What wasps can tell us about sex

What wasps can tell us about sex

Researchers at ETH Zurich and the University of Zurich have discovered that a single gene in a particular aphid wasp decides whether the insects reproduce sexually or asexually. This is not only of interest for pest control, but could also help answer a central question of evolutionary biology.

Why does sex exist? Evolutionary biologists have yet to find a satisfactory answer to this simple question. Asexual reproduction would be more “economical” because, with separate sexes reproducing sexually, only some organisms will produce offspring. Yet, in the course of evolution, sexual reproduction has become the predominant mode. Various theories have sought to explain why this is so, but they all have to contend with the problem that they are difficult to test empirically. It is true that certain animal species reproduce both sexually and asexually; however, in these species, individuals of sexual origin generally also differ in other respects from those of asexual origin, so that only limited conclusions can be drawn from direct comparisons.

Christoph Vorburger, SNSF Professor of Evolutionary Ecology at the ETH in Zurich and at Eawag in Dübendorf, and Christoph Sandrock, a doctoral student at Zurich University, have now discovered that a particular species of parasitoid wasp is ideally suited for reproduction studies. The two researchers investigated the aphid parasitoid Lysiphlebus fabarum, which has long been known to be capable of reproducing both sexually and asexually. Vorburger and Sandrock showed that there is virtually no difference between asexual and sexual individuals of this species — apart, that is, from their different modes of reproduction. In other words, the genetic differences between individuals are not greater than would typically be expected within a population.

Laws of inheritance apply

In sexual populations of these wasps, females develop from fertilized eggs and males from unfertilized eggs. In asexual populations, females only produce daughters without fertilization. Vorburger and Sandrock aimed to identify the genetic factors which determine whether a wasp will reproduce sexually or asexually. Surprisingly, it turned out that this fundamental difference is controlled by a single gene. Using crossing experiments, the researchers also demonstrated that the trait is inherited recessively. Exactly 12.5% of the third-generation females were found to reproduce asexually — the precise proportion predicted by Mendel’s laws of inheritance for a recessive trait.

Vorburger and Sandrock do not yet know which gene determines the mode of reproduction. Vorburger explains: “We’ve only been able to show that the trait behaves like a single genetic factor, but we’ve already identified a microsatellite — a genetic marker — which is located close to the responsible gene. We’ll now be carrying out a further study to answer this question.”

More effective method of pest control?

However, these findings are not only of interest to evolutionary biologists: parasitoid wasps such as Lysiphlebus fabarum are used for biological pest control, as they deposit their eggs in aphids, which are then killed by the developing larvae. With sexual populations, only half of the individuals — i.e. the egg-laying females — actually contribute to pest control; this means that the effectiveness of the method could possibly be improved if asexual populations were used. But, as Vorburger points out, “It’s also possible that the method would only be more effective in the short term — specifically, if the wasps can’t adapt to environmental changes.” Asexual reproduction yields genetically uniform lines, and because there is no genetic mixing, adaptation to changing conditions is difficult. Ultimately — many evolutionary biologists believe — it is precisely the ability to adapt which explains why sexual reproduction predominates in nature, even though it appears at first glance to be less economical.

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Materials provided by ETH Zürich. Note: Content may be edited for style and length.


How Do Wasps Reproduce?

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When it comes to wasps, not all insects are created equal. Wasps include some 30,000 identified species, and come in various shapes, sizes and colors. How the insects reproduce depends on their specific genetic makeup and the subgroup in which they live.

While most people may consider wasps dangerous stinging pests, the majority actually are nonstinging and do far more good than harm. For example, most wasps help control problem insect populations that destroy plants and farm land.

Types of Wasps

There are two primary subgroups of wasps: social and solitary. The group in which wasps live determine how wasps will reproduce. Social wasps live in large colonies and are led by a queen. Such wasps make their nests in holes or above ground. Yellow jackets and hornets are two well-known species of social wasps.

