Polyploidy is the state of a cell or organism having more than two sets of chromosomes. There are mainly three different kinds of polyploids, namely, autopolyploids, allopolyploids and segmental allopolyploids.
Autopolyploids are those polyploids, which have the same basic set of chromosomes multiplied. For instance, if a diploid species has two similar sets of chromosomes or genomes (AA), an autotriploid will have three similar genomes (AAA) and an autotetraploid will have four such genomes (AAAA).
Origin and production of autopolyploids:
The autopolyploids may occur in nature or may be artificially produced. When they occur in nature, their autopolyploid nature is inferred mainly by their meiotic behaviour. One of the very common examples of natural autopolyploidy is Cynodon dactylon.
Autotriploids occur in watermelon, sugarbeet, tomato, grapes and banana, although in several of these cases the polyploids have been artificially produced. Similarly autotetraploids occur in corn, red clover, marigold, snapdragons, phlox, grapes, apple etc.
Since polyploids are normally larger and more vigorous, their role in crop improvement has been realised and techniques developed for artificial induction of polyploidy. Polyploid is mainly induced by treatment with aqueous solution of a drug termed as colchicine. This drug has the property of arresting and breaking the spindle so that a cell division without cell wall formation may be affected leading to doubling of chromosome number. The concentration of aqueous solution of colchicine may vary from 0.01% to 0.50 % and the treatment may be given in one of the following manners:
1. Soaking seeds for different durations in aqueous solution of colchicine is an effective method .
2. Injections of colchicine solution at seedling stage, into cortex tissue with the help of a hypodermic needle is another effective method.
3. Axillary bud treatment is also effective. Since bud is meristematically active, soaking cotton in colchicine and applying on the bud with continuous dropping of solution on the bud leads to the induction of polyploidy in the branch arising from the treated bud.
4. Shoot apex treatment with colchicine is also effective.
Effects of chromosome doubling
One of the important effects of polyploidy is to produce ‘gigantism’. The autopolyploids may be normally larger in size. Sometimes plants may be smaller than diploids, bud leaves, flowers and the cells themselves may be bigger in size. Following are some of the important effects of polyploidy:
- With increase in cell size, water content increases leading to decrease in osmotic pressure. This results into loss of resistance against frost etc.
- Growth rate decreases due to slower rate of cell division, this leads to a decrease in auxin supply and decrease in respiration.
- There is a delay in the time of blooming in the autopolyploids due to slow growth rate.
- At higher ploidy level the adverse effects are high and may lead to the death of plants.
Cytology of autopolyploids:
In an autopolyploid, there will be more than two sets of homologous chromosomes. This leads to formation of multivalents instead of bivalents as found in diploids. An important difference exists even between autotriploid and an autotetraploid, because while in the latter normal disjunction is possible giving rise to diploid gametes, in triploids it is not possible. In an autotriploid, there are three sets of homologous chromosomes. If these three sets pair normally , trivalents cannot disjoin normally and will either disjoin 2:1 chromosome to two poles or will disjoin 1:1 leaving one chromosome as a laggard .The number of chromosomes in gametes of triploid organism, therefore will vary from n to 2n.
In autotetraploids since there are four sets of chromosomes, there is the formation of quadrivalents, which disjoin in a normal 2:2 manner giving diploid gametes. Rarely however a quadrivalent may disjoin 3:1 or may leave a chromosome as a laggard at anaphase 1. Therefore autotetraploids also have a certain degree of sterility, although it will not be as high as in autotriploids.
Allopolyploids are those whose chromosome sets are derived from two different genomes. Such polyploids are the result of doubling of chromosomes in F1 hybrids derived from two distinctly different species. The commonest example of polyploidy is Raphanobrassica. Russian geneticist G.D. Karpechenko (1927) made a cross between Raphanus sativus (2n=18) and Brassica oleracea (2n=8). The F1 hybrid was sterile, because chromosome sets of both these plants were dissimilar and could not pair during meiosis.
Allopolyploidy developed artificially
Some allopolyploids were also produced artificially. F1 hybrid of these were sterile as usual due to lack of pairing. After colchicine treatment the plants were fertile. Following are some of the common examples:
- Hexaploid wheat, i.e., Triticum spelta (2n=42) produced by crossing Triticum dicoccoides (2n=28) with Aegilops squarrosa (2n=14) and then by treating F1 hybrid with colchicine.
- Gossypium hirsutum (cotton), produced by crossing Gossypium herbaceum (2n=26) with Gossypium raimondii (2n=26) and then treating F1 hybrid with colchicine.
- A man made cereal Triticale, produced by crossing (i). Triticum durum (2n=28) with Secale cereale (2n=14) and then treating F1 hybrid with colchicine to obtain hexaploid Triticale and (ii).Crossing Triticum aestivum (2n=42) with Secale cereale (2n=14) and then treating F1 hybrid with colchicine to produce octoploid Triticale.
In some allopolyploids, the different genomes which are present are not quite different from one another. Consequently, in these polyploids chromosomes from different genomes do pair together to some extent forming multivalents. This means that segments of chromosomes and not the whole chromosomes are homologous. Therefore, such allopolyploids are termed as segmental allopolyploids according to Stebbins (1943, 1950).
These segmental allopolyploids are intermediate between autopolyploids and allopolyploids and can be identified by their meiotic behaviour.
Most of the naturally occurring polyploids are neither true autopolyploids nor true allopolyploids but are rather segmental allopolyploids. Our common hexaploid bread wheat is also a segmental allopolyploid, because the three diploid genomes (A,B and D) are related (homoeologous) to each other.