In this article we will discuss about the meaning and classification of euploidy.
Meaning of Euploidy:
Most diploid sexually reproducing organisms have an alternation between a haploid and a diploid state in the life cycle. The haploid state, mostly confined to germ cells, is characterised by the presence of a single set of chromosomes (n).
When two haploid germ cells produced by a male and a female parent unite during fertilisation, the zygote formed contains two haploid sets of chromosomes and it becomes diploid (2n). Mitotic divisions of the zygote produce a diploid adult organism of which each cell contains one haploid set of chromosomes from the male parent, the other from the female parent.
Since both sets of chromosomes are morphologically identical, pairs of identical chromosomes exist in a cell nucleus. The two members of a pair are said to be homologous to each other. When meiosis starts in the germ mother cells, homologous chromosomes attract each other and start pairing. The exactness of pairing depends upon the identical nature of the homologues.
In euploidy an organism acquires an additional set of chromosomes over and above the diploid complement. If one additional set is present the condition is known as triploid (3n); if two then tetraploid (4n); addition of three sets is called pentaploid (5n), and of four sets hexaploid (6n) and so on. This is known as a polyploid series and the individuals are said to show euploidy.
Classification of Euploidy:
Depending upon the source of the additional chromosome set, euploids are classified into two types, auto-polyploids and allopolyploids which are described below:
(a) Auto-polyploids:
Auto-polyploids arise when the additional sets originate from the same species. For example, if the haploid set of a species is designated A, the diploid is AA, triploid AAA, tetraploid AAAA, and so on.
Autotetraploids can arise through one of the following ways:
(i) Fertilisation of an egg by two or more sperms giving rise to a zygote with three or more sets of chromosomes;
(ii) Normal mitotic division in the diploid zygote in which chromosomes duplicate but cell division fails to occur so that four haploid sets of chromosomes produce a tetraploid nucleus; if the same mitotic error occurs during embryo development it results in some tetraploid tissues in an adult diploid individual;
(iii) Failure of meiotic division in germ mother cells so that unreduced diploid gametes are formed instead of haploids.
Although auto-polypoids have homologous genomes, yet those having odd numbered sets of chromosomes such as 3n, 5n, 7n, 9n, and so on, show a high degree of sterility. This is because during meiosis, pairing between two homologous chromosomes only results in normal segregation of the haploid set into gametes.
If three homologues are present they may or may not become paired to form a trivalent (Fig. 11.1). Since pairing in any region is restricted to only two homologues at a time, third homologue may fail to pair and remain a univalent, or may pair at some places to form a trivalent.
During anaphasic segregation at meiosis I, two of these homologues may move into one daughter cell, and the third into the other daughter cell. Since all chromosomes of the haploid set have three homologues each, their random distribution or independent assortment will cause the resulting gametes to have varying numbers of different homologues.
In this way a true haploid or a true diploid gamete would be rarely formed. Instead, unbalanced gametes with chromosome numbers ranging between n and 2n would be formed. Such gametes are not viable and triploids consequently are sterile.
It is noteworthy that most popular varieties of seedless watermelons, bananas, Indian carpet grass, and European pears and apples are triploid. These triploid plants have resulted from fertilisation between diploid gametes from tetraploid plants and haploid gametes from diploid plants. Once formed, the triploid plants are healthy and robust and are propagated through asexual cuttings.
Autotetraploids are either fertile or only partially sterile. As there are no univalents and trivalents formed, the four homologues can segregate to form viable diploid gametes, so that tetraploids are often fertile. Groundnut, potatoes and coffee are well known examples of autotetraploid species.
Among cereals, autotetraploid rye is grown in Sweden and Germany. Some of the giant sized plants of Oenothera lamarckiana which De-Vries first noticed in Holland and attributed to a mutation, had later turned out to be autotetraploids.
Polyploidy is more common in plants than in animals. More than 50 per cent of angiosperms are known to be polyploids. There are some explanations for this. Plants are mostly hermaphrodites or bisexual organisms in which sex chromosomes do not play a significant role in normal growth and development.
