Double Fertilisation

After landing on a receptive stigma, a pollen grain absorbs moisture and germinates; that is, it produces a pollen tube that extends down between the cells of the style toward the ovary.

The nucleus of the generative cell divides by mitosis and forms two sperm. Directed by a chemical attractant, possibly calcium, the tip of the pollen tube enters the ovary, probes through the micropyle (a gap in the integuments of the ovule), and discharges its two sperm near or within the embryo sac.

The events that follow are a distinctive feature of the angiosperm life cycle. One sperm fertilises the egg to form the zygote. The other sperm combines with the two polar nuclei to form a triploid (3n) nucleus in the centre of the large central cell of the embryo sac. This large cell will give rise to the endosperm, a food–storing tissue of the seed. The union of two sperm cells with different nuclei of the embryo sac is called double fertilisation. Double fertilisation ensures that the endosperm will develop only in ovules where the egg has been fertilized, thereby preventing angiosperms from squandering nutrients.

The tissues surrounding the embryo sac have prevented researchers from being able to directly observe fertilization in plants grown under normal conditions. Recently, however, scientists have isolated sperm from germinated pollen grains and eggs from embryo sacs and have observed the merging of plant gametes in vitro (in an artificial environment). The first cellular event that takes place after gamete fusion is an increase in the cytoplasmic calcium (Ca2+) levels of the egg, as also occurs during animal gamete fusion. Another similarity to animals is the establishment of a block to polyspermy, the fertilization of an egg by more than one sperm cell. Thus, maize (Zea mays ) sperm cannot fuse with zygotes in vitro. In maize, this barrier to polyspermy is established as early as 45 seconds after the initial sperm fusion with the egg.

From Ovule to Seed

After double fertilisation, each ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s). As the embryo develops from the zygote, the seed stockpiles proteins, oils, and starch to varying extents, depending on the species. This is why seeds are such major sugar sinks. Initially, these nutrients are stored in the endosperm, but later in seed development in many species, the storage function of the endosperm is more or less taken over by the swelling cotyledons of the embryo.

Endosperm Development

Endosperm development usually precedes embryo development. After double fertilisation, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” having a milky consistency. This liquid mass, the endosperm, becomes multicellular when cytokinesis partitions the cytoplasm by forming membranes between the nuclei. Eventually, these “naked” cells produce cell walls, and the endosperm becomes solid. Coconut “milk” is an example of liquid endosperm; coconut “meat” is an example of solid endosperm. The white fluffy part of popcorn is also solid endosperm.

In grains and most other monocots, as well as many eudicots, the endosperm stores nutrients that can be used by the seedling after germination. In other eudicots (including bean seeds), the food reserves of the endosperm are completely exported to the cotyledons before the seed completes its development; consequently, the mature seed lacks endosperm.

Double Fertilisation

Development of angiosperm gametophytes and the process of pollination

Within the microsporangia (pollen sacs) of an anther are many diploid cells called microsporocytes, also known as microspore mother cells.

Each microsporocyte undergoes meiosis, forming four haploid microspores, each of which can eventually give rise to a haploid male gametophyte.

A microspore undergoes mitosis and cytokinesis, producing two separate cells called the generative cell and tube cell. Together, these two cells and the spore wall constitute a pollen grain, which at this stage of its development is an immature male gametophyte. The spore wall usually exhibits an elaborate pattern unique to the particular plant species. During maturation of the male gametophyte, the generative cell passes into the tube cell. The tube cell now has a completely free–standing cell inside it (the generative cell). The tube cell produces the pollen tube, a structure essential for sperm delivery to the egg. During elongation of the pollen tube, the generative cell usually divides and produces two sperm cells, which remain inside the tube cell. The pollen tube grows through the long style of the carpel and into the ovary, where it then releases the sperm cells in the vicinity of an embryo sac.

One or more ovules, each containing a megasporangium, form within the chambers of the ovary. One cell in the megasporangium of each ovule, the megasporocyte (or megaspore mother cell), grows and then goes through meiosis, producing four haploid megaspores.

The details of the next steps vary extensively, depending on the species. In most angiosperm species, only one megaspore survives. This megaspore continues to grow, and its nucleus divides by mitosis three times without cytokinesis, resulting in one large cell with eight haploid nuclei. Membranes then partition this mass into a multicellular female gametophyte—the embryo sac. At one end of the embryo sac are three cells: the egg cell and two cells called synergids. The synergids flank the egg cell and function in the attraction and guidance of the pollen tube to the embryo sac. At the opposite end of the embryo sac are three antipodal cells of unknown function. The remaining two nuclei, called polar nuclei, are not partitioned into separate cells but instead share the cytoplasm of the large central cell of the embryo sac. The ovule, which will eventually become a seed, now consists of the embryo sac and two surrounding integuments (layers of protective sporophytic tissue that eventually develop into the seed coat).

