Wild Genes Introgression for Trait Specific Improvement in Crop Plants
Author: Sanganamoni Mallesh

We can use term interspecific hybridization, when individuals from two distinct species of the same genus are crossed. At the same time intergeneric hybridization, when individuals being crossed belonging from species of two different genera. Another related term called distant hybridization in which hybridization between individuals of different species, belonging to same genus or to different genera (distantly related).

Introgression is the process of transfer of some genes from one species in to the genome of another species. Main objective of Introgression hybridization is to transfer disease resistance to cultivated varieties. Interspecific hybridization is used to improve crops by transferring desirable agronomic characters and some specific characters such as pest and stress resistance from wild species to cultivated species. Moreover, interspecific crosses in a genus also provide information about the relation between the taxa. Interspecific crossing is potential important for cucumber improvement, since wild species possess valuable character’s unavailable in the cucumber gene pool. Cucurbita papo, C. moschata and C. maxima are the most economically important three (out of five) cultivated species within the Cucurbita genus that include squashes, pumpkins and gourds, which represent several species in the same crop. These species are remarkably diverse in morphology, disease resistance and environment adaptability.

In the history first man to do distant hybridization was Thomas Fairchild (1717) in Carnation (Dianthus caryophyllus) and sweet williams (D. barbatus). Triticale was produced by Rimpau in 1890 (amphidiploid - wheat and rye). Followed by Karpechenko (Russian,1928) which is an inter generic hybrid produced by cross between Radish ( Raphanus sativus) and cabbage (Brassica oleraceae).

Role of wild species in crop improvement

Historically wild relatives are used for different important trait improvements like;

  1. 56% - Biotic stress
  2. 13% - Abiotic stress
  3. 10 % - Enhancing yield
  4. 11% - Qualitty improvement
  5. 4% - Male sterility and fertility restorations
  6. Economic value-US $ 115 billion/year
  7. Value of product derived-US $ 500-800 billion/year
Major genetic barriers for distant hybridization

1) The pre-fertilization barriers

a) Lack of pollen germination

b) Due to Slow pollen tube growth in style and inability to unite with egg cell

c) Arrest of pollen tubes in style itself

2) Post fertilization barriers.

a) Failure of zygote formation (or) Hybrid sterility.

b) Failure of zygote development (or) Hybrid inviability.

c) Failure of F1 seedling (or) Hybrid break down.

a) Failure of zygote formation / hybrid sterility

  1. Slow pollen tube growth
  2. Style may be longer than pollen tube
  3. Pollen tubes of polyploid species may be thicker than diploid species and
  4. Growth of pollen tube will be slower.
b) Failure of zygote development / hybrid inviability

  1. Unfavourable interaction between chromosome
  2. Disharmony between cytoplasm and nuclear gene
  3. Unfavourable interacation among embryo, endosperm and maternal tissue
c) Failure of hybrid seedling development

It is due to chlorophyll deficiency seedlings may die.

Techniques to overcome the barriers

First determine barrier to production of hybrid embryos

  1. Choice of parents : parents should be selected based on proper knowledge of cross ability and worth of cross for resistance for stresses and desirable characters.
  2. Bridge crosses : here when one species does not cross with other, then third species is taken for crossing. Here first species is crossed with third species and then it is crossed with second to get successful hybrid.
  3. Reciprocal crosses : Here changing maternal and paternal parents for crossing.
  4. Manipulation of ploidy : Ploidy manipulation by using of suitable parents and also by use of chromosome doubling thorough colchicines.
  5. Large number of crosses: Large number of crosses using many parents and successful crossable parents selected.
  6. Protoplast fusion : Fusion of different species protoplast to develop hybrid through further culture.
  7. Use of pollen mixtures : Pollen of many species or parents used and pollinated, selection of hybrid done based on desirable character.
  8. Use of growth regulators (NAA, IAA, 2,4-D, GA)- at the flowering stage increases seed set. [Pollen tube, flower abortion].
  9. Organic solvents (Hexane): Used to dissolve some of the barrier in fertilization, i.e. used to dissolve the stigmatic protein surfaces which inhibit the growth of pollen in stigma.
  10. Manipulation of pistil : Done by removing style length by cutting its size, so as easily the pollen tube get germinate and pollinates.
Embryo culture: Growing plant by culturing of hybrid embryos which are not able to germinate.

About Author / Additional Info:
I am pursuing PhD in Horticulture (Vegetable Science) from G.B. Pant University of Agriculture and Technology, Pantnagar (Uttarakhand), India.