Solitary wasps do not form colonies and live alone. The volume of solitary wasps is much larger than social wasps. They include digger wasps, sand wasps and mason wasps, among others.

Social Wasp Reproduction

Most species of social female and male wasps mate once a year. After mating, female wasps hibernate in the ground or in an enclosed space until the winter passes. The males die. In the spring, a fertilized female wasp starts her colony by laying eggs in cell-like pods. Larvae hatche from the eggs and are fed by the female. Adult workers emerge 10 days later and care for the additional eggs. Some female worker wasps are sterile, their sole purpose being to nourish the baby wasps and assist the queen.

As the spring progresses, more eggs, larvae and workers are produced. Near the end of summer, male wasps grow from the unfertilized eggs. Fertile females develop from the well-fed larvae. The insects mate and the process begins again.

Solitary Wasp Reproduction

For wasps that live solitary lives, all females are fertile. Female solitary wasps commonly lay their eggs near a spider or other insect that the wasp has paralyzed with venom. The mother wasp does this so that as larva develops, it can use the insect for food. Some solitary females watch over their nest, while others abandon them.

Male and female solitary wasps mate in the spring. Some male species of such wasps die shortly after mating, while others survive the summer. Unlike social wasps, the majority of solitary wasps, male and female, live through winter as pupae.

The Lifespan of a Wasp

The majority of wasps live one year or less. Worker wasps generally exist for several months, while queen wasps can survive for years. Wasps do not migrate. If temperatures drop, most wasps become dormant until the weather improves.

Social wasps have the ability to live longer than solitary wasps because of their pack power when in danger. Social wasps in trouble emit a pheromone that calls to nearby colony members. The result can be a stinging attack by hundreds or thousands of wasps. As opposed to bees, wasps can sting continuously. While social wasps activate their stingers for defensive purposes, solitary wasps rely on their stingers to hunt and feed their young.


Asexual Reproduction

Bacteria, cyanobacteria, algae, most protozoa, yeast, dandelions, and flatworms all reproduce asexually. When asexual reproduction occurs, the new individuals are called clones, because they are exact duplicates of their parent cells. Mosses reproduce by forming runners that grow horizontally, produce new stalks, and then the runner decomposes, leaving a new plant which is a clone of the original.

Starfish can regenerate and eventually produce a whole new organism from one of its severed appendages.

Phases of mitosis. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Duplication of organisms, whether sexually or asexually, involves the partitioning of the genetic material (chromosomes) in the cell nucleus.

During asexual reproduction, the chromosomes divide by mitosis, which results in the exact duplication of the genetic material into the nuclei of the two daughter cells. Sexual reproduction involves the fusion of two gamete cells (the sperm and ova) which each have half the normal number of chromosomes, a result of reduction division known as meiosis.

Bacteria reproducing asexually double their numbers rapidly, approximately every 20 minutes. This reproduction rate is offset by a high death rate that may be the result of the accumulation of alcohol or acids that concentrate from the bacterial colonies.

Yeasts reproduce asexually by budding, as well as reproducing sexually. In the budding process, a bulge forms on the outer edge of the yeast cell as nuclear division takes place. One of these nuclei moves into the bud, which eventually breaks off completely from the parent cell. Budding also occurs in flatworms, which divide into two and then regenerate to form two new flatworms.

Bees, ants, wasps, and other insects can reproduce sexually or asexually. In asexual reproduction, eggs develop without fertilization, a process called parthenogenesis. In some species the eggs may or may not be fertilized; fertilized eggs produce females, while unfertilized eggs produce males.

There are a number of crop plants which are propagated asexually. The advantage of asexual propagation to farmers is that the crops will be more uniform than those produced from seed. Some plants are difficult to cultivate from seed and asexual reproduction in these plants makes it possible to produce crops that would otherwise not be available for commercial marketing.

