What is hybrid technology? Explain with   
 Examples of important crop varieties

Outline:

1.Hybridization?

1.1. Introduction and history

 1.2 mechanism of hybrid variety and hybrid seed production

 1.3 hybridization: pros and cons

2. Examples of important hybrid crop varieties

Introduction

Plant breeding is a crucial phenomena which results in improved varieties of plants with better adaptability to environment and other advantageous characteristics. For better varieties and improved properties with higher yield from plants different breeding techniques are adopted by breeders, based on the objective set for breeding, experience of farmers and available resources these techniques are continuously updated and modified for desired results. One of the common breeding method is “hybrid technology” for producing desired characteristics as well as genetic diversity in plants. Hybridization is defined as, “sexual reproduction cross between plants of different species producing a plant that carry genetic material from both parents.”

After cross between different plant species resulting F1 will be heterozygous and possess the genetic makeup from paternal and maternal genome (C. Neal Stewart, Jr. University of Tennessee Knoxville, Tennessee, 2008). Natural hybridization and polyploidization has resulted in speciation and evolution of lineages. Hybridization is used to create novel phenotypes, new adaptive lineages and to take advantage of hybrid vigor of plants (Payseur and Rieseberg, 2016)

Hybridization process has played very important role in the development of many important crops. Many important crop varieties have developed by the combination of genomes or by the hybridization of haploid gametes. For example, common vegetable cooking oil Canola (Brassica napus) consists of genome of two species Brassica rapa having genome AA and Brassica oleracea having genome CC as shown in figure1.1. Also bread wheat (Triticum aestivum) consists of genome AA, BB, DD of three different varieties, as a result of hybridization (also called alloploidy speciation of common wheat)

Emmer wheat (Triticum turgidum) + Goat Wheat (Aegilops tauschii) = Bread Wheat

(C. Neal Stewart, Jr. University of Tennessee Knoxville, Tennessee, 2008)

Source:Plant Biotechnology And Genetics Published By John Wiley & Sons, Inc., Hoboken, New Jersey Edited ByC. Neal Stewart, Jr. University Of Tennessee Knoxville, Tennessee, Chapter 2, Page 41)

 

Hybrid crops and hybrid vegetables:

Production of hybrid crops for achieving desired traits involve:

·         Hybrid development

·         Hybrid seed production

Hybrid variety:

First or second progeny of crosses between two inbred lines is called hybrid variety. Earlier in history hybrid varieties were produced as a result of double crosses as [(A×B) ×(C×D)]. But in modern agriculture and farming hybrid seed, produced by cross pollination in two different species, is used to produce hybrid variety.

Source:Christine Arncken (-Karutz) Hansueli Dierauer

Coop Naturaplan Fund Organic Seed Project, Module 1.4

             Page 08

Hybrid Seed technology:

Productionof seed by cross pollinated plants. When two pure lines with desired traits are bred together by hand. When these seeds planted by the farmer will produce same plants, but the seeds produced from these hybrids will not uniformly exhibit same characteristics as that hybrid.

Source:Plant Biotechnology And Genetics Published By John Wiley & Sons, Inc., Hoboken, New Jersey Edited By C. Neal Stewart, Jr. University Of Tennessee Knoxville, Tennessee, Chapter 3, Page 73)

Procedure of hybridization:

a)      Set up your objective (select a plant specie for a desirable trait e.g., disease resistance, producing high yield or drought resistance etc.) the main purpose behind hybridization process is to create a plant variety that is more adapted to environment, resistant to disease and produce high yield than parent. Based on objective farmers choose parent plant specie.

b)      Select parent plant specie

c)      Assessment of parents

d)     Emasculation and dusting

e)      Bagging

f)       Harvesting and storage of seeds

Central mechanism of hybrid variety and hybrid seed production involves following main steps:

1.      Production of inbred lines. In case of self-sterile plants such as rye the self-pollination in plants is forced to produce number of inbred lines. This is done by covering ears from external pollens with greaseproof paper bag. Seeds produced from this process are grown and selfing is repeated for many years until the fairly vigorous inbred lines are produced. These are further improved by crossing and selection.

