This is the thirteenth article in a series of brief progress reports on the application of the science of ge- Mal e -S t e ri I e To ma to es netics to commercial agriculture. unfruitful mutants offer several advantages for the production of hybrid seed Charles M. Rick Male-sterile tomatoes offer advan- ties, but it is entirely feasible to obtain tages in the production of hybrid seed- a male-sterile mutant in any variety from which substantially higher yields merely by searching for it, provided a than those of parent varieties have been sufficiently large planting-20 to 30 acres reported. -is available for inspection. Because tomatoes are naturally self- Male-sterile plants show the following pollinated, they are highly inbred and characteristics: 1 , pollen is nonviable or resemble corn in respect to the vigor of absent; 2, anthers are reduced in size to hybrids between inbred lines. Dominant varying degrees and are often modified disease resistance or other desirable in color; and 3, the whole flower may be dominant characters of one or both par- slightly smaller than that of normal ents can be combined in the hybrids. plants. In all other features the plants Fertile flower at left; male-sterile flower at appear to be normal. These advantages have not been fully ex- right. Note that male-sterile anthers-in center ploited because large-scale production of of flower-are shorter and more slender. If viable pollen is applied to flowers hybrid tomato seed by the usual methods of male-sterile plants, fruits and seeds is too costly. circumvented By usiiig male-sterile plants, are produced in normal fashion. Many To cross-pollinate the flowers of two in which the anthers are effectively ster- similar examples of male sterility are known in corn, onions and many other different tomato plants necessary to ilized by the action of a single gene. it is plants. take precautions against self-pollination of the female parent. This is accom- Mutants plished in fertile plants by removing the Male-sterile mutants occur naturally Inheritance anthers-pollen-bearing organs-before in tomato varieties as one of the types the flower bud opens, by a tedious hand Male sterility is inherited very simply, of extremely unfruitful plants-the so- operation. Corn, to the contrary, can be being controlled in each mutant thus far called, bull, male, or rogue plants. investigated by a single recessive gene. emasculated very simply and quickly by These very unfruitful plants appear at merely pulling out the tassel before it The hybrid between a male-sterile and variable rates, with a mean of about one a fertile plant-F,, first generation- sheds pollen. The risk of self-pollination per every 1,000 fruitful plants-rates so of the female parent in tomatoes can be would be expected to be fertile. Self- low that they do not appreciably decrease pollination of this hybrid yields a second total yields. In spite of this very low fre- generation-F,-consisting of approxi- quency, they are very easily found, even mately 75% fertile and 25% sterile GENE- TYPES at a considerable distance, since they RATION AND FREQUENCIES plants; and the backcross between the F, grow many times larger and higher than hybrid and the sterile plants produces ap- surrounding fruitful plants, which are weighted to the ground by their fruit Continued on page 12 load. In the late harvest season it is a very simple procedure to collect and examine GENE- TYPES AND FREQUENCIES a large number of unfruitful plants in a RATlPN short time; about 200 have been collected 6 100 70 and studied to date. Inherited male sterility causes unfruit- fulness in 5% to 10% of the unfruitful plants-that is, 0.005% to 0.01% of all [GENOTYPE: Msrn plants. Abnormal chromosome numbers and many other inherited defects account for the sterility of the other plants. Fourteen different male-sterile mutants have been discovered in the varieties -x- Earliana, Early Santa Clara, Pearson and + San Marzano. These differ somewhat from each other in the contents of their anthers and, to some extent, in external GENOTYPE: 50%V.