The process of producing plants asexually is called vegetative propagation and is used for such crops as potatoes, bananas, raspberries, pineapples, and some flowering plants used as ornamentals. Farmers plant the so-called ” eyes” of potatoes to produce duplicates of the parent. With banana plants, the suckers that grow from the root of the plant are separated and then planted as new ones. With raspberry bushes, branches are bent and covered with soil. They then grow into a separate plant with their own root system and can eventually be detached from the parent plant. A colored transmission electron micrograph (TEM) of the Salmonella typhimurium bacterium reproducing by binary fission. Photograph by Dr. Kari Lounatmaa/Photo Researchers, Inc. Reproduced by permission.


Wasps and bee did it long before Dolly’s day

WHEN the news broke a few weeks ago about Dolly, the cloned sheep, I thought of the day last summer when I called to see my mother. I found her cloning plants in the back garden. Had I been wearing my Superman spectacles, that allow me to see incredibly tiny things, I would have seen that the whole garden was a hive of cloning activity.

Bacteria were cloning themselves all over the place. I would have seen aphids busily cloning themselves by the hedge, and, down in the far corner, the bees and the wasps were also at it. Cloning is “old hat” to Mother Nature.

Biological organisms reproduce either by sexual or asexual methods, and some can employ both. In asexual reproduction, the new [individual (offspring) derives genetic material from a single parent only. In sexual reproduction, the new individual usually derives half its genetic complement from one parent and half from the other parent. The recent cloning of Dolly the sheep, in Scotland, is a special example of artificially contrived reproduction, where all the genetic content of the offspring comes from the mother.

The genetic content of an organism is contained in its chromosomes. A biological clone is defined as an individual that is genetically identical to its single parent.

For example, when simple single celled organisms wish to reproduce, they do so by dividing in two. The chromosomes of the “parent” duplicate prior to cell division, and each daughter cell receives an identical copy. Both daughters are genetically identical to the parent and are therefore clones of the original.

The asexual production of clones is also common in horticulture. The type of cloning my mother was doing involves sticking twigs (“cuttings”), from plants she landed while out for a stroll, into the ground. The cutting takes root and grows into a new plant identical to the one from which it was cut.

Asexual reproduction can also take place in multicellular animals. The more primitive the animal, the more likely it is that asexual reproduction can take place.

For example, you could cut a sponge or a flatworm into small parts, and each part, if kept in its usual environment, would grow into a complete organism. These new organisms could be considered clones of the original.

Many insects, including bees, wasps and aphids, also commonly produce clones of themselves.

Technically this is classified as a form of sexual reproduction, because a sex cell, the egg, is involved. However, the process differs radically from the normal form of sexual reproduction seen in the higher animals.

This special process in insects is called parthenogenesis. In aphids, an unfertilised egg cell containing, only a half set of chromosomes develops into an embryo, and subsequently into an adult insect, without the assistance of any sperm cell. The egg’s half set of chromosomes duplicates itself to produce a full sea the egg divides and continues to divide to become a fully developed organism. All of the chromosomes come from the female parent. So, again we are dealing with a kind of clone.

I once heard a well known Irish feminist declare at a public meeting: “We (women) don’t need men, we can have children by parthenogenesis.” She may have been joking, but she sounded serious. If she was serious, she could scarcely have made a nastier statement.

Could a woman conceive by parthenogenesis? I don’t know the answer to this question. In most forms of life an egg requires activation by a sperm in order to start dividing and to develop into an embryo. However, in several cases it has been shown that it is possible to spark this activation experimentally and to obtain parthenogenetic reproduction.

For example, sea urchin eggs can be stimulated to divide by placing them in strong salt water. Frog eggs have been stimulated similarly by pricking them with “needles. There is a report that “rabbit eggs were also successfully stimulated in this manner. A breed of white turkeys produce eggs (unfertilised) that sometimes develop into embryos when incubated. Some of these embryos eventually hatch and go on to produce adults – turkeys without fathers.