2.      To find best hybrid different test crosses are performed between different inbred lines.

3.      Commercial Production of hybrid seeds. For this purpose process of self-pollination must be prevented in maternal lines. This is achieved by emasculation (means removal of stamen in self-pollinating species) and introducing pollens from another plant. This is called target pollination. This is achieved by following mechanisms:

·         Mechanical means

·         Genetic means

Emasculation by Mechanical means:

Male inflorescence (panicle) of plant is cut off by a machineor by hand. Adjacent to each other seed parents and pollinizer are grown on a strip.

Example: maize seed hybrid is produced by this strategy. Because of fluorescence morphology of maize which is monoecious in which male and female parts are in the same plant but separated physically, this is easiest method which ensures that seed produced is result of desired cross. (Christine Arncken (-Karutz) Hansueli Dierauer)

Hybrid seeds of many vegetable crops are produced by hand emasculation and hand pollination method. These crops are with hermaphrodite flowers like tomato, chilli, brinjal, French bean, pea, sweet pepper. Emasculation of these bisexual flowers is done at afternoon when corolla are separate and pollination is performed in early morning in next day when anthesis occur. One day before anthesis, the hermaphrodite flowers of female parent are protected by bagging after emasculation. On other side male flowers of desired parent plant are collected a day before anthesis and preserved in moist bags of polythene. In morning time of next day pollination is done and again bagging is repeated. (Nishi, 1967)

Genetic male sterility:

This is under control of a recessive gene. Genetic system controls the pollen sterility. The certain nucleic genes which are in conjunction with mitochondrial genes are responsible for the pollen sterility. These gene with other nucleic genes in other plants are responsible for fully fertile plants. Those nucleic genes which are responsible for male sterility are called maintainers and those which are responsible for pollen fertility are called restorers, and restorers are dominant. By this method hybrid seed can be produced economically at large scale. To produce all the progeny pollen-sterile, maternal line is pollinated with paternal maintainer line. But in case if  hybrid seed is crop then hybrid variety should not pollen sterile, so that in a hybrid seed development paternal line must be restorer for normal development of seed.

Advantages of hybrid varieties:

·         Hybrid vigour:

When genetically different plants are cross pollinated their progeny is high yielding, more vigorousthan parent. In early 18^th century many scientists noticed this phenomena, an American geneticist G.H. SHULL (1914) coined the term “heterosis” to describe this. (Christine Arncken (-Karutz) andHansueli Dierauer)

·         Reduce plant stresses:

Hybrid varieties of plants suffer less due to environmental stresses than conventional varieties. As hybrids are produced keeping in mind the enhanced and improved properties of plants along with higher yield. So these are diseased resistant as well as resistant to other environmental stresses drought etc.

·         Grow faster

·         Uniformity in overall hybrid variety than an open-pollinated variety

·         High yield results in higher earnings

·         Other qualitative characteristics in hybrid variety than parent

Criticism on hybrid technology:

·         More expensive

·         Difficult and impossible to save and replant seeds

·         Former’s dependency on seed making companies

·         Ethical issues-patenting

·         Genetic depletion due to loss of recessive  genes

(Christine Arncken (-Karutz) andHansueli Dierauer)

2. Examples of important hybrid crop varieties:

2.1 Hybrid wheat and seed:

Due to global challenge of food shortage faced by world, sustainable development of high yielding crop varieties is under consideration. Hybrid Wheat seed if implemented, in fields will produce a remarkable high yield and will be very beneficial as wheat is 3th cereal crop of whole world. Okada et al. (2017) described the developmental physiology of wheat florets which opens after some days of anthesis. It facilitates out-crossing. Hybrid seed production of wheat is illustrated in following figure:

A)  Field arrangement of male pollen donor line and female receptor line which are commonly used for hybrid seed production.

B) Second opening of male sterile floret, which is produced genetically or chemically,

In relevance to a floret with fertile anthers. In this floret with fertile anthers, pollens by anther are received to stigma and fertilization occur and grain development is started. But in male sterile floret radial swelling of the ovary open floret and pollen from a male donor line are received by a female line receptor.

Source: “A new opening for wheat seed production” by

Laura E. Dixon, Stefano Bencivengaand Scott A. Boden

 

2.2 Hybrid rice and seed:

Hybrid rice is successfully cultivated in many areas of world. Hybrid rice gives 10-20% more     yield and exhibit many other improved traits. Hybrid rice seed produce when egg of parent seed plant is fertilized with a pollen containing male parent from another variety.

Common male sterile methods used are

·         Cytoplasmic male sterility method

·         Genic male sterility

·         Chemically induced male sterility