Asexual Reproduction
Asexual reproduction is by which a single parent produces a progeny. The genes are inherited from that parent only. Since there is only one parent, there is no exchanging of genetic information, and the offspring are clones of the parent; an individual of a clone is called a Ramet (Picket et al., 1982). The offspring here are usually produced by mitosis (Figure 1.1).
Asexual reproduction can also be referred to as agamogenesis. Agamogenesis is the type of reproduction which does not involve the fusion of gametes. This type of reproduction is the primary form of reproduction for single-celled organisms like archarea, bacteria and protists.
Table 1. The Advantages and Disadvantages of Asexual Reproduction.
- There are many variations of asexual reproduction however the most common forms include: Binary Fission, Multiple Fission, Budding, Fragmentation and Parthenogenesis.
Figure 1.1. Diagram showing cell division by mitosis
Figure 1.2. Diagram showing a cell undergoing meiosis
(Biophilia, 2016)
Sexual Reproduction
Sexual reproduction occurs when two morphologically distinct types of specialised reproductive cells (gametes) fuse together. Each gamete only contains half the number of chromosomes of normal cells (Nasmyth, 2001).
These are produced through a specialised type of cell division known as meiosis (Figure 1.2.). Meiosis begins with chromosome duplication. The cell then undergoes two consecutive rounds of nuclear divisions, producing four daughter cells from one parent cell. Each of these daughter cells has half the number of chromosomes as the original parent cell (Kleckner, 1996).
Researchers now estimate that over 99.99% of eukaryotes today currently reproduce sexually (at least on occasion).
Table 2. The Advantages and Disadvantages of Sexual Reproduction.
- Two types of sexual reproduction: External and Internal Fertilisation (Figure 2).
External fertilisation is most seen in fish and most water dwelling animals. Most fish produce a large number of sex cells which are released into the water. Courtship rituals make sure that the male and female sex cells are released near each other, but even so the chances of fertilisation are very low, so that huge numbers of eggs and sperm are needed to ensure that enough young are produced.
Internal fertilisation is seen in terrestrial animals. This is necessary since sperm need fluid in which to swim to the egg. The chances of fertilisation are much greater since the sex cells are closer together when released, and this means that fewer sex cells need to be produced. There is a correlation seen in the number of eggs which any animal needs to produce for a successful reproduction, the chance of fertilisation and the degree of parental care given to the young. Therefore it is usually the case that the lower the degree of parental care given or the low chance of fertilisation the greater number of eggs that are produced.
Figure 2. The advantages and disadvantages to internal and external fertilisation.
