The consequences of this are far-reaching, in particular because polyandry females mating with multiple males within the same reproductive cycle is quite common in nature. That means the competition is not so much among the sperm within an ejaculate but among the sperm of ejaculates of several males.
Summing up all of the above we can deduce that potential reproductive rates for males are typically higher than those of females. It is energetically cheaper to produce a male gamete than a female gamete.
In general, the sperm stores in males are continually replenished; females, on the contrary, are normally born with a fixed number of ova that cannot be increased, just managed such as by producing an ovum periodically.
Donkeys produce billions of sperm per ejaculate. A male donkey could therefore theoretically father hundreds or thousands of descendants over a lifetime. The reproductive success of a female donkey is limited by the number of ova she produces and the higher effort she puts into rearing the offspring.
The sex that produces small and numerous gametes that is, the male will more-often-than-not have higher potential reproductive rates. The primary consequence of anisogamy is therefore that reproductive competition for mates — i. Sexual selection shapes traits in males that help them to obtain mates and fertilise as many ova as they can, either because these traits e. It also helps to explain why, across species, females are often the sex that tends to be more discerning in their choice of mate.
Of course, none of this is news. The answer to our hypothetical survey is simple: gametes are the difference between males and females. Exactly what a male and female looks like, at the end of the day, and how each behaves their sex roles depends largely on the ecological theatre in which their evolutionary history has been played out.
Festival of Social Science — Aberdeen, Aberdeenshire. Edition: Available editions United Kingdom. Become an author Sign up as a reader Sign in. We use a range of hormone-induced indicators to determine who is male and who is female on a daily basis. European Parliament. Genetic variation is important in allowing a population to adapt via natural selection and thus survive in the long term. Improve this page Learn More.
Skip to main content. Module 8: Cell Division. Search for:. Genetic Variation in Meiosis Learning Outcomes Understand how meiosis contributes to genetic diversity. Try It. Did you have an idea for improving this content? It is known as male gamete or sperm cell.
In humans, each gamete contains 23 chromosomes and their fusion results in the formation of a diploid zygote containing 46 chromosomes. In animals, these reproductive cells are produced in respective male and female gonads or reproductive organs. In seed-bearing plants, the male gametes are the pollen while the female gametes are enclosed in the ovules of the plant. However, in plants, the gamete may or may not be always a haploid cell.
What are examples of gametes? What are the different types of gametes? The gametes involved in fertilization can be similar referred to as isogamy or the two gametes may be different referred to as anisogamy. Isogamy : gametes with similar morphology i.
Examples: gametes of unicellular algae, Chlamydomonas reinhardtii and Carteria palmata. Anisogamy : gametes with dissimilar morphology i. The gamete with a smaller size is known as sperm or male gamete whereas the gamete with a larger size is known as ova or egg or female gamete. Furthermore, these gametes can be motile as well as non-motile. In the case of red algae, Polysiphonia , both the gametes are non-motile. The non-motile sperm fuses with the non-motile egg to form a zygote.
Non-motile male gamete or sperm is known as spermatia. This is also seen in certain flowering plants wherein both the gametes are non-motile and are present in the gametophyte. The non-motile male gamete in plants is known as pollen. Oogamy : in humans and mammals, one of the gametes, male gamete or sperm, is motile and the other gamete, egg, or female gamete is non-motile.
This condition is known as oogamy , wherein a large non-motile egg is fertilized or will fuse with a small and motile sperm to form the zygote. Another way of classification of the gametes is based on their size. Depending on the size, gametes can be categorized as:.
During the process of gametogenesis, a diploid 2n cell undergoes meiosis to produce four haploid n cells. In general, the process of gametogenesis usually starts with gametogonia. Gametogonia are derived from the primordial germ cells PGCs. These germ cells proliferate by the process of mitosis. In the late embryonic stage, these cells are transferred to the gonadal ridge where they are known as gametogonia. Once gametogonia have developed, further gametogenesis will result in the formation of egg or sperm, depending on the sex of the individual.
