2.2Cell Multiplication Through Cell Division
Genome and Polyploidy
Under normal growth conditions, unicellular organisms proliferate by cell division and therefore are said to undergo asexual reproduction. In addition to cell proliferation, many eukaryotes undergo sexual reproduction in which a new cell with novel genetic infomation is formed by fusion of two cells derived from different individuals (Fig. 2-3). Generally, vegetative growth refers to the process in which cells proliferate by obtaining nutrients, and reproductive growth refers to the process in which sexual reproduction occurs. The conversion from vegetative growth to reproductive growth in plants or unicellular organisms occurs in response to a change in environmental conditions such as nutrition or light, but in higher level animals, it occurs following developmental programming.
If two cells simply undergo fusion, the number of chromosomes will double; therefore, the cells must first undergo meiosis in which the number of chromosomes is halved. Since a normal cell should have two sets of homologous chromosomes, it is represented as 2n, following the custom to represent one set of chromosome as n. This is the idea of the nuclear phase. The variable represented by the letter n stands for one set of chromosomes in classical biology, i.e., the genome. As the actual chromosomes differ according to sex (described later), this is an ambiguous term. However, this notation is often used for convenience (Refer to Chapters 7, 10, and 24 for contemporary definitions of the genome).
In the nuclear phase, a 2n cell (diploid) undergoes meiosis, the number of chromosomes is halved, and an n cell (haploid) known as a gamete is formed. In a gamete, male–female morphological differences may (eggs and sperm; heterogamy, anisogamy) or may not clearly arise (homogamy, isogamy). For example, in humans, who are heterogamous, males have two sets of autosomes (22 pairs of autosomes) and one pair of sex chromosomes (X and Y sex chromosomes) (notated as 2n also in this case). Thus, each sperm has one set of each chromosome from 1 through 22 and one X or Y chromosome (n nuclear phase). Because females have two sets of autosomes (22 pairs of autosomes) and two X chromosomes (2n), the egg has one set of each chromosome 1 through 22 and an X chromosome (n nuclear phase). Sexual reproduction will be described in greater detail in see Chapter 18. The significance of sexual reproduction is that the combination of chromosomes and recombination within the chromosomes results in the combination of various genes, which is believed to increase the chances of survival of a group of organisms in response to sudden environmental changes (see Column at the bottom). Moreover, because of the large amount of genetic material in eukaryotes, although often very low, errors in replication are inevitable. Sexual reproduction is believed to play a very important role in eliminating replication errors to prevent their accumulation.
Significance of Sexual Reproduction
What exactly is the purpose of sexual reproduction? “Sex” can have different connotations. In mammals, males and females are clearly differentiated. However, in fish, some may switch their sex during development. In plants such as cucumbers, male and female flowers may occasionally even share the same plant (monoecism). Sexes can also be found in unicellular organisms. In other words, what is more important than the presence of individual males and females is the mechanism of exchanging genetic material between different cells, which is the broad meaning of the word sex. The process for this exchange of genetic material is called sexual reproduction.
Proliferative efficiency is better achieved with asexual reproduction than with sexual reproduction. Sexual reproduction requires the encounter of two individuals. If we only consider the number of offspring produced, it seems pointless to have males making up half of the whole group. It is most efficient for a single individual to produce offspring by self-replication. Bacteria and microbes proliferate through simple cytokinesis. It is evident that the proliferative rate of asexual reproduction is superior. Asexual reproduction is not limited to lower level organisms. Insects called aphids (Column Fig. 2-1), which subsist by sucking plant sap, are all females. They proliferate by parthenogenesis. Many plants also undergo asexual reproduction by proliferating without the formation of flowers, i.e., through root division or tubers. Nevertheless, why most eukaryotes, especially higher level organisms, continue to proliferate basically through sexual reproduction? Although asexual reproduction may have a better proliferative efficiency, this efficiency only applies in a stable growth environment. In the presence of sufficient nutrition and absence of competitors, it is natural that asexual reproduction proceeds faster. However, in reality, natural environment can never be purely ideal. Organisms not only have to compete with other organisms for food but also face the risk of falling prey to other organisms. Moreover, environmental conditions are constantly changing. The geneticist Hermann Mueller showed in 1964 that groups of asexually reproducing bacteria accumulate mutations that they cannot eliminate. This is a negative aspect of asexual reproduction.
Sexual reproduction involves genetic recombination during meiosis. This refers not only to simple genetic recombination but also to the possibility of the removal of deleterious alleles. In sexual reproduction, various combinations of chromosomes are formed and recombination occurs within the chromosomes to eliminate fatal recessive mutations and increase genetic diversity. Therefore, the chance that at least a small number of offspring will have the combination of features necessary for survival in an unfavorable environment is increased. Additionally, traits that are unfavorable under normal growth conditions (see Chapter 3) may be advantageous under specific environmental conditions. Moreover, genetic recombination is believed to be advantageous for survival against pathogenic infections.
According to a slightly more quantitative theoretical reasoning, the upper limit of the maintainable genome size is believed to be determined by the error rate in replication. It is hypothesized that meiotic recombination is important in lowering the error rate.
Let us take a closer look at asexual reproduction. Asexual reproduction may be a single phrase but its types are varied. Although unicellular organisms proliferate by cell division, under poor nutritive conditions, they may proliferate by forming spores (e.g., Bacillus subtilis, a type of bacterium used to make nattō).
Asexual reproduction also occurs in multicellular organisms. Parthenogenesis is a form of asexual reproduction in which the eggs produced by females develop in the absence of fertilization. Such ova may be diploid or haploid. For example, Aphids (insects) produce diploid eggs, while honeybee males are produced by the parthenogenesis of an unfertilized egg (haploid). A more universal type of asexual reproduction is vegetative reproduction (or vegetative propagation). Natural vegetative reproduction is exemplified by proliferlation through propagules (small axial shoots that develop into independent, individual plants) in species such as Dioscorea japonica, Begonia grandis, and Lilium lancifolium or minitubers. In case of potato and sweet potato, tubers (potato is a stem tuber and sweet potato is a root tuber) can be cultivated at high yields by cutting the stem and replanting them with buds attached. Bulbs, such as scaly bulbs of lilies, are also a form of vegetative structure. Plants such as bamboo and horsetails multiply through a network of subterranean stems. Kalanchoe plants can be cultivated by planting a fleshy leaf in the ground. Plant cells exhibit totipotency, i.e., a single plant cell can develop into an individual plant. In mammals, only reproductive cells have the ability to give rise to new individuals, and it is difficult to form new individuals from specialized somatic cells differentiated into organs or tissues. However, differentiation can be reprogrammed (iPS cells) by controlling expression of certain genes. The differentiation and regenerative ability of cells will be described in greater detail in Chapters 18, 19, and 22.