Euphytica 7 (1958) : 119-122

THE BREEDING OF HYBRID VARIETIESAND THE PRODUCTION OF HYBRID SEED IN SPINACH

J. SNEEPVegetable Research Station, Alkmaar ; formerly Head,

Division of Vegetables,Institute of Horticultural Plant Breeding, Wageningen

Received 18 April 1958

ABSTRACT

In spinach the breeding of hybrid varieties is attractive because this may result ina heterosis and/or a transgressive effect . Resistance to downy mildew [Peronosporaspinaciae (MONT.) DE BY] and against mosaic (Cucumis virus, 1) can be combinedrather easily, since in both cases the resistance depends on one dominant gene .Besides a hybrid variety excludes the growing of later generations .

However, a careful removal of all pollen-producing plants demands so much labourthat a production of hybrid seed on a commercial scale involves great difficulties .The creation of an entirely female variety would be a solution for this problem . Anoutline has been given of the way in which such a variety can be developed, startingfrom purely female and from intersexual plants .

It is possible to simplify the production of hybrid seed by using a round-seededfemale parent and a prickly-seeded male parent. Moreover this will prevent errorsall plants can then be harvested and threshed at the same time and in cleaning theprickly seed can be separated mechanically from the round-seeded commercial seed .The use of a prickly-seeded male parent will be an extra check on the possible growingof later generations by others .

INTRODUCTION

In a previous article (3) the writer has given a survey of spinach breeding in general .In this article his views are given on the possibilities of creating hybrid varieties

and the production of hybrid seed on a commercial scale .

THE AIMS OF PRODUCING FIRST-GENERATION HYBRID SEED

The aims of breeding hybrid varieties are1 . to produce a heterosis and/or a transgressive effect .2. to achieve a combination of favourable characters that are governed by dominant

genes .3. to obtain a protection of the breeding product .

sub. 1 . In the literature some data are mentioned stating that F,'s produce a highertotal leaf yield than the better parent (see 2, references 80, 53, 121) . Various crosses ofinbred parent plants in our own experiments also gave very productive F 1 's . But it is

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still an open question whether this yield increase is the result of heterosis, of trans-gression, or of a combination of both .

sub . 2 . Characters that should be considered for combination are in the first placeresistance against downy mildew [Peronospora spinaciae (MONT.) DE BY] and theresistance against mosaic (Cucumis virus 1), which are both governed by one dominantgene .

The combination of a downy mildew-resistant variety with a non-resistant one mayalso offer fair prospects in regions and in methods of cultivation where no mosaicoccurs .

sub 3. A hybrid variety will segregate when later generations are grown and it mayalso cause inbreeding degeneration . Therefore the breeder need not fear that his"variety" is grown by others in later generations .

DIFFICULTIES INVOLVED IN THE MOST OBVIOUS PROCEDURE

At first sight spinach seems a very suitable crop for the breeding of hybrid varieties,on account of its dioecious character : emasculation is not necessary, only the pollenproducing plants of the mother variety must be removed . However, there are seriousobjections against this procedure because :

1. A careful removal of all the pollen producing plants from the mother varietydemands much labour in a short space of time . Experiments showed that, even invarieties with few intersexual plants, at least 200 hours of a trained worker are neces-sary per ha of the mother variety . And all this work must be done within three weeks,in eight times, with intervals of two to five days (dependent on the weather) .

2. The fairly small quantity of seed that is harvested per ha in seed production andthe fairly large quantity of seed used by the growers per ha require that the costinvolved in the production of hybrid seed should not be too high .

The drawbacks mentioned above are so serious that it is highly questionablewhether with a mother variety consisting of female, male and intersexual plants it willbe ever possible to grow highly valuable hybrid seed on a commercial scale .

THE CREATION OF A FEMALE VARIETY

It is, therefore, no wonder that the suggestion has been made to investigate thepossibility of developing an entirely female variety . Attempts have been made toapproach this ideal by means of physiological procedures and by genetical analysis .The first-mentioned method has not made much progress, for the influencing of sexhas hardly any prospects . However, the results obtained by the genetic analysis of sexare encouraging .

