Manipulation of Sex Expression in Vegetables for Hybrid Seed Production

What are Hybrids?

Basically hybrids are the superior progenies derived from the crossing of two or more parents. Actually the hybrids possess the best traits from both the parents.

Sex forms and Hybrids:

We know that there are many vegetables like cucurbits, asparagus, amaranth, spinach etc. produces various sex forms in nature in both cultivated and wild forms. The original sex forms in most of these vegetables are hermaphrodite but due to continuous evolution in nature as a result of natural mutation and preferred selection by human new forms in nature appeared. In natural occurring sexes, some of it alone are very useful and some are of less important.

Sex forms of Cucurbits

Most interesting part is the sex expression of these plant can be altered or modified in favour of the aimed crosses. Sex manipulation is the process of alteration. In simple words, we can alter the sex expression externally to produce hybrids with desirable traits. Moreover in some cases traditional hybrid production by hand emasculation and hand pollination is a tough job and is not economical. In that case sex alteration can be a useful and efficient tool for hybrid production. A brief in regard to this topic is pointed below.

A.   Cucurbits

Among vegetable groups, one of the largest group showing diversity is Cucurbitaceous vegetables. The group comprised of almost 900 species classified under 130 genera. The cucurbits shows a fascinating range of sex forms. A wide range of variation in sex forms owed to evolve from primitive sex form hermaphrodite could lead to evolution of predominant sex form i.e. monoecious and advanced sex form i.e. gynoecious.

How sex forms in cucurbits could be modified?

·         Controlling Environment

·         Nutritional and Cultural Practices

·         Growth Regulators

·         Chemical Hybridizing Agents (CHAs)

·         Biotechnological interventions.

Effect of Boron on Cucurbit sex expression

Breeding systems in cucurbits:

Gynomonoecious and gynoecious lines:

Gynomonoecious and gynoecious sex forms are most important sex forms in commercial production of hybrids, especially in cucumber.

Method 1 (Use of gynomonoecious line): Generally. Gynomonoecious lines are grown with monoecious lines in alternate rows with a ratio of 4:1. The Gynomonoecious lines give rise to basically three types of plant, viz. a) Gynoecious (important as seed parent), b) Monoecious (important as pollen donor) and c) Intermediate (not important). At 10th node stage when male flowers appears first the monoecious derived from gynomonoecious parent and intermediate forms are removed (uprooted) from the breeding blocks. Then the gynoecious plants are left to set hybrid seed with the desired monoecious lines.

Schematic representation of hybrid production in cucumber using Gynomonoecious and Monoecious line

Maintenance of Gynomonoecious line:

For maintenance of gynoecious & gynomonoecious lines hermaphrodite forms plays key role. Kubicki proposed how hermaphrodite plants can act as complementary maintainers. Here gynoecious (MMFF) lines are crossed with an andromonoecious line (mmff). After several generation of back crossing stable hermaphrodite forms (mmFF) are produced. This hermaphrodite forms are again back crossed several times with the gynoecious (MMFF) line as recurrent parent. This results into production of two isogenic line, viz. gynoecious and hermaphrodite. When these two isogenic lines were crossed all gynomonoecious lines were produced.

Hybrid production in cucumber using gynoecious line: As proposed by Kubicki

Method 2 (Use of gynoecious line):

2a. Gynoecious line crossed with Monoecious inbreds:

This is one of the efficient method of hybrid seed production in cucumber, as it ensures the less chance of self-pollination and assures 100% of hybrid seeds. The F₁ hybrid produced are gynoecious when gynoecious lines are used as female parent and hermaphrodite forms are used as pollen parent. But the main drawback is it cannot be utilised at commercial scale.

2b. Use of two separate gynoecious lines:

Any one line of the two gynoecious line is treated with Gibberellic Acid or AgNO₃ to produce male flower for pollination.

Hybrid production using gynoecious line in cucumber

Hybrids with all pistillate flowers have been developed for maximizing the yield in cucumber. In this case some pollen producing plants (act as pollinator for fruit development in hybrids) have to grow with this hybrids. But in this case yield actually reduced as in later stage fruit growth was inhibited due to the development of seeds.

To solve this problem, both in slicing and pickling cucumber parthenocarpic gynoecious lines have been developed.

In muskmelon, hermaphrodite (aagg) and monoecious (AAGG) lines are crossed to produce gynoecious (AAgg) and hermaphrodite (aagg) types. For self-seeds in hermaphrodite plants application of MCEB (5-methyl-7chloro-4-ethoxycarboxylmethoxy-2,1,3-benzothiodiazole) is reported to be effective at 4^th true-leaf stage followed by hand pollination.

Maintenance of gynoecious line:

Gynoecious lines are mainly maintained by the application of Gibberellic Acid (GA) and AgNO₃ at small seedling stage (2-4 true-leaf stage). Foliar application of GA @1500-5000 ppm induces male flower in gynoecious line. Even GA₃ at low concentration and AgNO₃ at low concentration (@50-500 ppm) found to be more effective in inducing staminate flowers.

Method 3 (use of Growth Regulator):

In most of the monoecious cucurbits like bottle gourd, Bitter Gourd, Cucumber, Muskmelon normally staminate flowers appear first at early node followed by the pistillate or a mixture of staminate & pistillate at later stage. Now it has been proved that Ethrel @50-100 ppm at 2-3 true leaf stage reverse the normal sex expression of monoecious cucurbits viz. induces pistillate flower at early nodes followed by the staminate and mixed form at later stage. Using this principle hybridization techniques in monoecious cucurbits can be deployed.

Firstly selection of two desirable monoecious lines or any two line one of which is monoecious are done. Those two lines are grown at 4:2 (monoecious : pollen parent) ratio. Now the monoecious plants are treated with Ethrel @ 50-100 ppm at 2-3 true leaf stage induces female flowers at early nodes. Simultaneously pollen parent produces their male flowers and pollen from these fertilise the female flowers and produces sufficient hybrid seed, after fruit-set all pollinator rows are destroyed.

Use of Growth Regulator in Cucurbits Sex Expression

Method 4 (Manual pollination):

This method is generally followed in dioecious plants like pointed gourd, ivy gourd etc. where hand pollination are done between two desirable lines manually. It’s very cost intensive operation for commercial scale.

B.   Spinach:

Spinach is monoecious and a wide range of sex expression from nearly complete female to nearly complete male was observed. The complete female line can be produced through proper selection. Single Cross hybrids and Double Cross Hybrids are most potent in hybrid seed production of Spinach. In Single Cross highly male and highly female lines are used. In Double Cross highly female monecious F₁ and highly female monoecious lines are used.

The technique involves growing of 5-6 rows of highly female monecious lines are grown alternatively with 1-2 lines of pollinators. In female lines roguing of few male plants are done as soon as they appear and identified. Pollination between female and male monoecious lines give rise to hybrids.

Even dioecious population can be used. In this case male and females are distinguished by sparse foliation and early simultaneous bolting.

In Spinach Prima and Echo hybrids are developed through this method.

C.   Asparagus:

Asparagus are generally dioecious. There are female (YY) and two type of male, viz. normal male (XY) and Supermale (XX). Supermale are not much productive in respect to the production but on the breeding point of view it is valuable. Generally supermale occurrence is erratic, sometime produced in functional female flower naturally or through pollen culture. In asparagus, segregating families from hermaphrodite lines may be utilised for hybrid production. In this case high proportion of male plants are desired. Double Cross hybrids are popular in asparagus.