This form of parthenogenesis is more common in plants. For most organisms that reproduce the first way, through automixis, the offspring typically gain two X chromosomes from their mother. Two X chromosomes, the primary sex-linked genetic storehouse, give rise to only female offspring. But on rare occasions, animals such as aphids can produce fertile male offspring that are genetically identical to their mother except for lacking a second X chromosome.
These males are usually fertile, but because they are only able to produce sperm containing X chromosomes, all their offspring will be female. For millions of years animals have reproduced via parthenogenesis, which first emerged in some of the smallest, simplest organisms.
For more advanced animals like vertebrates, scientists think that the ability to reproduce asexually came about as a last-ditch effort for species facing adverse conditions. That may explain why parthenogenesis is possible in so many desert and island species.
Most animals that procreate through parthenogenesis are small invertebrates such as bees, wasps, ants, and aphids, which can alternate between sexual and asexual reproduction. Parthenogenesis has been observed in more than 80 vertebrate species, about half of which are fish or lizards.
No mammals are known to reproduce this way because unlike simpler organisms, mammals rely on a process called genomic imprinting. Like a molecular stamp, imprinting labels which genes are from mom and which are from dad. For mammals such as humans, this means that certain genes are switched on or off depending on the contributing parent. If there were only a single parent, some genes would fail to activate altogether, making viable offspring impossible. In some very rare cases, animal species reproduce via parthenogenesis exclusively.
One such species is the desert grassland whiptail lizard , all of which are female. In certain insects, salamanders, and flatworms, the presence of sperm serves to trigger parthenogenesis.
Sperm cells launch the process by penetrating the egg, but the sperm later degenerates, leaving only the maternal chromosomes. The ability to reproduce asexually allows animals to pass on their genes without spending energy finding a mate, and so can help sustain a species in challenging conditions.
If a Komodo dragon arrives on an uninhabited island, for example, she alone could create a population through parthenogenesis. However, because every individual would be genetically identical, Komodo dragon mothers and their daughters would be more vulnerable to disease and environmental changes than a genetically-varied group.
In areas of New Mexico, for example, some populations of female whiptail lizards share nearly-identical genetic profiles. All rights reserved. That resembles conceptualizations of sex from the s! Compared with gynogenesis, androgenesis is extremely rare, only known from a few insects and molluscs. The bottom line is that, except for mammals, asexual reproduction is quite common, but is often really self-sexual reproduction. This article is republished from The Conversation under a Creative Commons license.
Interested in writing a piece? Please contact Steven Reid or sign up to become an author. Karim Karim spends much of his time out of the public eye - teaching, researching and writing. It's not often that researchers in psychology and geochemistry share the same spotlight.
Read on to read up on two noteworthy research Despite treatment, she passed away By Root Gorelick. But, as we will see, in many species, only meiosis is needed. Some form of sex It seems that all animals eventually need to engage in some form of sex. No sperm required We often only consider one egg nucleus fusing with one sperm nucleus to form a zygote nucleus and the other nuclei are discarded.
Some salamanders that have only one biological sex reproduce asexually. Shutterstock As another alternative, a spontaneous doubling of all chromosomes can occur just before meiosis, in which case the four egg nuclei at the end of meiosis will have the normal number of chromosomes and therefore one of them can form a zygote without fertilization. Sexual reproduction is the combination of reproductive cells from two individuals to form genetically unique offspring.
The nature of the individuals that produce the two kinds of gametes can vary, having for example separate sexes or both sexes in each individual. Sex determination, the mechanism that determines which sex an individual develops into, also can vary. Hermaphroditism occurs in animals in which one individual has both male and female reproductive systems. Invertebrates such as earthworms, slugs, tapeworms, and snails Figure Hermaphrodites may self-fertilize, but typically they will mate with another of their species, fertilizing each other and both producing offspring.
Self-fertilization is more common in animals that have limited mobility or are not motile, such as barnacles and clams. Many species have specific mechanisms in place to prevent self-fertilization, because it is an extreme form of inbreeding and usually produces less fit offspring.
Mammalian sex is determined genetically by the combination of X and Y chromosomes. In mammals, the presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants. Bird sex determination is dependent on the combination of Z and W chromosomes.
Notice that this system is the opposite of the mammalian system because in birds the female is the sex with the different sex chromosomes. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects such as butterflies and moths , and reptiles use the ZW system. More complicated chromosomal sex determining systems also exist. For example, some swordtail fish have three sex chromosomes in a population.
The sex of some other species is not determined by chromosomes, but by some aspect of the environment. Sex determination in alligators, some turtles, and tuataras, for example, is dependent on the temperature during the middle third of egg development. This is referred to as environmental sex determination, or more specifically, as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females, while in many other species of turtles, the reverse is true.
In some crocodiles and some turtles, moderate temperatures produce males and both warm and cool temperatures produce females. Individuals of some species change their sex during their lives, switching from one to the other. The wrasses, a family of reef fishes, are all sequential hermaphrodites. Some of these species live in closely coordinated schools with a dominant male and a large number of smaller females.
If the male dies, a female increases in size, changes sex, and becomes the new dominant male. The fusion of a sperm and an egg is a process called fertilization.
This can occur either inside internal fertilization or outside external fertilization the body of the female.
Humans provide an example of the former, whereas frog reproduction is an example of the latter. External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization takes place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male s release sperm in the same area, at the same time.
The spawning may be triggered by environmental signals, such as water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans such as crabs and shrimp , mollusks such as oysters , squid, and echinoderms such as sea urchins and sea cucumbers. Frogs, corals, mayflies, and mosquitoes also spawn Figure
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