In this article we will discuss about:- 1. Introduction to Haploid 2. Meaning of Haploid 3. Origin 4. Types 5. Uses.
Introduction to Haploid:
The chromosome number is of three types, viz., haploid, diploid and basic number. Haploid number represents half of the somatic chromosome number of a species and is denoted by n. Since haploid chromosome number is found in the gamete, it is also known as gametic number.
Diploid number refers to somatic chromosome number of a species and is represented by 2n. Since diploid chromosome number is found in the zyotic and somatic cells, it is also referred to as zygotic or somatic number. Basic number refers to gametic chromosome number of a true diploid species and is denoted by x. It is the minimum haploid chromosome number of a crop species.
A basic set of chromosomes of an individual is called genome. In a genome, each type of chromosome is represented only once. Haploid individuals have single genome, diploid species two, triploid species three and so on. Any change in the chromosome number from the diploid state is referred to as heteroploidy and such individuals are called heteroploids.
Depending upon the change in the genome, heteroploidy is of two types, viz.:
(1) Euploidy, and
(2) Aneuploidy.
Euploidy refers to numerical change in the entire genome. It includes haploids and polyploids. Aneuploidy refers to change in one or few chromosomes of the genome. It includes, monosomics, nullisomics and polysomies.
Meaning of Haploid:
An individual with gametic chromosome number is known as haploid and such condition is referred to as haploidy. There is difference between haploids and monoploids. Monoploids have the basic chromosome number (x) of a species. In other words, monoploids contain single set of chromosomes (i.e., single copy of the genome). Haploids have gametic chromosome number (n).
In a true diploid species, both monoploid and haploid chromosome number is the same (n = x). Thus a monoploid can be a haploid but all haploids cannot be monoploids. The main differences between monoploids and haploids are presented in Table 26.1. In monoploids proper meiosis does not occur due to lack of pairing partners of chromosomes.
Origin of Haploids:
In crop plants, haploids may originate in several ways, viz.:
(1) By parthenogenesis,
(2) From interspecific crosses,
(3) As a member of twin embryos,
(4) By semigamy,
(5) By alien cytoplasm, and
(6) By anther culture.
These are briefly discussed below:
(1) By Parthenogenesis:
Parthenogenesis refers to development of embryo from the egg cell without fertilization. Haploid embryos give rise to haploids plants. In maize, haploids occur spontaneously by parthenogenesis, though at a very low frequency. In maize, both gynogenetic and androgenetic haploids have been observed. Haploids also occur in several fruit crops by parthenogenesis. The frequency of gynogenetic haploids is 1 in 1000 in maize. For androgenetic haploids it is 1 in 80,000.
(2) From Interspecific Crosses:
Interspecific hybridization refers to mating between two different species of the same genus. In some crops, haploids arise from interspecific crosses. In such crosses, the pollen may not effect fertilization, but stimulates the development of egg cell into haploid embryo. Haploids from interspecific crosses have been observed in potato and many other crops.
(3) From Twin Embryos:
Development of more than one embryo in seed is known as poly-embryony. Twin embryos develop by this technique (poly-embryony). In twin embryos, atleast one embryo is haploid which gives rise to haploid plant. Development of haploids through poly-embryony has been reported in Linum, pepper and several horticultural crops.
(4) By Semigamy:
Semigamy is a form of apomixis in which the sperm nucleus enters the egg cell but fails to fuse with the egg nucleus. In semigamy, the sperm and egg nuclei develop independently resulting in formation of seeds with maternal and paternal chimeras of haploid tissues. Semigamy can be used for production of haploids at will in Egyptian cotton (Gossypium borbadense).
(5) By Alien Cytoplasm:
Alien cytoplasm refers to cytoplasm of related species. In wheat, haploids have been obtained through the use of alien cytoplasm. About 30% haploids were obtained when nucleus of bread wheat was placed into the cytoplasm of Aegilops caudata.
(6) By Anther Culture:
Haploids can be obtained by anther culture i.e. regeneration of plants from anthers in the nutrient medium. Such haploids have been obtained in tobacco, wheat, rice and many other crops.
Now various methods are known by which haploids can be produced at will.
These methods include:
(i) Pollination with foreign pollen,
(ii) Delayed pollination,
(iii) Use of X-ray irradiated pollen for pollination,
(iv) Temperature shock,
(v) Treatment with chemicals like colchicine,
(vi) Interspecific and intergeneric crosses, and
(vii) Anther and pollen culture.
The anther and pollen culture technique was first used for the production of haploids in Datura. Now this technique is used for the production of haploids in tobacco, rice, and several other crops.
Types of Haploids:
Depending upon the origin, haploids are of two types, viz., euhaploids and aneuhaploids. Euhaploids develop from a euploid species and have complete chromosome set. Euhaploids are of two types, viz., monohaploid — which develop from a normal diploid species; and polyhaploids — which develop from polyploid species.
The polyhaploids are again of two types, namely allohaploids — which arise from allopolyploid species; and autohaploids — which develop from autopolyploid species. When a haploid develops from a tetraploid species, it is called dihaploid.
Aneuhaploids develop from aneuploid species and have either one additional or missing chromosome. Aneuhaploids include disomic haploids (n + 1), nullisomic haploids (n – 1), substitution haploids (n – 1 + 1), mis-division haploids, etc. The mis-division haploids have an iso-chromosome which is produced by vertical division of centromere. Generally, centromere divides longitudinally. Aneuhaploids are generally inviable.
Uses of Haploids in Crop Improvement:
Haploids have several applications in plant breeding.
They are used for:
(1) Development of pure-lines,
(2) Disease resistance,
(3) Development of inbred lines, and
(4) Indirect uses.
These aspects are briefly described below:
(1) Development of Pure-Lines:
Pure-lines can be obtained through chromosomal doubling of haploids. This is a short cut method of obtaining homozygous lines.
Doubled haploids can be used in two important ways, viz.:
(i) As a parent in the hybridization, and
(ii) As a cultivar.
In maize, doubled haploids have been successfully used in the development of commercial hybrids and in barley for developing new cultivars. In Brassica napus, a doubled haploid called Maris Haplona has been released from the cultivar ‘Oro’. The new variety is superior to the parent variety in oil yield.
(2) Disease Resistance:
In tobacco, doubled haploids obtained from anther culture have been used to improve the disease resistance of the leading Japanese flue cured cultivar Mc 1610.
(3) Development of Inbreds:
In dioecious plants, development of inbreds requires several generations (6-7) of selfings. The haploid method is a useful tool for obtaining inbred lines within two crop seasons in such crop species.
(4) Indirect Uses:
There are some indirect uses of haploids as listed below:
(i) In wheat, a complete series of nullisomics, monosomics and trisomics was developed from haploids. These aneuploids provided the basic material for extensive studies about breeding, cytogenetics and evolution of wheat crop.
(ii) In wheat, a haploid deficient for chromosome 5B provided the evidence that homoeologous pairing was prevented by the activity of genes on the 5B chromosome. This concept of diploidization created interest about pairing in other polyploids.
(iii) In potato and alfalfa, haploids have provided convincing evidence for the polysomic nature of these tetraploid crops.
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