Scientific Data Surah 92 · Ayah 3
The Sex of Offspring Is Determined by Particular Chromosomes
The Sex of Offspring Is Determined by Particular Chromosomes
The Sex of Offspring Is Determined by Particular Chromosomes
In humans and many other animal species, sex is determined by specific chromosomes. How did researchers discover these so-called sex chromosomes? The path from the initial discovery of sex chromosomes in 1891 to an understanding of their true function was paved by the diligent efforts of multiple scientists over the course of many years. As often happens during a lengthy course of discovery, scientists observed and described sex chromosomes long before they knew their function.
An idea inspired by the "X element"
By the 1880s, scientists had established methods for staining chromosomes so that they could be easily visualized using a simple light microscope. With this staining method, scientists were able to observe cell division and to identify the steps that occurred during both mitosis and meiosis (Figure 1).
Figure 1: Cell division observed through the microscope (left) is redrawn to show the action of chromosomes (right). Arrows indicate the axis along which the cell divides.
The first indication that sex chromosomes were distinct from other chromosomes came from experiments conducted by German biologist Hermann Henking in 1891. While using a light microscope to study sperm formation in wasps, Henking noticed that some wasp sperm cells had 12 chromosomes, while others had only 11 chromosomes. Also, during his observation of the stages of meiosis leading up to the formation of these sperm cells, Henking noticed that the mysterious twelfth chromosome looked and behaved differently than the other 11 chromosomes. Accordingly, he named the twelfth chromosome the "X element" to represent its unknown nature. Interestingly, when Henking used a light microscope to study egg formation in female grasshoppers, he was unable to spot the X element.
Based on his observations, Henking hypothesized that this extra chromosome, the X element, must play some role in determining the sex of insects. However, he was unable to gather any direct evidence to support his hypothesis.
Before Her Time
• The Life of Nettie Stevens
Figure 2: The darkling beetle, Tenebrio molitor.
More than a decade after Henking's work, Nettie Stevens surveyed multiple beetle species and examined the inheritance patterns of their chromosomes. In 1905, while studying the gametes of the beetle Tenebrio molitor(Figure 2), Stevens noted an unusual-looking pair of chromosomes that separated to form sperm cells in the male beetles. Based on her comparisons of chromosome appearance in cells from male and female beetles, Stevens proposed that these accessory chromosomes were related to the inheritance of sex.
Over time, other scientists studied the appearance of chromosomes in a wide variety of animal species, and it became clear that there was a relationship between the physical appearance and number of chromosomes in gametes and somatic cells from males and females of a given species.
The variety of sex determination systems
Figure 3: Example set of male human chromosomes. In the image, the X and Y chromosomes are indicated by arrows.
In humans, females inherit an X chromosome from each parent, whereas males always inherit their X chromosome from their mother and their Y chromosome from their father. Consequently, all of the somatic cells in human females contain two X chromosomes, and all of the somatic cells in human males contain one X and one Y chromosome (Figure 3). The same is true of all other placental mammals — males produce X and Y gametes, and females produce only X gametes (Figure 4). In this system, referred to as the XX-XY system, maleness is determined by sperm cells that carry the Y chromosome.
Figure 4: Sex determination in humans.
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Figure 5: Sex determination in insects.
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Many people do not realize, however, that the XX-XY sex determination system is only one of a variety of such systems within the animal kingdom. In fact, sex determination can be very different between different organisms. For example, in the XX-XO system found in crickets, grasshoppers, and some other insects, sperm cells that lack an X chromosome (referred to as O) determine maleness. Here, females carry two X chromosomes (XX) and only produce gametes with X chromosomes. Males, on the other hand, carry only one X chromosome (XO) and produce some gametes with X chromosomes and some gametes with no sex chromosomes at all (Figure 5).
Figure 6: Sex determination in birds.
Figure Detail
Despite the previous examples, males are not always the sex with the mismatched chromosome pair. For example, the ZZ-ZW sex determination system used in birds, snakes, and some insects relies upon females to carry the mismatched chromosome pair (ZW) and males to carry the identical pair (ZZ) (Figure 6).
