CHAPTER 1
What is reproduction?

• Reproductive strategies
  • Mammals reproduce sexually
  • Both natural and sexual selection operate in mammals
  • Fertilization in mammals is internal
  • Oviparity versus viviparity
  • Parental care
  • Reproduction strategies: summary
• Reproductive life cycles
  • The somatic and social life cycles
  • The generative life cycle
  • The epigenetic cycle
• Conclusions
• Key learning points
• Clinical vignette
• Further reading

The ability to reproduce is a defining feature of all living organisms. Through reproduction, we pass our genes to a new generation. Each new generation in turn reproduces or dies out. The survivors are 'selected', by disease resistance and by successful competition for resources and mates, for their 'fitness' to live and to reproduce. In this way, the gene pool of surviving species is constantly adapting to the prevailing environment to provide the best available 'fit'. Thus, reproduction has been central to our evolution as the species Homo sapiens.

However, humans transmit more than simply their genes across generations. Humans have evolved high levels of sociability through which cultures are formed. Cultural practices are also transmitted across generations, and reproduction itself lies at the very heart of many of our cultural practices and taboos (see Chapters 5, 6, 20 and 23). Human society, by influencing socially and/or medically who survives to reproduce and with whom, is itself now part of the 'selection' process. This pivotal position of reproduction in our culture makes it a sensitive subject for study. Indeed, scientific enquiry into human reproduction was relatively late to the modern research scene and even today can provoke hostility, embarrassment or distress.

In this opening chapter, human reproduction is introduced and contextualized: in relation to other species - reproductive strategies, and in relation to time - the reproductive life cycle.

Reproductive strategies

Most organisms reproduce asexually (or vegetatively). For example, many unicellular organisms reproduce themselves mitotically, just like the individual cells of our body (Figure 1.1). Mitotic divisions generate two offspring that are genetically identical to each other and to their single parent. Among multicellular organisms, some shed cells or even body parts from which another genetically identical individual can be generated - a process called regeneration. Others, including some complex vertebrates such as lizards, reproduce themselves by setting aside a special population of egg cells that can differentiate into conceptuses in the absence of a fertilizing spermatozoon. This type of asexual reproduction is called parthenogenesis, and generates a completely new organism with the same gene complement as its parent.

Figure 1.1 Mitosis and meiosis in human cells. Each human cell contains 23 pairs of homologous chromosomes, making 46 chromosomes in total. Each set of 23 chromosomes is called a haploid set. When a cell has two complete sets, it is described as being diploid. In this figure, we show at the top a single schematized human cell with just two of the 23 homologous pairs of chromosomes illustrated, each being individually colour-coded. Between divisions, the cell is in interphase, during which it grows and duplicates both its centriole and the DNA in each of its chromosomes. As a result of the DNA replication, each chromosome consists of two identical chromatids joined at the centromere. Interphase chromosomes are not readily visible, being long, thin and decondensed (but are shown in this figure in a more condensed form for simplicity of representation).

Lower left panel: In mitotic prophase, the two chromatids become distinctly visible under the light microscope as each shortens and thickens by a spiralling contraction; at the end of prophase the nucleoli and nuclear membrane break down. In mitotic metaphase, microtubules form a mitotic spindle between the two centrioles and the chromosomes lie on its equator. In mitotic anaphase, the centromere of each chromosome splits and the two chromatids in each chromosome migrate to opposite poles of the spindle (karyokinesis). During mitotic telophase division of the cytoplasm into two daughters (known as cytokinesis) along with breakdown of the spindle and the reformation of nuclear membranes and nucleoli occurs, as does the decondensation of chromosomes so that they are no longer visible under the light microscope. Two genetically identical daughter cells now exist where one existed before. Mitosis is a non-sexual or vegetative form of reproduction.

Lower right panel: Meiosis involves two sequential divisions. The first meiotic prophase (prophase 1) is lengthy and can be divided into several sequential steps: (1) leptotene chromosomes are long and thin; (2) during zygotene, homologous pairs of chromosomes from each haploid set come to lie side by side along parts of their length; (3) in pachytene, chromosomes start to thicken and shorten and become more closely associated in pairs along their entire length at which time synapsis, crossing over and chromatid exchange take place and nucleoli disappear; (4) in diplotene and diakinesis, chromosomes shorten further and show evidence of being closely linked to their homologue at the chiasmata where crossing over and the reciprocal exchange of DNA sequences have occurred, giving a looped or cross-shaped appearance. In meiotic metaphase 1, the nuclear membrane breaks down, and homologous pairs of chromosomes align on the equator of the spindle. In meiotic anaphase 1, homologous chromosomes move in opposite directions. In meiotic telophase 1, cytokinesis occurs; the nuclear membrane may re-form temporarily, although this does not always happen, yielding two daughter cells each with half the number of chromosomes (only one member of each homologous pair), but each chromosome consisting of two genetically unique chromatids (because of the crossing-over at chiasmata). In the second meiotic division, these chromatids then separate much as in mitosis, to yield a total of four haploid offspring from the original cell, each containing only one complete set of chromosomes. Due to chromatid exchange and the random segregation of homologous chromosomes, each haploid cell is genetically unique. At fertilization, two haploid cells will come together to yield a new diploid zygote.

Mammals reproduce sexually

Parthenogenesis is simply not an option available to mammals. Thus, although it is possible to activate a mammalian egg (including a human egg) in the complete absence of a spermatozoon, such that it undergoes the early processes of development and may even implant in the uterus, these parthenogenetic conceptuses always fail and die eventually (see page 10 for an explanation as to why this is).

Reproduction in mammals is invariably sexual. Sex is defined formally in biology as a process whereby a genetically novel individual is formed as a result of the mixing of genes from two individuals. So, the essential feature of mammalian sexual reproduction is that each new individual receives its chromosomes in two roughly equal portions: half carried in a male gamete, the spermatozoon (see Chapter 7), and half in a female gamete, the oocyte (see Chapter 9). These gametes come together at fertilization (see Chapter 12) to form the genetically novel zygote. In order to reproduce subsequently, the individual formed from that zygote must transmit only half its own chromosomes to the new zygotes of the next generation. In sexually reproducing species, therefore, a special population of germ cells is set aside. These cells undergo the division process of meiosis, during which the chromosomal content of the germ cells is reduced by half and the genetic composition of each chromosome is modified as a result of the exchange of pieces of homologous chromosomes (Figure 1.1). The increased genetic diversity that is generated within a sexually reproducing population offers a richer and more varied source of material on which natural selection can operate. The population therefore shows greater resilience in the face of environmental challenge. In Chapters 3 and 4, we examine how the two sexes are formed and matured.

Both natural and sexual selection operate in mammals

Asexually reproducing organisms do not need to find a sexual partner. Whether or not they reproduce depends entirely on their survival - natural selection operates simply at this level. Sexual reproduction introduces a complication since it involves two individuals. These have to come together and synchronize their egg and sperm production and shedding: spatial and temporal coordination is highly desirable to optimize fertility. The conjunction of two sexes also provides opportunities for mate selection. For...