Pollination, the transfer of pollen from anther to stigma, is the first step in a chain of events that can lead to fertilisation. This step is accomplished in various ways. In some angiosperms, including grasses and many trees, wind is a pollinating agent. In such plants, the release of enormous quantities of pollen compensates for the randomness of this dispersal mechanism. At certain times of the year, the air is loaded with pollen grains, as anyone plagued with pollen allergies can attest. Some aquatic plants rely on water to disperse pollen. Most angiosperms, however, depend on insects, birds, or other animals to transfer pollen directly to other flowers.

The reproductive cycle of the human female

The Ovarian Cycle. 1 The cycle begins with the release from the hypothalamus of GnRH, which 2 stimulates the pituitary to secrete small amounts of FSH and LH. 3 The FSH (true to its name) stimulates follicle growth, aided by LH, and 4 the cells of the growing follicles start to make oestrogen. Notice in Figure(d) that there is a slow rise in the amount of oestrogen secreted during most of the follicular phase, the part of the ovarian cycle during which follicles are growing and oocytes maturing. (Several follicles begin to grow with each cycle, but usually only one matures; the others disintegrate.) The low levels of oestrogen inhibit secretion of the pituitary hormones, keeping the levels of FSH and LH relatively low.

The levels of FSH and LH, however, shoot up sharply when 5 the secretion of oestrogen by the growing follicle begins to rise steeply. Whereas a low level of oestrogen inhibits the secretion of pituitary gonadotropins, a high concentration has the opposite effect: It stimulates the secretion of gonadotropins by acting on the hypothalamus to increase its output of GnRH. 6 You can see this response in Figure (b) as steep increases in FSH and LH levels that occur soon after the increase in the concentration of oestrogen, indicated in Figure (d). The effect is greater for LH because the high concentration of oestrogen also increases the sensitivity of LH–releasing cells in the pituitary to GnRH. By now, the follicles can respond more strongly to LH because more of their cells have receptors for this hormone. The increase in LH concentration caused by increased oestrogen secretion from the growing follicle is an example of positive feedback. The LH induces final maturation of the follicle. 7 The maturing follicle develops an internal fluid–filled cavity and grows very large, forming a bulge near the surface of the ovary. The follicular phase ends, about a day after the LH surge, with ovulation: The follicle and adjacent wall of the ovary rupture, releasing the secondary oocyte.

8 Following ovulation, during the luteal phase of the ovarian cycle, LH stimulates the transformation of the follicular tissue left behind in the ovary to form the corpus luteum, a glandular structure (c). (LH is named for this “luteinising” function.) Under continued stimulation by LH during this phase of the ovarian cycle, the corpus luteum secretes progesterone and estrogen (see Figure(d). As the levels of progesterone and oestrogen rise, the combination of these hormones exerts negative feedback on the hypothalamus and pituitary, inhibiting the secretion of LH and FSH. Near the end of the luteal phase, the corpus luteum disintegrates, causing concentrations of estrogen and progesterone to decline sharply. The dropping levels of ovarian hormones liberate the hypothalamus and pituitary from the inhibitory effects of these hormones. The pituitary can then begin to secrete enough FSH to stimulate the growth of new follicles in the ovary, initiating the next ovarian cycle.

The Uterine (Menstrual) Cycle. The hormones secreted by the ovaries—oestrogen and progesterone—have a major effect on the uterus. Oestrogen secreted in increasing amounts by growing follicles signals the endometrium to thicken. In this way, the follicular phase of the ovarian cycle is coordinated with the proliferative phase of the uterine cycle (see Figure (e ). Before ovulation, the uterus is already being prepared for a possible embryo. After ovulation, 9 oestrogen and progesterone secreted by the corpus luteum stimulate continued development and maintenance of the endometrium, including enlargement of arteries and growth of endometrial glands. These glands secrete a nutrient fluid that can sustain an early embryo even before it actually implants in the uterine lining. Thus, the luteal phase of the ovarian cycle is coordinated with what is called the secretory phase of the uterine cycle.