The process of gametogenesis is completely different in males and females. The male reproductive cell or gamete is known as the sperm cell. In animals, anisogamy is seen, wherein the structure of the male gamete is different from the female gamete. In animals, including humans, sperm is small in size and is motile. The motile organ of the sperm is known as the flagellum.
The sperm cells have a limited life span and cannot divide. The sperms in mammals have two distinct structures enclosed in a single membrane Figure 2. Connecting the two parts, i. Mitochondria is very essential for the sperm cell as it provides all the energy for the movement of the sperm.
Mitochondria produces the required ATP for the movement of the sperms. Apart from the mitochondria, the neck also has centrioles. Sperm is a haploid gamete and in humans, it contains 23 chromosomes. In male humans, the process of spermatogenesis occurs in the testes and is initiated only at the onset of puberty. However, once it starts, spermatogenesis is a continuous process that occurs throughout a lifetime unlike oogenesis.
Spermatogenesis occurs in the tubular structure called seminiferous tubules. Spermatogonia, which are immature germ cells, are positioned on the outer edge of the seminiferous tubules near the basal lamina. These germ cells multiply continuously by the process of mitosis.
Some of these multiplying cells do not proliferate and are converted to primary spermatocytes. These primary spermatocytes then undergo the first meiotic phase wherein, each paired homologous chromosomes contribute to cross-over and eventually undergo division I of meiosis resulting in the formation of two secondary spermatocytes, which contain 22 duplicated autosomal chromosomes it can be a duplicated X or a duplicated Y chromosome.
These secondary spermatocytes then undergo phase II of meiosis resulting in the formation of haploid spermatids , which further undergoes differentiation to form sperms.
Eventually, these sperms then move into the lumen of the seminiferous tubule. Later sperms move into a coiled tube overlying the testes known as the epididymis, where these sperms further mature and are stored in a liquid known as semen. This whole process takes around 70 days and can survive for almost 5 days in the female reproductive tract.
However, outside the body, they may not survive more than a few hours. Sperms can be stored for months or years in a frozen state and retain its capacity to fertilize eggs upon thawing. Sperm acquire energy for motility from the fructose present in the semen.
Plants like, ferns, cycads and ginkgo also have flagellated sperms. Sperms of nematodes are amoeboid. They exhibit crawling movement rather than swimming. The non-motile sperms depend on the environmental conditions for their dispersal and eventually reaching the egg cells, for example, non-motile sperms of Polysiphonia , which is a red alga, spread by water currents after released.
Flies, butterflies, and insects act as a carrier for these non-motile sperms. Sperms have acquired some major adaptations, which makes them efficient cells. Some of these adaptations are as follows:. The function of sperm is to reach the egg and fuse or fertilize it to form a zygote, and in that process, transfer the male genetic material and centriole which eventually determines the microtubule cytoskeleton.
The genetic material of the sperm is responsible for the traits of the progeny like the color of eyes, hair, and skin.
Sperm contains X and Y chromosome that determines the sex of the progeny. The egg cell or the ovum is the ovoid or spherical non-motile gamete produced in the female reproductive structure known as the ovaries. The size of an egg is larger than the sperm. A human egg usually has a diameter of about 0. In fish and frogs, it is aroundmm. Egg or ova is a haploid gamete and in humans, it contains 23 chromosomes. Egg contains cytoplasm which is known as ooplasm.
The cytoplasm of the egg has two parts i. The human egg is described as alecithal as it contains a very small quantity of nutritive yolk. This is a contrasting feature to the eggs of birds wherein the cytoplasm is rich in nutritive yolk which is made up of lipoproteins, pigment granules, and water. The cytoplasm contains the nucleus of the egg, known as the germinal vesicle, and a vacuole known as the germinal spot.
The nucleus of the ovum is large, swollen with nucleoplasm , and is eccentric in position.
0コメント