Heredity is the passing on of characters from parents to their progeny. In some organisms, like humans, a parent passes on one copy per gene to their offspring. As a result, the progeny gets two copies in total from its parents. Diploid organisms refer to those that have two copies per parent (2n). In contrast, polyploid organisms transmit more than one copy per parent. Different types of polyploidy exist in nature, autopolyploidy and allopolyploidy.

So, what is the difference between autopolyploidy and allopolyploidy? Autopolyploidy appears when an individual has more than two sets of chromosomes, both from the same parental species. Allopolyploidy, on the other hand, occurs when the individual has more than two copies but from different species.

Definitions and Descriptions of Polyploids

To describe the two types of polyploidy, some researchers have focused on their origins (namely, the nature of parentage) to distinguish them. Others focus on their genetic characteristics (their chromosomal profile and behavior). Polyploids are termed polysomic if duplicated chromosomes are completely homologous and arise from multivalent or random bivalent segregation during meiosis. On the other hand, polyploids are referred to as disomic if duplicated chromosomes are homoeologous and strictly result from bivalent homologous chromosomes. Nonetheless, these polyploids, share many features—such as high levels of gene duplication and heterozygosity. Due to outcrossing, autopolyploids have high levels of heterozygosity compared to diploids.

In the formation of allopolyploids, it is important to note that the parental species do not have to have the same number of chromosomes in order for this polyploid to form. Even when the number of chromosomes does not match allopolyploid hybrids still do form.

Below we will go through how polyploidy arises, tolerance for it in Eukaryotes, advantages and disadvantages of polyploidy (allopolyploidy and autopolyploidy) which will also be compared and contrasted between the two polyploids.

Mechanism for Polyploidy

Polyploid species result through either single species (diploid) genome duplication (autopolyploidy) or the fusion of two or more diploid genomes from different species (allopolyploidy). However, there seems to be bias towards certain species pairing for the formation of certain allotetraploid species; where some species preferentially mate to form specific allotetraploids repeatedly.

Natural and Induced Polyploidy

So, how does polyploidy come about? Polyploidy can occur naturally through mysterious meiotic or mitotic errors. Alternatively, it can be chemically induced using substances like colchicine. Polyploids arise when a mitotic or meiotic anomaly occurs such as nondisjunction. Nondisjunction arises when sister chromatids or homologous chromosome pairs fail to split properly during meiosis or mitosis.

To induce this in the lab, a chemical Colchicine disrupts the separation of chromatids during mitosis or meiosis, resulting in cells with tetraploid chromosomes. This principle applies to both autopolyploid and allopolyploid formation, with the difference being the origin of the chromosomes.

Stability and Chromosome Multiples

Generally, even-numbered polyploids, like tetraploids and hexaploids, are stable enough that they make it in the population; while odd-numbered ones (e.g. triploids) tend to be less fortunate and are either aborted as zygotes or become sterile individuals. As a result, many polyploids found in nature usually contain chromosome-pair multiples.

Genetic Characteristics of Polyploids

Every locus in the genome of autopolyploids is homozygous because they result from a duplication event. On the other hand, allopolyploids display varying degrees of heterozygosity depending on the parental genomes. Allopolyploids produce a more diverse set of gametes because of the allelic diversity they possess.

Tolerance of Polyploids

Polyploids are common. Very few mechanisms can instantly result in speciation, polyploidization is one of them. Speciation by polyploidy has become a popular topic in the scientific community. For some reason, some groups tolerate it, some strive on while others are strictly against polyploidization. Recent polyploidization events are nearly unheard of in groups such as vertebrates but are very popular in plant groups such as angiosperms. In higher vertebrates such as humans, polyploids are strongly selected against. Indeed, 10% of spontaneous abortions in humans are thought to be a result of polyploidy in the zygotes. In contrast, at least, 50% of angiosperms are polyploids.

It is also noteworthy that highly diverse families also seem to have very high numbers of polyploids.

Advantages and Disadvantages of Autopolyploidy and Allopolyploidy

Advantage: Hybrid Vigor and Increased Diversity

The high prevalence of polyploidy in some groups clearly suggests that this must somehow confer a competitive advantage for the taxon. In plants, the allopolyploid progeny may come out healthier and fitter than either parent. This is usually a problem from a biodiversity standpoint because it means reduced diversity should the hybrid threaten the prevalence of either or both parents. When a hybrid is stronger than the parental species, this is a hybrid vigor or heterosis. Ideally, you want the hybrid to be just as fit as either parent. This means increased diversity and a possibility of another evolutionary trajectory. Indeed, this is one of the ways explosive radiation of some lineages can arise.

Hybrid vigor can arise in several ways. One possibility is that when the two species interbreed, more selective pressure is placed upon the recessive mutation. Thus, this reduces their propagation in the polyploid hybrids. Recombination also means that the hybrids can produce more diverse progenies, with high possibilities of heterozygosity. The redundancy of many copies of each gene means that there is increased protection against deleterious mutations. The reason the haploid phase is vulnerable to developmental failure is that deleterious recessive alleles are not masked. This increases the chance of something going wrong; while in allopolyploids the presence of a healthy version can mask this effect. This advantage of allopolyploidy is crucial in small inbreeding populations and can be the difference between population failure and flourishing.