10 The rapid drop in the level of ovarian hormones when the corpus luteum disintegrates causes spasms of the arteries in the uterine lining that deprive it of blood. The upper two–thirds of the endometrium disintegrates, resulting in menstruation—the menstrual flow phase of the uterine cycle—and the beginning of a new cycle. By convention, the first day of menstruation is designated day 1 of the uterine (and ovarian) cycle. Menstrual bleeding usually persists for a few days. During menstruation, a fresh batch of ovarian follicles are just beginning to grow.

Cycle after cycle, the maturation and release of egg cells from the ovary are integrated with changes in the uterus, the organ that must accommodate an embryo if the egg cell is fertilised. If an embryo has not implanted in the endometrium by the end of the secretory phase of the uterine cycle, a new menstrual flow commences, marking day 1 of the next cycle.

In addition to the roles of oestrogen in coordinating the female reproductive cycle, this hormone family is responsible for the secondary sex characteristics of the female. Oestrogen induces deposition of fat in the breasts and hips, increases water retention, affects calcium metabolism, stimulates breast development, and influences female sexual behaviour.

Menopause. After about 450 cycles, human females undergo menopause, the cessation of ovulation and menstruation. Menopause usually occurs between the ages of 46 and 54. Apparently, during these years the ovaries lose their responsiveness to gonadotropins from the pituitary (FSH and LH), and menopause results from a decline in estrogen production by the ovary. Menopause is an unusual phenomenon; in most species, females as well as males retain their reproductive capacity throughout life. Is there an evolutionary explanation for menopause? Why might natural selection have favoured females who had stopped reproducing? One intriguing hypothesis proposes that during early human evolution, undergoing menopause after having some children actually increased a woman′s fitness; losing the ability to reproduce allowed her to provide better care for her children and grandchildren, thereby increasing the survival of individuals bearing her genes.

Spermatogenesis and Oogenesis

In humans and other mammals, a complex interplay of hormones regulates gametogenesis

How exactly are gametes produced in the mammalian body? The process, gametogenesis, is based on meiosis, but details differ in females and males. Oogenesis, the development of mature ova (egg cells). Spermatogenesis, the production of mature sperm cells, is a continuous and prolific process in the adult male. Each ejaculation of a human male contains 100 to 650 million sperm cells, and males can ejaculate daily with little loss of fertilising capacity. Spermatogenesis occurs in the seminiferous tubules of the testes.

Oogenesis differs from spermatogenesis in three major ways. First, during the meiotic divisions of oogenesis, cytokinesis is unequal, with almost all the cytoplasm monopolized by a single daughter cell, the secondary oocyte. This large cell can go on to become the ovum; the other products of meiosis, smaller cells called polar bodies, degenerate. By contrast, in spermatogenesis, all four products of meiosis develop into mature sperms. Second, although the cells from which sperm develop continue to divide by mitosis throughout the male′s life, this is thought not to be the case for oogenesis in the human female. Third, oogenesis has long “resting” periods, in contrast to spermatogenesis, which produces mature sperm from precursor cells in an uninterrupted sequence.

Genetic (Introduction)

Genetic 1

Eyes of brown, blue, green, or gray; hair of black, brown, blond, or red—these are just a few examples of heritable variations that we may observe among individuals in a population. What genetic principles account for the transmission of such traits from parents to offspring?

One possible explanation of heredity is a “blending” hypothesis, the idea that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paints blend to make green. This hypothesis predicts that over many generations, a freely mating population will give rise to a uniform population of individuals. However, our everyday observations and the results of breeding experiments with animals and plants contradict such a prediction. The blending hypothesis also fails to explain other phenomena of inheritance, such as traits reappearing after skipping a generation.

An alternative to the blending model is a “particulate” hypothesis of inheritance: the gene idea. According to this model, parents pass on discrete heritable units—genes—that retain their separate identities in offspring. An organism’s collection of genes is more like a deck of cards or a bucket of marbles than a pail of paint. Like cards and marbles, genes can be sorted and passed along, generation after generation, in undiluted form.

Alleles, alternative versions of a gene. A somatic cell has two copies of each chromosome (forming a homologous pair) and thus two alleles of each gene, which may be identical or different. This figure depicts an F1 pea hybrid with an allele for purple flowers, inherited from one parent, and an allele for white flowers, inherited from the other parent

Mendel’s law of segregation. This diagram shows the genetic makeup of the generations in Figure 14.3. It illustrates Mendel’s model for inheritance of the alleles of a single gene. Each plant has two alleles for the gene controlling flower colour, one allele inherited from each parent. To construct a Punnett square, list all the possible female gametes along one side of the square and all the possible male gametes along an adjacent side. The boxes represent the offspring resulting from all the possible unions of male and female gametes.