If the three systems discussed above are compared in side-by-side Punnett squares (Figure 7), it is easy to see that sex determination is simply a matter of gamete assortment. Determinations of male and female character arise from a variety of different gamete combination patterns, all of which are the result of gender coding in sexually reproducing organisms.
Figure 7: A side-by-side comparison of sex determination systems in humans, insects, and birds.
Figure Detail
More on sex determination
• In some animals, sex can be determined by environmental conditions
• Sex Determination in Honeybees
• Scientists report sex reversal in a transgenic mous
The variety of inheritance patterns described in this article illustrate that sex determination is a complex and varied feature among organisms. The XX-XY, XX-XO, and ZZ-ZW systems are only a sample of the wide variety of sex determination systems that scientists have documented in the wide world of living beings, however.
The cry of "It's a boy" or "It's a girl" marks the newborn child's first and most basic label of personal identity. But researchers' understanding of sex is undergoing profound and surprising changes due to new insights gained from sociology, biology, and medicine. The differences between females and males, once believed black and white--or pink and blue--now appear like a blurred rainbow of confusion. Researchers are learning, for example, that the Y chromosome has degenerated over the centuries. They have found that, in mice, some genes involved in early stages of sperm production are on the female X chromosome; and they have identified the gene that can produce ambiguous genitalia.
Genetic studies are revealing that men and women are more similar than distinct. So far, of the approximately 31,000 genes in the human genome, men and women differ only in the two sex chromosomes, X and Y, and only a few dozen genes seem to be involved. Moreover, it's now known that the Y has only about 30 genes and many of those are involved in basic housekeeping duties or in regulating sperm production. The X has hundreds of genes with a vast array of roles.
Strong evidence exists that these two chromosomes were once a matching pair of Xs, says Jennifer Graves, a genetics researcher at La Trobe University, in Australia. According to Graves, it's unclear why the male sex chromosome, the Y, shrunk and shed most of its genes over time. Humans are not alone in this. The Y chromosome's degeneration is well documented in fruit flies and "is clearly an ongoing process in all animals," says Sherman Silber, a medical doctor and director of the Infertility Center of St Louis.1
Past assumptions regarding these sex chromosomes are being challenged: It's recently been discovered that in mice, nearly half of all genes involved in the earliest stages of sperm production are found on the X. "Scientists and non-scientists alike are comfortable thinking about the Y chromosome as a specialist in male characteristics," says David Page, who headed the discovery team at the Whitehead Institute for Biomedical Research in Boston.2 "By default, we've traditionally thought of the X chromosome as sexually neutral or as a specialist in female characteristics," Page says. "Our findings indicate that the X chromosome has a specialty in sperm production, much like the Y chromosome does."
Sex-Determining Genes
Detailed molecular and embryological studies are revealing how genes determine the anatomical sex of a fetus and how that process can and does go awry. Andrew Sinclair, who now heads the Centre for Hormone Research, University of Melbourne, was part of the British-based team led by Peter Goodfellow, of the Imperial Cancer Research Fund, which in 1990 discovered a crucial gene, known as SRY.3 It usually occurs only on the male Y chromosome. Using this discovery, researchers at Britain's National Institute for Medical Research then showed that a fertilized female mouse egg will become male when injected with SRY. The animal in question, named Randy, was the first sex-reversed mouse ever produced in this way, Sinclair says. The testes were small but Randy otherwise grew up male in every discernible way, despite being conceived as a genetic female. Placed in a cage with some females, Randy behaved in a typically male-mouse way, mating up to six times a night. "He thought he was male, they thought he was male, and we thought that was pretty good evidence," Sinclair says. "It tells you that SRY is the only gene you need on the Y chromosome to develop testes and become male."
