Sexual development (zoology)

While some lower forms of life with no recognizably different sexes exchange genetic material in a form of sexual reproduction, sexual reproduction in most organisms involves individuals with some obviously different physical and behavioral features. Biologically, the real difference between males and females is the type of sex cells they produce—whether large eggs specialized to support embryonic development or tiny sperm specialized for moving to the egg. Eggs and sperm are produced in gonads—the ovaries of females and the testes of males. The gonads of higher animals also produce sex hormones, chemical messengers that affect both embryonic and adult sexual development.

Even these basic sex distinctions are rather flexible in some organisms. Sometimes, sex is determined entirely by the environment. One kind of marine worm becomes a female unless it attaches as a larva to an adult female, whereupon it becomes a male—probably because of hormones secreted by the female. Temperature can control sex development in some animals, such as mosquitoes and amphibians. Sex may also be determined by size. Since it takes more energy to produce eggs than sperm, when food is scarce, it may be more adaptive to be male. The European oyster begins adult life as a male, changes to a female as it grows larger, and reverts to being a male after shedding eggs.

In territorial animals, being a large male may be an advantage. A tropical wrasse, or “cleaner fish,” travels with a harem of smaller females. If he is removed, the largest female becomes a male within a few days. Many organisms, including earthworms, snails, and some fish, are hermaphrodites—functional males/females that can fertilize themselves or exchange sperm with others. Some insects, worms, crustaceans, goldfish, whip-tailed lizards, and even turkeys lay eggs that can develop without fertilization, a process called parthenogenesis. This strategy is not sound in the long run since it does not promote genetic diversity. It is an advantage for an organism living under good conditions, however, where an all-female population can exploit the ideal environment most efficiently.

The Origin of Sex Differentiation

Sex differentiation probably originated as differential growth of either the ovary or the testis, mediated in various ways by hormones or other environmental factors. Later in evolution, it came under genetic control, which made the process more independent of the environment and made possible the development of more complex reproductive structures and behavior.

The genetic sex of an animal is determined by the father at fertilization. In most species, females have two matching X chromosomes, males have an unmatched X and a smaller Y chromosome. If a normal egg with one X chromosome is fertilized by an X-bearing sperm, the XX embryo is genetically female. A Y-bearing sperm will produce a genetic male, XY. In butterflies, fishes, and birds, however, females have XY chromosomes, and males have XX. Initially, XX and XY embryos look identical and, in a sense, are still sexually bipotential. Their gonads are “indifferent,” that is, able to form either an ovary or a testis. Each has two sets of undifferentiated sex ducts. One set, the Wolffian ducts, will become the sperm ducts and other male structures. The other set, the Müllerian ducts, form the female oviducts, uterus, and vagina.

Soon, however, genes on the Y chromosome direct the inner part of the indifferent gonad to become a testis, which then produces the male sex hormones (androgens) and Müllerian-inhibiting substance (MIS), which control further events in male development. An androgen called testosterone causes the male duct system to persist and develop, and MIS makes the female duct system degenerate. Testosterone has other developmental effects, as indicated by the fact that in monkeys, male behavior is linked to the length of embryonic exposure to testosterone.

Without the influence of the Y chromosome, an XX gonad begins to develop into an ovary. The role of female sex hormones in development is unclear since, in mammals, female embryonic development can occur in the absence of female hormones. The mammalian embryo tends to develop in the female direction unless specific influences prevent it. The Wolffian ducts are remnants of a drainage system from a temporary embryonic kidney that disappears before birth. Only the presence of male sex hormones will keep these tubes from disintegrating. The Müllerian ducts, on the other hand, tend to persist unless acted upon by the anti-Müllerian substance. In birds, the embryonic ovary is the dominant gonad, and it actively feminizes the reproductive tract. It has been suggested that early male development in mammals is necessary to allow male differentiation in the female-hormone-rich uterine environment.

Until differentiation begins, both sexes also have the same vaguely female-looking external sex structures or genitalia. In both sexes, a small protuberance called the genital tubercle is found toward the front or belly side of the embryo. Behind the genital tubercle is a slitlike opening, the urogenital groove; it is flanked by two sets of paired folds or swellings, like a river valley paralleled by two sets of ridges on either side. In the female, the genital tubercle will form a small structure called the clitoris. The urogenital groove will remain open, forming a vestibule into which the vagina and the urethra open, which empties the urinary bladder. The folds on either side of the groove will remain relatively unchanged to form labial folds.