An increase in the number of chromosomes is therefore desirable as it increases phenotypic variability and magnifies the expression of some favourable traits. In animals on the other hand, polyploidy leads to a disturbance in the balance between sex chromosomes and autosomes. An increase in the number of sex chromosomes markedly affects sexual development.
Due to this, polyploidy in animals is restricted to those species which are hermaphrodites such as leaches and earthworms, or those which develop partheno-genetically (without fertilisation of egg), as in shrimps, aphids and some lizards. A second reason why polyploidy is more prevalent in plants is that the problem of sterility is easily overcome through asexual methods of reproduction in plants.
Moreover, if one portion of a diploid plant becomes polyploid, for instance a branch bearing fruits, it is possible to propagate that branch through budding and grafting for raising new plants. Such techniques are obviously not applicable to animals except that individual polyploid cells can be excised and cultured in the laboratory. In mammals liver cells are often polyploid.
Even germ cells such as primary spermatocytes are sometimes polyploid in mouse (Fig. 11.2) and in man. In plants cells of the tapetum which nourishes the male gametophyte, and endosperm cells which support the growing embryo, are also polyploid. The root nodules of leguminous plants frequently show polyploidy.
From evolutionary point of view polyploidy has played a significant role in evolution of plant species. The origin of some important crop plants such as barley, potatoes, grass (Dactylis glomerata), lotus and many ornamental plants is due to polyploidy.
(b) Allopolyploidy:
This is the second type of euploidy where the additional set of chromosomes comes from a different species. For example, suppose a diploid species with two chromosome sets AA crosses naturally or artificially with another species BB. The offspring produced would be AB which is viable but sterile.
This is because during meiosis the chromosomes belonging to the set A do not find homologous partners in chromosomes of B. Due to failure of pairing at anaphase I, the chromosomes move at random towards the two poles. Thus each gamete gets an unbalanced mixture of A and B chromosomes and sterility results.
There is one way of restoring fertility to a sterile hybrid (AB). If during mitotic division in the AB hybrid all the chromosomes are allowed to divide but cell division is inhibited, the result would be a tetraploid nucleus with two sets of A and two sets of B chromosomes (AABB).
Therefore, when meiosis starts, all chromosomes belonging to one set of A will find homologous partners with the remaining A chromosomes and perfect pairing will result. Similarly, the two sets of B chromosomes will pair with each other and viable fertile gametes would be formed. Such an allopolyploid individual is called an amphidiploid.
It is possible to induce amphidiploidy artificially by treating young buds or seeds with the alkaloid colchicine, a mitotic poison which inhibits spindle formation, consequently cell division. This leads to all the duplicated chromosomes becoming included in a single tetraploid nucleus.
Raphanobrassica is an interesting example of a newly synthesised genus for illustrating allopolyploidy. In 1927 a Russian geneticist Karpechenko made a cross between Raphanus sativus (radish) and Brassica oleracea (cabbage) with the aim of producing a new plant that would have the roots of radish, and in the aerial portions would bear cabbage. The hybrid that was actually formed had the roots of cabbage and tops of radish plant.
The hybrid produced between radish and cabbage proved useless economically. But it proved very important genetically. Both radish and cabbage plants are diploid with 18 chromosomes.
Thus gametes from each parent had 9 chromosomes, and their union produced the F1 hybrid with 18 chromosomes. This hybrid was sterile because the 9 chromosomes of radish did not pair with the 9 chromosomes of cabbage. Sometimes however, viable pollen and ovules were produced having all 18 chromosomes.
Fusion of such unreduced gametes produced tetraploid (4n = 36) plants with 18 chromosomes of radish and 18 of cabbage. Pairing took place amongst the radish chromosomes to form 9 pairs; similarly the cabbage chromosomes also formed 9 pairs.
Normal segregation gave rise to viable gametes. The hybrid therefore became fertile and was given the name of a new genus Raphanobrassica. This is a beautiful demonstration of how a new genus can be artificially synthesised through allopolyploidy.