Advantage: Promotion of Novel Genes

Polyploidy can promote the appearance of novel genes. Novel genes can arise from the extra copies without disrupting their original function because the other duplicate copies will still be there to provide that function. The presence of additional copies can also aid in establishing self-compatibility in species with selection against it. In both plants and animals, the presence of asexual reproduction has been associated with polyploidy presence. Asexual reproduction of allopolyploids would not be as bad as when diploids self because they have gene diversity within and, therefore, their propagules would still have a chance of heterozygosity.

Disadvantage: Selfing

Selfing poses a similar problem in autopolyploids as with diploids. One can even argue that it has worse consequences in autoploids than in diploids. This is because autopolyploids contain alleles that are copies of each other. Thus, this increases the risk of propagating deleterious mutations and reducing diversity within a population. However, autopolyploids can still promote the formation of novel genes. In fact, this might be even more important than it is in allopolyploids as the genes in autopolyploids are homozygous in any case so are better off becoming novel genes instead.

Disadvantage: Increase in Biomass and Cell Size

One major problem all polyploids share is the increase in biomass with the increase in chromosome number. Now, we all know how important DNA packaging and organization are. All that genetic information has to fit within a cell and still leave enough space for other cell components. So, imagine what happens when more chromosomes are added to a cell that, when diploid, is already organized just right to accommodate everything without hindering cell functions. The obvious solution is to increase cell size. However, in the known polyploids cell size has been reported to only increase by 1.6-fold. This obviously then poses a danger to cell processes. As a result, the positioning of the telomeric and centromeric heterochromatin, amongst other things, can be compromised due to less space in the nuclear envelope being available.

Disadvantage: Negative Effects on Mitosis and Meiosis

Ironically, mitosis and meiosis can also be affected negatively when polyploid cells enter cell division. It’s funny, isn’t it? Meiosis and mitosis are well capable of creating polyploids, but when polyploid cells enter mitosis or meiosis, meiosis and mitosis can’t handle them. Due to the confusing number of chromosomes, more spindles may be formed in the cell. This can lead to chaos during segregation. As a result, the daughter cells can have irregular chromosome numbers (aneuploidy).

Examples of Polyploidy (Allopolyploidy and Autopolyploidy)

Polyploidy in Agriculture

Examples of polyploidy in nature are plentiful. They are also purposely induced or bred for purposes including aquaculture and agriculture. Examples in the agricultural sector include autopolyploids such as alfalfa and potato; and allopolyploids such as wheat and coffee). In some instances, some lineages show evidence of having polyploidy in their ancestry; for example, soybean, and cabbage (paleopolyploids). Polyploids usually have some advantageous traits such as increased drought tolerance, pest resistance, organ size, and biomass. These present polyploids with opportunities to expand their range and exploit new niches.

Bananas are among the famous sterile polyploids. Banana fruits develop parthenocarpically. Parthenocarpically refers to the ovary of a flower developing or an egg developing without pollination or fertilization, respectively. Consequently, fruits that developed in this way are typically seedless. Some seedless fruits come from sterile triploid plants (parents: tetraploid (4n) and diploid (2n) plants). The cultivated banana, Musa x paradisiaca (Musaceae), however, is a triploid hybrid from two diploid Asian species, M. acuminata and M. balbisiana.

Polyploidy in the Animal Kingdom

There are a number of polyploid species in the animal kingdom. In the Class Mammalia, the first tetraploid discovered was in the red vizcacha rat, Tympanoctomysbarrerae (2n = 102). This species’ polyploid chromosome content shows evidence of being allopolyploid. It also possesses some characteristics associated with genome duplication such as a size increase in the spermatozoa and different somatic cell lines. In “lower vertebrates,” some Asian carp individuals present an interesting allopolyploid profile with parentage down to four species; the bighead carp, the black carp, the grass carp, and the silver carp.

Wrapping Up the Differences between Autopolyploidy and Allopolyploidy

Polyploidy is an important phenomenon in the diversification of the angiosperm lineage. Some lineages, however, may have adverse consequences leading to deleterious genetic conditions. Allopolyploids and autopolyploids are crucial for the diversification of groups and present the opportunity to suppress lethal recessive properties. Allopolyploids seem to provide an ideal setting for asexual reproduction while costing the species less diversity loss than would diploids. Autopolyploids present a crucial platform for novel genes and novel gene expression in having spare duplicates per gene.

Looking for more Biology practice?

Check out our other articles on Biology.

You can also find thousands of practice questions on Albert.io. Albert.io lets you customize your learning experience to target practice where you need the most help. We’ll give you challenging practice questions to help you achieve mastery in Biology.

Start practicing here.

Are you a teacher or administrator interested in boosting Biology student outcomes?

Learn more about our school licenses here.