In the male, the genital tubercle will become the tip of the penis, and the innermost urogenital folds will fuse together to form the body of the penis; the “scar” of this joining may be seen on the underside of the penis. This fusion closes off the urogenital groove and encloses the male’s urethra within the tubelike penis. The outer pair of ridges will fuse to form the scrotum, the sac that encloses the two testes, which descend into the scrotal sac before birth. Another androgen, dihydrotestosterone, may be responsible for the development of these external male structures.

Studying Sexual Development

Many sex-determining mechanisms can be studied by simple modification of the environment. For example, by varying temperature, hormone level, social-group composition, or other environmental factors, it is possible to reverse the sexes of some invertebrates, fish, and amphibians. Castration experiments are commonly used to study the effects of hormones on the sexual development of birds and mammals. For example, castrated mammals of either sex develop in the female pattern.

Sex development can be studied with naturally occurring hormone imbalances, as in freemartin calves—sterile, masculinized females whose male twin exposed them before birth to male sex hormones. The same masculinizing effect on female fetuses can be achieved by injecting a pregnant mammal with androgens or even by growing an embryonic ovary and testis together in an organ culture outside the body. In each case, the female structures are masculinized by the male hormones.

Sex chromosome mutants in animals as diverse as fruit flies, mice, and birds can be used to study chromosomal influences in sex determination. To use a human example, there are sterile XX men with a tiny piece of the short arm of the Y chromosome attached to one X. There are also XY females who show a deletion of the same short arm of the Y. These observations have led geneticists to think that the testis-determining genes are on the short arm of the Y since without it an individual—even one with a Y—is female.

The Evolutionary Advantage of Sexual Reproduction

Despite its great biological costs, sexual reproduction is practiced by almost every kind of living thing. It confers tremendous evolutionary advantage on a species by producing a new individual with the genetic characteristics of two parents but with unique combinations of features that may make the offspring more successful than either parent. The advantage of having separate sexes for reproduction is that it permits the development of extremely specialized reproductive organs for the very different requirements of sperm or egg production and, when needed, intrauterine support for the embryo. Though some organisms show great sexual flexibility, higher animals and plants have tended toward sexual stability, probably because of the high cost of sex reversal for organisms with highly specialized sexual structures.

The study of sex differentiation helps advance scientific knowledge in many areas. Modes of sex determination often provide clues to evolutionary relationships among groups of organisms. In addition, the development of sex differences makes a good model system for the study of more general questions. For example, one might use the control of sexual size differences to attack the broader question of what makes mice smaller than elephants. The control of sex differentiation by environmental factors, to use another example, might provide geneticists with a way to study how genes are turned on by hormones, temperature, or other external influences.

For embryologists, the stepwise determination and differentiation of the mammalian reproductive system is an excellent general development model; it involves a genetically controlled sequence of events that includes both the preservation of one embryonic structure (the Wolffian duct) and the removal of another (the Müllerian duct). Hormonally controlled events include a wide variety of developmental sequences, from the externally visible large-scale changes involved in the shaping of the external genitalia to the biochemical differentiation that programs the brain hypothalamus for its complex control of the menstrual cycle.

In 2020, scientists discovered the oldest known vertebrate sex-determining system—a female-specific genome region in a 180-million-year-old sturgeon. The discovery was exceptional for species conservation, evolutionary biology, and gene mapping. However, in 2024, scientists discovered an older sex chromosome in octopuses. The Z chromosome was found in two California two-spot octopuses (Octopus bimaculoides), and scientists established its existence to between 248 and 455 million years ago. This discovery was a first among cephalopods. Scientists noted that the Z chromosome is evolutionarily much different from other chromosomes, and it evolves much more slowly.

Additional research concerning sexual development from an evolutionary perspective found that gene-level sex differences occur much more quickly in animals than cell-level differences. These sex differences are formed in each species in different ways in terms of developmental stages and sex organs.

Principal Terms

Androgens: The general term for a variety of male sex hormones, such as testosterone and dihydrotestosterone

Genital Tubercle: A small swelling or protuberance toward the front of an embryo’s genital area; it is destined to become the penis tip or clitoris

Genitalia: The external sex structures

Gonad: The structure that produces eggs or sperm cells and sex hormones; the ovary or the testis

Hermaphrodite: A single organism that produces both eggs and sperm

Labial Folds: The paired ridges of tissue on either side of the embryo’s genital area, which become penis and scrotum in males and labia in females

Müllerian Ducts: The embryonic ducts that will become the female oviducts or Fallopian tubes, uterus, and vagina

Parthenogenesis: The development of an unfertilized egg

Urogenital Groove: A slitlike opening behind the genital tubercle that will become enclosed in the penis but remain open in females

Wolffian Ducts: An embryonic duct system that becomes the internal accessory male structures that carry the sperm

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