The genus Triticale demonstrates the efforts of man to create a new cereal by crossing wheat and rye. A hexaploid Triticum (2n + 44) is crossed to the diploid Secale (2n = 14).
The tetraploid hybrid undergoes chromosome duplication to produce the octoploid Triticale which combines the characters of wheat and rye. It is resistant to diseases affecting both wheat and rye, and the flour made from its grains has very high protein content. Therefore, efforts are being made to develop it for commercial use as a crop plant.
A number of cultivated plants are allopolyploids. One of the most important cereals, wheat, represents an allopolyploid series of diploid, tetraploid and hexaploid species. The series is represented by three groups designated Einkorn (single seeded), Emmer and Vulgare.
The einkorn group consists of two primitive diploid (2n = 14) species, namely Triticum monococcum and the wild T. boeoticum. Although not of much use for human consumption because the grain is tightly enclosed in the glumes, the einkorn species are useful as fodder. In some parts of Europe and the Middle East they are used for making dark breads.
The Emmer group consists of seven species of tetraploid wheats of which the most important are Triticum dicoccum (Persian emmer wheat) and T. durum.
The origin of emmer wheats took place through hybridisation between an einkorn wheat and a wild species Aegilops (goat grass) as explained below:
Most of the emmer wheats are grown for animal feed, one T. durum has a high gluten content and is particularly useful for making chapattis in India and noodles in western countries. The vulgare group consists of five species of hexaploid wheats (2n = 42) including the economically important bread wheat T. aestivum.
It is said to have originated through hybridisation between T. dicoccum (A’A’B’B’) and a different species of goat grass Aegilops squarrosa (DD) followed by chromosome doubling. The true bread wheat of today therefore, contains three genomes from three different wheats (A’A’B’B’DD; 2n = 42).
Bread wheat has 21 pairs of chromosomes which show an interesting behaviour during meiosis. Normally, chromosomes of wheat species coming from different origins do not pair at meiosis. But the chromosomes belonging to A, B and D genomes that are present in hexaploid bread wheat pair with each other under one condition that chromosome No. 5 of B genome should be absent.
Thus chromosome I of A pairs with chromosome I of B; chromosome IA can also pair with ID; and chromosome ID can pair with IB. Similar combinations of pairs exist for other chromosomes of A, B and D genomes. Such chromosomes which belong to different genomes, yet show pairing are called homologous.
Apparently there is a gene on the long arm of chromosome 5B of wheat which suppresses homologous pairing. Riley in 1974 have given the name pairing homologous or ph to this gene. They have also found that at the beginning of meiosis the positions of the chromosomes on the nuclear membrane are determined by this gene, thereby affecting their pairing behaviour.
In cotton it has been possible to trace the origin of American cottons from hybridisation in the past between New World and Old World cottons. The American cultivated cottons have 52 chromosomes whereas the wild American cottons have only 26 chromosomes. The Indian cultivated cottons also have 26 chromosomes but these are morphologically different from the 26 chromosomes of the wild New World varieties.
It appears that sometime in the past the American wild cotton must have crossed with the Old World cultivated cotton to produce a hybrid with 13 New World and 13 Old World chromosomes. Chromosome duplication in this hybrid gave rise to the present day tetraploid (2n = 52) cultivated cottons in America.
Clausen and Goodspeed synthesised a new species of Nicotiana (tobacco) by induction of polyploidy as follows:
Like auto-polyploids, allopolyploids are also of common occurrence in plants. Among animals they are extremely rare for some well-defined reasons. There are usually no fertilizations of the interspecific type due to different behavioural patterns. Even if fertilisation is induced artificially, the hybrids formed show defects and do not grow normally.
They cannot reproduce vegetatively and do not live long enough to allow chromosome doubling to take place in their germ mother cells to form diploid gametes. Nevertheless, allopolyploidy is found in some animals such as lizards (Cnemidophorns), fishes (Poeciliopsis) and salamanders (Ambystoma), all of which are parthenogenetic.