Sexual Reproduction

Sexual reproduction is a method for producing a new individual organism while combining genes from two parents. A single sperm and egg fuse dur­ing fertilization, and their genomes combine in the new zygote. Sperm are small and contain little more than the father’s genes. Eggs are large and contain the mother’s genes and all cellular components necessary for the early development and nutrition of the embryo. The sexual dimorphism in gamete size is echoed in many other traits of adults and has resulted in the evolution of different male and female reproductive strategies. Sexual re­production is widespread in almost all groups of multicellular organisms, but the reasons for its evolution and prevalence are not well known.

Common frogs in amplexus. Sexually reproducing individuals spend a considerable amount of time and energy locating mates, exchanging genetic material, and often caring for young.

Fertilization

Much scientific knowledge about the steps of fertilization comes from ob­servations on sea urchins and other marine invertebrates. In these animals, sperm cells that contact the jelly coat surrounding the egg react with large carbohydrates in the jelly. These carbohydrates cause the sperm to release protein-digesting enzymes that erode a path through the jelly coat and stimulate the sperm to burrow into the egg. Once the sperm reaches the egg surface, a protein called bindin on the sperm membrane attaches to a receptor molecule on the egg membrane. Following this attachment, the egg and sperm membranes fuse and fertilization is complete.

Fertilization usually must involve only one egg and one sperm. Fusion of additional sperm is prevented by a change in the electrical voltage of the egg cell membrane within a second or two of the first sperm fusing with it. The change in voltage results from sodium ions moving into the egg cyto­plasm, but how it prevents additional sperm from fusing is not well known. Multiple fertilizations are further prevented by chemical reactions that change the receptivity of the egg’s outer layers.

Successful fertilization must involve gametes (sperm and egg) from the same species. In many animals with internal fertilization, courtship behav­iors and reproductive anatomy prevent fertilization between species. In some animals with external fertilization (like marine invertebrates that re­lease their gametes into the water around them), fertilization involves species-specific chemical interactions. For example, in many sea urchins the sperm-activating carbohydrates in the jelly and the bindin and bindin- receptor proteins are very species-specific, thereby ensuring conspecific fer­tilizations.

Male and Female Sexual Strategies

Because their gametes are rare and energetically costly to produce, females suffer a greater consequence of mating with the wrong species or with a low-quality mate than do males. This disparity between the sexes imposes different selective pressures on males and females. Females usually increase their evolutionary fitness (number of surviving offspring) by mating with high-quality males. Males usually increase their fitness by mating frequently to increase the chances that their sperm will encounter a rare egg. Conse­quently, females have often evolved mechanisms for choosing the fathers of their children, while males have often evolved mechanisms for gaining ac­cess to females and their eggs.

Males may gain access to females by competing with other males. The enormous size of bull elephant seals and the head-slamming contests of mountain goat rams are familiar examples of attributes that increase an in­dividual male’s access to females. More cryptically, competition for access to eggs rather than to females can occur among sperm even after mating has occurred. For example, boars and some promiscuous monkeys produce copious amounts of semen to displace the sperm left in the female’s vagina by other males. A male damselfly will remove a previous male’s sperm from a female before depositing his own. Male snakes insert a plug into the fe­male’s reproductive tract after mating to prevent insemination by subse­quent males. Many rodents have evolved penises with hooks and spines to dislodge the plug left by a previous male. And mammalian semen contains prostaglandins that stimulate the uterus to contract, thereby pumping the semen toward the egg and hastening fertilization.

Females can choose mates on the basis of material offerings or partic­ular male traits. Female hangingflies mate with males that present them with large prey items, while peahens choose peacocks with showy tails, and fe­male frogs choose males with energetic calls. As with sperm competition in males, females may also exercise cryptic choice in deciding which males will fertilize their eggs even after mating with them. Examples include beetles in which the female contracts muscles in her reproductive tract to prevent males from inserting their sexual organs completely, lionesses that delay ovulation after their pride is taken over by new males, zebras that eject se­men from their vaginas, and female spiders that transport more or less sperm from a given male down their reproductive tracts depending on the vigor of his courtship.

Evolution of Sexual Reproduction

Many organisms reproduce asexually; that is, they produce genetically iden­tical clones. All of an asexual individual’s offspring can also produce off­spring, but for a sexual female that produces both daughters and sons, only the daughters can bear young. If an asexual individual and a sexual female each produce the same total number of offspring in an unchanging envi­ronment, then the asexual individual will have twice as many grandchildren as will the sexual female (since only half of the sexual female’s children will bear young), four times as many great-grandchildren, and so on. In this sense, sex is evolutionarily very costly; that is, it appears to have a lower fit­ness than a strictly asexual strategy. Sex also carries other costs such as en­ergy expenditures associated with finding and competing for mates and the risk of exposure to sexually transmitted diseases. So why has sex evolved and why does it persist?

Most explanations for sex are based on the fact that sexual reproduction results in genetically variable offspring, whereas asexual reproduction does not. Genetic variation among offspring is valuable, particularly when envi­ronments change over time. If the environment changes for the worse, an asexual mother may lose all of her offspring, while a sexual mother is likely to have at least some of her offspring survive the new conditions. Environ­ments usually do change, particularly in terms of the adaptations of other organisms with which a species interacts. In such uncertain environments sexual reproduction should be favored by natural selection. But as in much of biology, there is no single widely accepted answer for the evolution and persistence of sex in all organisms.

References

Alcock, John. Animal Behavior: An Evolutionary Approach, 6th ed. Sunderland, MA: Sinauer Associates, Inc., 1998.

Catton, Chris, and James Gray. Sex in Nature. New York: Facts on File Publications, 1985.

Eberhard, William. Female Control: Sexual Selection by Cryptic Female Choice. Prince­ton, NJ: Princeton University Press, 1996.

Forsyth, Adrian. A Natural History of Sex. Shelburne, VT: Chapters Publishing, 1986.

Gilbert, Scott. Developmental Biology, 5th ed. Sunderland, MA: Sinauer Associates, Inc., 1997.