~+ 50%msm CROSS 50k 50& 50& appearance. Mutants in at least three BACK- ‘*OS 1PHENOTYPE: other varieties have been found by other I Diagram of inheritonce of mole-sterile condi- workers. tion. Flowers with plump anthers represent fer- Diagram of inheritance of male-sterile ron- tile type; those with shrunken anthers represent It is not necessary to breed the male dition. Plump onthers represent fertile type, sterile type. sterility from mutants into desired varie- shrunken anthers represent male-sterile type. CALIFORNIA AGRICULTURE, APRIL, 1950 5 3. Most striking is the fact that yield HYBRIDS ject to relatively high rates of cross-polli- was not significantly reduced at pH's as nation and at the same time, are readily Continued from page 7 low as 3.5. The pH of the heavy sulfur identified and pollinated. strip averaged 3.7 at harvest in 1949 and proximately equal numbers of fertile and Charles M. Rick is Associate Professor of total yield was no different from the sterile plants. Truck Crops and Associate Geneticist in the checks on either side at pH 7.0. By backcrossing in this manner, male- Experiment Station Davis. The effect of pH on potatoes is now sterile plants can be propagated indefi- being studied to determine whether sul- nitely and in as large numbers as desired. fur toxicity, rather than low pH, is re- It is also expedient for special purposes OLIVES sponsible for some of the crop failure to propagate the male-sterile mutant as a after sulfur applications. clone by rooting cuttings. Continued from page 4 A second set of sulfur soil trials in One curious and exceptional mutant- duced total yield and at the same time September 1948-at rates of application designated as msj-produces small quan- pay for the cost of thinning, the income similar to those of the February 1948 tities of fertile pollen in occasional flow- per tree was calculated from the yield experiments-brought out the fact that ers and can therefore be propagated by records and size grades. oxidation of elemental sulfur in the soil self-pollination to produce 100% male- After thinning and harvesting costs by the sulfur bacteria is dependent upon sterile progenies. the early-thinned trees gave a return of abundant soil moisture. Male-sterile mutants are useful for any $11.56 per tree in comparison with $7.06 In the absence of any appreciable rain- project that demands large-scale cross- per tree from the check trees, an increase fall, the soil remained dry until the pre- pollination. Their use eliminates not only of $4.50 per tree or approximafely $225 planting irrigation in February. The pH the need for costly emasculation but also an acre. of all treatments remained neutral until the possibility that contamination by self- The trees that were thinned later in February and then dropped abruptly. pollination might occur if flowers of fer- the summer, on July 19th and August The strips receiving the 2,500-pound ap- tile plants were not properly emasculated. 16th, failed to show any appreciable ben- plication dropped from pH 6.8 to 4.5 in Hybrid seed production is being facili- efits. While fruit sizes in the later thinned 40 days. tated also by the invention by an Austra- trees were greater than in the unthinned Many older potato fields have a reac- lian worker of a simple mechanical pollen trees, apparently the yield was reduced to tion of about pH 6.0-due to the applica- collector. such an extent by the later thinning that tion of ammonium sulfate fertilizers Certain hybrids are very difficult to the increased fruit sizes failed to offset which tend to change the soil reaction to obtain and may require the pollination this reduced yield. the acid side. In the experimental field of great numbers of flowers. In transfer- Another fruit thinning experiment with initial pH was 7.5 and 2,500 pounds of ring desired characters from species of olives was conducted in an orchard near sulfur were effective on scab with no wild forms to cultivated tomatoes, it is Davis in 1949. harmful effects on potato yields. It is not important that accidental self-pollination Two Mission variety trees were used, likely that 2,500 pounds of sulfur per of fertile plants be avoided. The contami- about 25 years old and of moderate size, acre could be safely applied to fields-with nation of such crosses is misleading and growing in a border row. Both trees were pH 6.0. Many instances of poor stands can be pretented by utilizing, as female heavily loaded with fruit, and the thin- and yields have been reported where sul- parents, male-sterile plants planted to- ning was performed on June 30th. fur applications have been too great. gether in a plot well isolated from other Individual branches on the same tree Since it apparently takes two years to tomatoes. were given different amounts of thinning renovate scabby land with sulfur, it seems Male-sterile mutants also serve effici- to determine how localized the thinning advisable not to crop the land to potatoes ently to measure rates of natural cross- effect was. Fruits and leaves were counted the first year after the treatment. No con- pollination because all fruits and seeds following thinning to obtain a leaf-fruit trol of scab could be expected and the that they produce must issue from pollen ratio. Each treatment was given to three infestation of the fungus would be build- transferred to them from surrounding branches.