An organism having a pair of identical alleles for a character is said to be homozygous for the gene controlling that character. A pea plant that is true–breeding for purple flowers (PP ) is an example. Pea plants with white flowers are also homozygous, but for the recessive allele (pp ). If we cross dominant homozygotes with recessive homozygotes, as in the parental (P generation) cross, every offspring will have two different alleles—Pp in the case of the F1 hybrids of our flower–colour experiment. An organism that has two different alleles for a gene is said to be heterozygous for that gene. Unlike homozygotes, heterozygotes are not true–breeding because they produce gametes with different alleles—for example, P and p in the F1 hybrids. As a result, those F1 hybrids produce both purple–flowered and white–flowered offspring when they self–pollinate.

Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition. Therefore, we distinguish between an organism’s traits, called its phenotype , and its genetic makeup, its genotype . In the case of flower colour in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes

The Law of Independent Assortment

Mendel derived the law of segregation by performing breeding experiments in which he followed only a single character, such as flower color. All the F1 progeny produced in his crosses of true–breeding parents were monohybrids , meaning that they were heterozygous for one character. We refer to a cross between such heterozygotes as a monohybrid cross.

Mendel identified his second law of inheritance by following two characters at the same time. For instance, two of the seven characters Mendel studied were seed colour and seed shape. Seeds may be either yellow or green. They also may be either round (smooth) or wrinkled. From single–character crosses, Mendel knew that the allele for yellow seeds is dominant (Y ) and that the allele for green seeds is recessive (y ). For the seed–shape character, the allele for round is dominant (R ), and the allele for wrinkled is recessive (r).

Imagine crossing two true–breeding pea varieties differing in both of these characters—a parental cross between a plant with yellow–round seeds (YYRR ) and a plant with green–wrinkled seeds (yyrr ). The F1 plants will be dihybrids , heterozygous for both characters (YyRr ). But are these two characters, seed color and seed shape, transmitted from parents to offspring as a package? Put another way, will the Y and R alleles always stay together, generation after generation? Or are seed colour and seed shape inherited independently of each other?

The figure illustrates how a dihybrid cross, a cross between F1 dihybrids, can determine which of these two hypotheses is correct.

The F1 plants, of genotype YyRr, exhibit both dominant phenotypes, yellow seeds with round shapes, no matter which hypothesis is correct. The key step in the experiment is to see what happens when F1 plants self–pollinate and produce F2 offspring. If the hybrids must transmit their alleles in the same combinations in which they were inherited from the P generation, then there will only be two classes of gametes: YR and yr. This hypothesis predicts that the phenotypic ratio of the F2 generation will be 3:1, just as in a monohybrid cross.

The alternative hypothesis is that the two pairs of alleles segregate independently of each other. In other words, genes are packaged into gametes in all possible allelic combinations, as long as each gamete has one allele for each gene. In our example, four classes of gametes would be produced by an F1 plant in equal quantities: YR, Yr, yR, and yr. If sperm of the four classes are mixed with eggs of the four classes, there will be 16 (4 × 4) equally probable ways in which the alleles can combine in the F2 generation, as shown in the Punnett square. These combinations make up four phenotypic categories with a ratio of 9:3:3:1 (nine yellow–round to three green–round to three yellow–wrinkled to one green–wrinkled). When Mendel did the experiment and “scored” (classified) the F2 offspring, his results were close to the predicted 9:3:3:1 phenotypic ratio, supporting the hypothesis that each character—seed color or seed shape—is inherited independently of the other character.

Mendel tested his seven pea characters in various dihybrid combinations and always observed a 9:3:3:1 phenotypic ratio in the F2 generation. Notice in Figure 14.8, however, that, if you consider the two characters separately, there is a 3:1 phenotypic ratio for each: three yellow to one green; three round to one wrinkled. As far as a single character is concerned, the alleles segregate as if this were a monohybrid cross. The results of Mendel’s dihybrid experiments are the basis for what we now call the law of independent assortment , which states that each pair of alleles segregates independently of other pairs of alleles during gamete formation.

Strictly speaking, this law applies only to genes (allele pairs) located on different chromosomes—that is, on chromosomes that are not homologous. Genes located near each other on the same chromosome tend to be inherited together and have more complex inheritance patterns than predicted by the law of independent assortment. All the pea characters studied by Mendel were controlled by genes on different chromosomes (or behaved as though they were); this fortuitous situation greatly simplified interpretation of his multi–character pea crosses.