But other genes complicate the process of sex determination. One known as DAX1, for example, is thought to act as an anti-testis gene, promoting ovary development. Another, called SOX9, combines with SRY to promote the formation of testicular cells in a male embryo. A third gene known as WNT4 and found on chromosome 1, seems to prevent the development of Leydig cells in male testes.4 Researchers led by Eric Vilain, assistant professor of human genetics, University of California, Los Angeles, recently found that when WNT4 occurs twice it can convert an embryo from male to female, often resulting in ambiguous genitalia.5
Neutral Starting Position
It is often argued that being female is the default state for mammals; that an embryo will develop in a female way unless male genes impose themselves on the process. It now seems just as possible, however, that the default state is merely a sexless one, a case of dual potential. In the first six weeks after conception, a human embryo develops a simple gonad that is neither a testis nor an ovary, but more of a neutral sex bud that can bloom either way. The bud has two parts, the medulla and cortex. At about seven weeks, when the fetus is thumbnail size, the SRY gene, if it is present, starts to work its magic. It switches on and the cells in the medulla start to multiply and form into a testis, while the cells in the cortex regress and virtually disappear.
Every significant distinction between men and women apparently stems from that event. As Graves puts it: "Of all the differences between male and female mammals, the primary one seems to be the development of the testis in males. Early in development the mammalian embryo is ready for anything. Male and female embryos are morphologically indistinguishable ... and the embryo is equipped with both male and female internal ducting." The development of the testis triggers a cascade of hormone-controlled changes, so once this decision--testis or no testis--has been taken, the sex of the embryo is determined. Within a week the new testes soon start producing their lifelong supply of testosterone, which floods the embryo and kicks off the development of typical masculine physical traits. If no SRY gene is present, the embryonic gonad waits until the 13th week of gestation to commit itself to femaleness. The medulla shrinks away and the cortex develops into an ovary, which starts to produce estrogen and feminizes the rest of the body.
The differentiation process in the womb can be affected, however, by genetic problems, infections, or exposure to toxins, drugs, or maternal hormones. In that context, it's remarkable to note that SRY's discovery was made possible with the help of French scientists who identified four unusual men who had sought treatment for infertility, Sinclair says. Chromosome tests revealed that they were in fact genetic females, with an XX female sex-chromosome pattern. Yet all four were anatomically male and had testes. Further studies showed that they had a small fragment of Y chromosome containing the SRY gene, tacked onto one of their X chromosomes. It was just a tiny glitch, enough to reverse their sexual anatomy but not endow them with other male genes to enable their testes to make sperm.
Many other such variants are now being discovered, along with true hermaphrodites and pseudo-hermaphrodites. There are people with missing sex chromosomes, extra sex chromosomes, or tiny genetic fragments tacked onto or missing from chromosomes that seem to have nothing to do with sex determination.
Delving into the very fundamentals of sexual genetics and biochemistry and making personal contact with people whose sex and gender are ambiguous has made Sinclair see the whole issue in a fresh light. "I think humans like things to be ordered, and they get bothered about gray areas and when things become less clear-cut," he says. "But these days I don't think so much in black and white about male and female. Now I think of it all as being on a spectrum."
Genders of plants -
Angiosperms, or flowering plants as they are commonly known, dominate the plant kingdom with over a whopping 3.5 lakh species. Unlike the bisexual plants that are predominant, where both the reproductive structures are present in the same flower of the plant, some have the male and female flowers in different plants. Such plants are called dioecious plants, and there are over 15,600 species of them across the world. Many of these plants have been used traditionally as food, medicine and timber for over thousands of years. In a recent study, researchers from the University of Oslo, Norway, and The Institute of Trans-Disciplinary Health Sciences and Technology, Bangalore, have explored some aspects of the bond between folk medicine and the sex of the plants used for the same.
The earliest record of medicinal herbs such as opium come from the Sumerian civilisation, followed by the Chinese who used ginseng and cinnamon bark as food and in the preparation of medicines. Excavations in the Gangetic region in central India present evidence for the use of medicinal plants since 4000 years. The Vedas too have a record of spices such as nutmeg, cloves and pepper used as medicinal plants.
