Polyploidy in plants
One of the remarkable features of living material is their ability to perpetuate themselves. However, the ever dynamic nature of the surrounding environment has imposed upon plants, much like other organisms, various evolutionary and selective bottlenecks necessitating the adoption of ways and means by organisms to keep their "race going". A concept which gains prominence in this regard is that of hybridization and has been an area of much fascination since the eighteenth century. Stebbins (1958) defined hybridization as the "crossing between individuals belonging to separate populations which possess different adaptive norms". Polyploidy, a prime facilitator of speciation and evolution in plants and to a lesser extent in animals, is associated with intra and inter-specific hybridization.The purpose of this paper is to give a broad overview of the phenomenon of polyploidy in its entirety in plants ranging right from a brief historical background to where it stands today. Given the importance of polyploidy in speciation this will be looked into a little more in detail compared to other aspects of polyploidy.
So what is polyploidy?
It refers to a definite arithmetic relationship between the chromosome numbers of related organisms. It has been defined as the possession of three or more complete sets of chromosomes and has been an important feature of chromosome evolution in many eukaryotic taxa including plants, yeasts, insects, amphibians, reptiles, fishes and even the mammalian genome. Polyploidy is present to at least to some extent in most members of the plant kingdom being more common in some and rather rare in others. The fact that it is widespread many plants is also kind of exemplified by the wide variations in chromosome numbers with chromosome numbers ranging from 2n = 4 to 500 in angiosperms to 2n=6 to 226 in monocots. Thousands of angiosperm species have 14 to 15 pairs of chromosomes.
The phenomenon of polyploidy gained much of what it is today during the early part of the twentieth century. One of the early examples of a natural polyploid was one of De Vries's original mutations of Oenothera lamarckiana (mutat. gigas). The first example of an artificial polyploid was by Winkler (1916) who in fact introduced the term polyploidy. Winkler was working on vegetative grafts and chimeras of Solanum nigrum and found that callus regenerating from cut surfaces of stem explants were teratploid. Digby(1912) had discovered the occurrence of a fertile type Primula kewensis from a sterile inter-specific hybrid through chromosome doubling but failed to realize its significance in the context of polyploidy. Though unaware of the 'Primula type" fertile hybrid, Winge (1917), from his studies on the chromosomal counts of Chenopodium and Chrysanthemum found that chromosome numbers of related species were multiples of some common basic number; he subsequently proposed a hypothesis that chromosome doubling in sterile inter-specific hybrids is a means of converting them into fertile offsprings. This was subsequently verified by various workers in artificial inter-specific hybridizations of Nicotiana, Raphanobrassica and Gaeleopsis. Finally the colchicine method of chromosome doubling was developed by Blakeslee and Avery (1937) and became an important tool for the experimental study of polyploidy.
Before going ahead what is the main significance of polyploidy in brief? As pointed out at the very outset it is recognized as one of the main process in the evolutionary history of plants and to some extent other organisms. Though differences do exist with regard to the nature of its role and the relative importance of different kinds of polyploidy, an understanding of the ways in which polyploidy operated in the past to produce new species and races may provide useful insights to improving our cultivated plants. In fact many of our crop species, including wheat, maize, sugar cane, coffee, cotton and tobacco, are polyploid, either through intentional hybridization and selective breeding (e.g. some blueberry cultivars) or as a result of a more ancient polyploidization event(e.g. maize).
Added to it, technological advances in the analysis of genome structure and function has made it possible to better analyze the genetic consequences of genome duplication. The importance of polyploidy in such diverse fields such as cytogenetics, physiology, breeding, cytotaxonomy and biogeography in conjunction with new possibilities put forth by various molecular techniques has all spurred a resurgence of interest in issues of origin and establishment of lineages.
Origin of polyploids
There are various modes for the origin of polyploids. These mainly include mechanisms such as somatic doubling during mitosis, non-reduction in meiosis leading to the production of unreduced gametes, polyspermaty (fertilization of the egg my two male nuclei) and endoreplication( replication of the DNA but no cytokinesis). Endoreduplication however, is more similar to somatic doubling and is therefore not viewed as a separate mechanism by some authors. These mechanisms are reviewed in greater detail below.
Chromosome doubling can occur either in the zygote to produce a completely polyploid individual or locally in some apical meristems to give polyploid chimeras. Somatic polyploidy is seen in some non-meristematic plant tissues as well.(eg: tetraploid and octoploid cells in the cortex and pith Vicia faba). In somatic doubling the main cause is mitotic non-disjunction.This doubling may occur in purely vegetative tissues(as in root nodules of some leguminous plants) or at times in a branch that may produce flowers or in early embryos (and may therefore be carried further down) Spontaneous somatic chromosome doubling is a rare event and the only well documented instance of the same was in case of tetraploid Primula kewensis which arose by somatic doubling in certain flowering branches of a diploid hybrid. The phenomenon of chromosome doubling in the zygotes was best described from heat shock experiments in which young embryos were briefly exposed to high temperatures. Zygotic chromosome doubling was first proposed by Winge and the spontaneous appearance of tetraploids in Oenothera lamarckiana and amplidiploid hybrids in Nicotiana were shown to be a result of zygotic chromosome doubling.
A second major route of polyploid formation involves gametic "non-reduction" or "meiotic nuclear restitution" during microsporogenesis and megasporogenesis resulting in unreduced 2n gametes. Non reduction could be due to meiotic non-disjunction (failure of the chromosome of separate and subsequent reduction in chromosome number), failure of cell wall formation or formation of gametes by mitosis instead of meiosis.The classic example, Raphanobrassica, originated by a one step process of fusion of two non- reduced gametes. The production of non- reduced gametes has been shown to be rather common in Solanum sps..
Another route may involve non-reduction occurring in one of the germ lines (pollens or the eggs). A tetraploid individual can then result from a two-step process (sometimes referred to as a triploid bridge mechanism) from the fusion of an unreduced 2n gamete with a reduced 1n gamete to give a 3n zygote followed by the subsequent fusion of a 3n gamete with a normal 1n gamete in the next generation to give rise to a tetraploids individual (as in artificial Galeopsis tetrahit). This is a more common route of polyploid formation from unreduced gametes (though the frequent sterility of most triploid hybrids has lead to a questioning of this method by some authors) rather than the former one.
The production of non-reduced gametes is also a function of the environment and genotype. eg: adverse growing conditions were shown to favor an increase the number of non-reduced gametes in Gilia. An example of the influence of genotype in modulating the production of non-reduced gametes can be seen in case of maize wherein the gene "elongate" on chromosome 3 was found to increase the proportion of diploid eggs produced.
Studies on unreduced gametes in both plants and animals are getting easier with the use of rapid screening techniques such as flow cytometry, chromosme painting and other genomic techniques.
Polyspermy is observed in many plants, but its contribution as a mechanism for polyploid formation is rather rare except perhaps in some orchids.
Endoreduplication is a form of nuclear polyploidization resulting in multiple uniform copies of chromosomes. It has been known to occur in the endosperm and the cotyledons of developing seeds, leaves and stems of bolting plants. In animals it occurs in certain tissues such as the liver cells, and megakaryocytes ( the cells which give rise to the thrombocytes).
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