Biology · Ch 1 — Reproduction in Organisms
Sexual Reproduction
Sexual Reproduction
Sexual Reproduction: An Overview
Sexual reproduction involves the formation of male and female gametes — either by the same individual or by two individuals of opposite sex — followed by the fusion of these gametes to form a zygote, which then develops into the new organism. Compared with asexual reproduction, it is a slower, more elaborate and more complex process. Because it combines genetic material from two gametes, sexual reproduction produces offspring that are genetically different from their parents and from one another, unlike the identical clones produced asexually.
Despite the enormous diversity among plants, animals and fungi in body form, internal structure and physiology, their sexual reproduction follows a broadly similar pattern.
Juvenile, Reproductive and Senescent Phases
Every sexually reproducing organism must first pass through a period of growth and development before it becomes capable of reproducing — this is the juvenile phase (called the vegetative phase in plants), and its length varies widely between species. In higher plants, the end of the vegetative phase and the start of the reproductive phase is marked by flowering. Annual and biennial plants show distinct vegetative, reproductive and senescent phases, but these are harder to separate in long-lived perennials. A few species show unusual flowering behaviour — some bamboos flower only once, after 50–100 years, produce a large crop of fruit, and then die, while Strobilanthus kunthiana (neelakuranji) flowers only once every twelve years, transforming hillsides in Kerala, Karnataka and Tamil Nadu into a sea of blue when it does.
In animals, the juvenile phase is followed by morphological and physiological changes that precede active reproductive behaviour, and the length of the reproductive phase again varies by species. Wild birds typically lay eggs only in a particular season, though captive birds (as in poultry farms) can be induced to lay throughout the year for commercial reasons. Female placental mammals show cyclical changes in ovarian activity and hormone levels during their reproductive phase — called the oestrus cycle in most non-primate mammals (cows, sheep, dogs and similar animals) and the menstrual cycle in primates (monkeys, apes and humans). Mammals that cycle only during particular favourable seasons are called seasonal breeders, while those that remain reproductively active throughout their reproductive phase are called continuous breeders.
The end of the reproductive phase marks the beginning of the senescent phase (old age), during which body processes such as metabolism slow down; senescence ultimately ends in death. Hormones, working together with environmental cues, control the transitions between the juvenile, reproductive and senescent phases in both plants and animals.