Conventional sexual reproduction is the norm for human beings and for organisms generally that are at our level of complexity. Sexual reproduction is effected by the union of a sperm cell from the father with an egg cell from the mother. The new individual receives an equal amount of genetic material from each.

The genetic material – genes – reside in the chromosomes, located in the nucleus of the cell. Our bodies are composed of cells of two types – somatic cells and germ cells. The somatic cells make up our tissues – including kidney, liver, etc. The germ cells are involved in procreation – sperm cells in the male, and egg cells in the female. Somatic cells contain 23 pairs of chromosomes, one set coming originally from the father and the other set from the mother.

In order for tissues to grow and maintain themselves, somatic cells increase by each dividing into two. Prior to cell division (mitosis), the, chromosomes duplicate themselves, and each daughter cell receives the full complement of 23 pairs of chromosomes.

However, when germ cells are formed, each cell receives, not 23 pairs, but 23 individual chromosomes. Union of the sperm with the egg restores the somatic situation of 23 pairs of chromosomes in the fertilised egg.

The subsequent development into an adult consisting of billions of cells is effected by countless cell divisions beginning with division of the fertilised egg cell. Each somatic cell in the body contains the same 23 pairs of chromosomes which constitute the entire genetic blueprint of the organism.

Cells of different tissues in the body look different from each other and perform different tasks, even though every cell contains identical genetic information. The explanation is that differentiated cells use only part of the entire genetic blue print information. Parts not in use in a particular tissue are switched off.

The question has long intrigued biologists – since an ordinary somatic cell has identical genetic information to that in the original fertilised egg, would it be possible to trigger a somatic cell to develop into a new organism, into a clone of the parent cell? Many attempts have been made over the years to achieve such cloning, and with, some success.

The phenomenon of parthenogenesis shows that the environment within the egg can trigger a programme of development in the chromosomes that results in the eventual formation of an adult organism. In attempting to clone an organism from a differentiated somatic cell, the approach has been to extract the nucleus from such a cell, then remove the nucleus from an egg cell and replace it with the nucleus from the differentiated somatic cell, and hope that the egg cell environment would trigger the developmental programme.

Success has been achieved in this manner in cloning tadpoles. However, not until Dolly had there ever been a success in triggering the chromosomes from a somatic cell from an adult mammal. Dolly was cloned from an udder cell taken from an adult sheep.

In an odd combination of the trite with the profound, it was decided to name the clone, Dolly, after the singer, Dolly Parton, who also displays impressive developments of mammary cells.

I will return next week to the subject of cloning to discuss ethical issues raised by this recent development.


Solving an old mystery with wasps

Swiss researchers at ETH Zurich and the University of Zurich have discovered that a single gene in a particular aph >

Why does sex exist? Even now, evolutionary biologists have not yet found a satisfactory answer to this simple question. Asexual reproduction would really be much more efficient from an “economic” point of view, since in sexual reproduction with separate genders only a certain proportion of the organisms – the females – give birth to progeny. Nonetheless, sexual reproduction has become widely established during the course of evolution. Various theories attempt to explain why this is the case – so far without success. This is because these theories are very difficult to test empirically. In fact, there are animal species that reproduce both sexually and asexually; in these species, however, the sexually produced individuals also differ from those produced asexually in terms of other characteristics, so that an unbiased comparison is virtually impossible.

However, Christoph Vorburger, Swiss National Science Foundation (SNF) Professor of Evolutionary Ecology at the Institute of Integrative Biology of ETH Zurich and the EAWAG (Swiss Federal Institute of Aquatic Science and Technology) in Dubendorf, along with his doctoral student Christoph Sandrock from the University of Zurich, have now discovered that an empirical study of this central question of evolutionary biology might be possible using a particular species of parasitic wasp. During their study, the two researchers examined the aphid wasp Lysiphlebus fabarum, which had long been known to be capable of reproducing in both ways. Vorburger and Sandrock have now shown that there is practically no difference between sexual and asexual individuals in this species of wasp – apart from the fact that they reproduce differently, of course. Thus the genetic differences between individuals are no greater than those which would exist in a population in any case.

