two polar nuclei to form a triploid primary endosperm nucleus (PEN).
- Significance: It forms both embryo and endosperm, restores the diploid condition, prevents polyembryony, and ensures proper seed development.
- Apomixis is the formation of seeds without fertilisation, where embryos develop without meiosis and syngamy.
- In apomixis, the embryo develops inside the ovule, and the ovule forms a seed without gamete fusion.
- Apogamy is the formation of an embryo from a gametophyte cell without fertilisation, while apospory is the formation of a diploid gametophyte from a sporophyte cell without meiosis.
- Recurrent apomixis produces a diploid embryo sac (e.g., diplospory and apospory), while non-recurrent apomixis forms haploid embryos that are usually sterile.
- Adventive embryony involves the formation of embryos from the nucellus or integuments along with a normal embryo, leading to polyembryony (e.g., mango, orange).
- Apomixis helps in producing genetically identical plants quickly and efficiently.
- Polyembryony is the formation of more than one embryo in a single seed, leading to multiple seedlings.
- It was first observed by Leeuwenhoek in Citrus (orange) seeds.
- Polyembryony can occur due to development from the zygote or other tissues like the nucellus and integuments.
- Types include true polyembryony (embryos in the same embryo sac) and false polyembryony (embryos in different embryo sacs).
- Cleavage polyembryony occurs when the zygote splits into many parts, while adventive polyembryony occurs from nucellus or integument cells.
- It is important in horticulture and increases chances of survival by producing multiple seedlings.
- Parthenocarpy is the formation of fruit without fertilisation, resulting in seedless fruits.
- The term was coined by Noll (1902).
- It occurs naturally in plants like bananas, pineapples, and papayas.
- It is often caused by plant hormones like auxin (IAA), which stimulate ovary development into fruit.
- Types of parthenocarpy include genetic, environmental (due to conditions like temperature or frost), and chemically induced (using hormones like IAA, NAA, gibberellin).