In the study published in the Journal of Ethnopharmacology, the researchers have documented the knowledge and use of dioecious plants by Malayalis—a hill-dwelling ethnic group living in the Eastern Ghats of Tamil Nadu. They have pondered on questions like—do folk healers have a preference for plant genders; if so, how are these gender differences in plants perceived; and how the Indian systems of medicine have documented the concept of plant gender and preferences for it.
“The Indian Systems of medicine (ISM) is one among the codified systems of medicine in the world. We need to validate the traditional knowledge of medicinal plants and its formulation via modern scientific tools”, says Mr Gopalakrishnan Saroja Seethapathy, a research scholar at the University of Oslo and an author of the study. “The world herbal trade is expected to reach USD HYPERLINK "https://www.sciencedirect.com/science/article/pii/S221080331100042X"7 HYPERLINK "https://www.sciencedirect.com/science/article/pii/S221080331100042X"trillion by the year HYPERLINK "https://www.sciencedirect.com/science/article/pii/S221080331100042X"2050. Although China has outperformed India in this, in HYPERLINK "HYPERLINK%20%22https://economictimes.indiatimes.com/news/politics-and-nation/chinas-export-of-herbal-products-is-a-challenge-for-us-narendra-modi/articleshow/30995997.cms%222014"HYPERLINK "https://economictimes.indiatimes.com/news/politics-and-nation/chinas-export-of-herbal-products-is-a-challenge-for-us-narendra-modi/articleshow/30995997.cms" HYPERLINK "HYPERLINK%20%22https://economictimes.indiatimes.com/news/politics-and-nation/chinas-export-of-herbal-products-is-a-challenge-for-us-narendra-modi/articleshow/30995997.cms%222014"2014, the Prime Minister of India admitted this as a challenge and emphasized the need to find ways to promote Ayurvedic medicines”, he adds, describing the current surge in the interest in alternative systems of medicines.
The researchers started off by compiling a comprehensive list of dioecious species used in traditional Indian medicine with data from the Indian Medicinal Plant Database. They then held interviews with folk healers—unlicensed practitioners of healing using conventional practices—and found that they have a gender preference for 13 species of plants including ten trees, two woody vines and one shrub. They differentiated the genders by looking at the plant size, and the presence and size of fruits. The interactions between the researchers and Ayurvedic doctors revealed that although Ayurvedic literature does show evidence of gender preference for medicine, the concept of gender among plants is mentioned in classical works like Charaka Samhita, Vriksh Ayurveda, and Raja Nighantu.
In several instances, the researchers noted that the use of the plants determined the preference of a particular gender. For example, in the case of toddy (palm wine), folk healers prefer female plants as they yield more toddy than male plants. Similarly, the timber of male palm trees like Borassus flabellifer (Palmyra palm) and Drypetes sepiaria (Hedge boxwood) is preferred for constructing houses, huts and making furniture because the wood is of the required size and is more durable than the timber from female trees. On the contrary, the wood of female Diospyros ebenum (Ceylon ebony) is preferred as carving the wood from the male plant is tough.
However, this sexual preference comes at a cost, say the researchers, who point out the threat to the existence of such dioecious species in a region. Also, selective logging of plants of a particular gender owing to demand can result in over-exploitation. As an attempt to conserve dioecious species that are heavily used, several conservation efforts are in place. “There are numerous schemes where Government of India supports the cultivation of medicinal plants. Cultivation is one way, and the other is creating awareness among plant collectors/harvesters about sustainable harvesting”, says Mr Seethapathy.
The study of plant diversity and the practical uses of plants through the knowledge of local people (ethnobotany) contribute to the fields of drug discovery and drug development immensely. These studies highlight the rate at which this valued knowledge is being forgotten and lost. As a boost to the field of ethnobotany, scientists around the world are attempting detailed molecular studies of male and female plants. The current research is a step in the same direction.
“One aspect is to devise measurements of selective logging by people and conservation of dioecious plants. The other is a metabolomics approach for dioecious plants to understand if there is any phytochemical variation between male and female plants. If true, it would be interesting to know the active compounds that vary between male and female plants and study its bioactivity”, remarks Mr Seethapathy before signing off.
https://researchmatters.in/news/study-shows-gender-matters-good-plants