Precise agreement with theory

In wasps that reproduce sexually, females originate from fertilised egg cells while males are produced from unfertilised egg cells (haplo-diploidy). In the case of asexual wasps, on the other hand, the females give birth only to daughters, without fertilization. Vorburger and Sandrock wanted to know which genetic factors decide whether a wasp will reproduce sexually or asexually. Astonishingly, it turned out that this fundamental difference is evidently controlled by just a single gene. Using crossbreeding experiments, the two scientists were also able to show that the trait is inherited recessively. In the third generation of their experiment, they found that exactly 12.5 percent of females reproduced asexually – which is precisely the number predicted by Mendel’s laws of inheritance for a recessive trait in haplo-diploids.

Vorburger and Sandrock still do not know which gene determines the mode of reproduction. “We could only demonstrate that the trait behaves like a single genetic factor, and we already know of one microsatellite, i.e. a genetic marker, which is located close to the decisive gene,” explains Vorburger. “We now want to answer this question in a further study”.

A real improvement in pest control?

Vorburger and Sandrock’s discoveries are interesting not only for evolutionary biology but also from the point of view of pest control. Aphid wasps such as L. fabarum are used for biological pest control because they lay their eggs in aphids, thus killing the pests. Since in the case of sexual wasps only half of the individuals contribute effectively towards pest control – namely the females which lay their eggs in the aphids – it might be possible to improve the efficiency of this method by using asexual wasps instead. “However, the increased effectiveness might be merely a short-term success,” says Vorburger, “because asexual reproduction yields only genetically identical lines. The lack of genetic exchange may constrain their ability to adapt to changes in ecological conditions.”. It is precisely this adaptability that many evolutionary biologists believe is ultimately the reason why sexual reproduction has become established in nature – even if, at first sight, it appears less economical.


How do wasp reproduce?

Best Answer: First stage
After emerging from hibernation during early spring, the young queens search for a suitable nesting site. Upon finding an area for their future colony, the queen constructs a basic paper fiber nest roughly the size of a walnut into which she will begin to lay eggs.

Second stage
The sperm that was stored earlier and kept dormant over winter is now used to fertilize the eggs being laid. The storage of sperm inside the female queen allows her to lay a considerable number of fertilized eggs without the need for repeated mating with a male wasp. For this reason a single female queen is capable of building an entire colony from only herself. The queen initially raises the first several sets of wasp eggs until enough sterile female workers exist to maintain the offspring without her assistance. All of the eggs produced at this time are sterile female workers who will begin to construct a more elaborate nest around their queen as they grow in number.

European paper wasp (Polistes dominula) evaporating water from a regurgitated droplet to cool itselfBy this time the nest size has expanded considerably and now numbers between several hundred and several thousand wasps. Towards the end of the summer, the queen begins to run out of stored sperm to fertilize more eggs. These eggs develop into fertile males and fertile female queens. The male drones then fly out of the nest and find a mate thus perpetuating the wasp reproductive cycle. In most species of social wasp the young queens mate in the vicinity of their home nest and do not travel like their male counterparts do. The young queens will then leave the colony to hibernate for the winter once the other worker wasps and founder queen have started to die off. After successfully mating with a young queen, the male drones die off as well. Generally, young queens and drones from the same nest do not mate with each other; this ensures more genetic variation within wasp populations, especially considering that all members of the colony are theoretically the direct genetic descendants of the founder queen and a single male drone. In practice, however, colonies can sometimes consist of the offspring of several male drones. Wasp queens generally (but not always) create new nests each year, probably because the weak construction of most nests render them uninhabitable after the winter.

Unlike most honey bee queens, wasp queens typically live for only one year (although exceptions are possible). Also, contrary to popular belief queen wasps do not organize their colony or have any raised status and hierarchical power within the social structure. They are more simply the reproductive element of the colony and the initial builder of the nest in those species which construct nests


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