The data gathered on the inheritance of sex can be best explained on the basis ofthe hypothesis of BEMIS & WILSON (1) : XYfor S and XX for Y, with two strongly-

HYBRID VARIETIES IN SPINACH

linked autosomal loci for modifying genes viz . AA for the tendency to become maleand GG for the tendency to become female . So long as these modifying genes arebalanced, there is a constant recurrence of dioecious plants : e .g . XXAAGG, XXaaggand XXAaGg for purely female and XYAAGG, XYaagg and XYAaGg for purelymale plants . If this balance between the male and female tendency is broken, thenintersexual plants will result, e .g . XXAAgg, X.YAagg, XYaaGG and XYaaGg.

Although there is an indication that more than two pairs of modifying genes areinvolved in sex inheritance, the representation given above is well suited for drawingthe outlines for the creation of an entirely female variety :

1 . First a cross is made between a purely female plant XXaagg and an intersexualplant XYaaGG :

XXaagg x XYaaGG -> XXaaGg and XYaaGg

2. The female plants obtained from this cross are once more crossed with an inter-sexual plant of the genotype XYaaGG:

XXaaGg x XYaaGG - XXaaGG, XXaaGg, XYaaGG and XYaaGg

3 . For increase the "super"-female plants XXaaGG are then crossed with an inter-sexual plant of the genotype XXAAgg . The seed obtained from this cross willcontain exclusively female embryos :

XXaaGG x XXAAgg > XXAaGg

4. The purely female plants XXAaGg can be used as mother plants for the product-ion of hybrid seed :

XYAAGGXXAaGg x XYaagg

-~ commercial seedX YAaGg

etc.

Purely female mother variety x male parent

PRACTICAL REALIZATION OF THE PROCEDURE

The procedure seems very simple on the face of it. However, a difficulty in carryingout the different stages is that the genotype of the plants XXaagg and XYaaGGmentioned sub 1, and sub 1 and 2 respectively, cannot be verified phenotypically . Thisis also the case with the genotype XXaaGG mentioned sub 2, since XXaagg andXXAAGG are identical phenotypically. And the same holds for XYaaGG andXYaaGg and for XXaaGG and XXaaGg .

For simplification of the growing of commercial seed, and also for avoiding errors,it is recommendable to use a round-seeded mother variety and a prickly-seeded fathervariety. All plants can then be harvested and threshed at the same time since thecleaners will later separate the prickly seed from the round commercial hybrid seed .

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SAMENVATTING

Het kweken van hybride-rassenen de produktie van hybride-zaad by spinazie

Bij spinazie is het kweken van hybride-rassen aantrekkelijk omdat een heterosisen/of transgressie-effect mogelijk is. Verder kan de resistentie tegen wolf [Peronosporaspinaciae (MONT.) DE BY] en die tegen mozalek (Cucumis virus 1) betrekkelijk een-voudig worden gecombineerd, daar ze in beide gevallen op een dominante factorberust. Een hybride-ras sluit bovendien nabouw uit.

Het zorgvuldig verwijderen van alle stuifmeelleverende planten uit het moederrasvergt echter zoveel arbeid, dat aan de produktie van hybride-zaad op commercieleschaal grote moeilijkheden zijn verbonden . Het kweken van een geheel vrouwelijkras zal hier de oplossing kunnen brengen . In het artikel is uitgestippeld hoe een derge-lijk ras, uitgaande van zuiver vrouwelijke en van gemengdslachtige planten, gekweektkan worden .

Een vereenvoudiging van de produktie van hybride-zaad is voorts mogelijk dooreen rondzadig moederras en een scherpzadig vaderras to gebruiken . Dit zal tevensvergissingen voorkomen : alle planten kunnen dan tegelijk geoogst en gedorst wordenen bij de reiniging scheidt men machinaal het scherpe zaad van het rondzadige han-delszaad. Het gebruik van een scherpzadig vaderras zal tevens nog een extra rem opeventuele nateelt leggen.

REFERENCES

1. BEMIS, W. P . and Wi soN, G. B ., A new hypothesis explaining the genetics of sex determination inSpinacia oleracea L ., J. Heredity 44 (1953) : 90-95.

2. SNEEP, J., De stand van de veredeling bij spinazie (The present position of spinach breeding) . ThesisWageningen 1957 . Also published in the series of the „Instituut voor de Veredeling van Tuinbouw-gewassen, Wageningen, Nederland", Mededeling 113, 1957, 198 pp., 41 figs. In this thesis manyreferences are mentioned.

3 . SNEEP, J., The present position of spinach breeding. Euphytica 7 (1958) : 1-8 .

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