Polyembryony

Key takeaways

Polyembryony belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Polyembryony to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Polyembryony from memory before moving on to harder problems.

Reference excerpt

Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg. Due to the embryos resulting from the same egg, the embryos are identical to one another, but are genetically diverse from the parents. The genetic difference between the offspring and the parents, but the similarity among siblings, are significant distinctions between polyembryony and the process of budding and typical sexual reproduction. Polyembryony can occur in humans, resulting in identical twins, though the process is random and at a low frequency. Polyembryony occurs regularly in many species of vertebrates, invertebrates, and plants.

Evolution of polyembryony The evolution of polyembryony and the potential evolutionary advantages that may entail have been studied. In parasitoid wasps, there are several hypotheses surrounding the evolutionary advantages of polyembryony, one of them being that it allows female wasps that are small in size to increase the number of potential offspring in comparison to wasps that are mono embryonic. There are limitations to monoembryony, but with this method of development, multiple embryos can be derived from each of the individual eggs that are laid. The potential advantages of polyembryony in competing invasive plant species has been studied as well.

Vertebrates Armadillos are the most well studied vertebrate that undergoes polyembryony, with six species of armadillo in the genus Dasypus that are always polyembryonic. The nine banded armadillo, for instance, always gives birth to four identical young. There are two conditions that are expected to promote the evolution of polyembryony: the mother does not know the environmental conditions of her offspring as in the case of parasitoids, or a constraint on reproduction. It is thought that nine banded armadillos evolved to be polyembryonic because of the latter. Chondrichthyans, namely the skates Beringraja binoculata and Beringraja pulchra, in addition to spotted catsharks, have exhibited multiple embryos in a single egg case.

Invertebrates

A more striking example of the use of polyembryony as a competitive reproductive tool is found in the parasitoid Hymenoptera, family Encyrtidae. The progeny of the splitting embryo develop into at least two forms, those that will develop into adults and those that become a type of soldier, called precocious larvae. These latter larvae patrol the host and kill any other parasitoids they find with the exception of their siblings, usually sisters. Obligately polyembryonic insects fall in two classes: Hymenoptera (certain wasps), and Strepsiptera. From one egg, these insects can produce over thousands of offspring. Polyembryonic wasps from the Hymenoptera group can be further subdivided into four families including Braconidae (Macrocentrus), Platygastridae (Platygaster), Encyrtidae (Copidosoma), and Dryinidae. Polyembryony also occurs in Bryozoa. Through genotype analysis and molecular data, it has been suggested that polyembryony happens in the entire bryozoan order Cyclostomatida.

Plants The term is also used in botany to describe the phenomenon of multiple seedlings emerging from one embryo. Around 20 genera of gymnosperms undergo polyembryony, termed "cleavage polyembryony," where the original zygote splits into many identical embryos. In some plant taxa, the many embryos of polyembryony eventually gives rise to only a single offspring. The mechanism underlying the phenomenon of a resulting single (or in some cases a few) offspring is described in Pinus sylvestris to be programmed cell death (PCD), which removes all but one embryo. Originally, all embryos have equal opportunity to develop into full seeds, but during the early stages of development, one embryo becomes dominant through competition, and therefore the now dormant seed, while the other embryos are destroyed through PCD. The genus Citrus has a number of species that undergo polyembryony, where multiple nucellar-cell-derived embryos exist alongside sexually derived embryos. Antonie van Leeuwenhoek first described polyembryony in 1719 when the seed in Citrus was observed to have two germinating embryos. In Citrus, polyembryony is genetically controlled by a shared polyembryony locus among the species, determined by single-nucleotide polymorphism in the genotypes sequenced. The variation within the species of citrus is based on the amount of embryos that develop, the impact of the environment, and gene expression. As with other species, due to the many embryos developing in close proximity, competition occurs, which can cause variation in seed success or vigor.

Illustrations

Worked examples

Example 1 — a first encounter with Polyembryony

Start with the simplest possible case. Write down what Polyembryony claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Polyembryony before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Polyembryony ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Polyembryony

In research: Polyembryony appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Polyembryony in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Polyembryony is common in secondary-school and first-year university syllabi. It links to neighbouring topics Embryology, Insect physiology, Plant reproduction, so understanding it makes those chapters shorter.
In everyday life: Look for Polyembryony outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Polyembryony in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Polyembryony means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Polyembryony out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Polyembryony in simple terms?

Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg. Due to the embryos resulting from the same egg, the embryos are identical to one another, but are genetically diverse from the parents.

Why does Polyembryony matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Polyembryony?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Polyembryony.