Heterosis...Hybrid Vigor

[jcm05]

I was hoping one of the guys could explain exactly what causes heterosis in hybrids. Thanks.

[bluelacedredhead]

Is that when they get bad breath from ingesting too much manure?

[gobmaters]

As the saying goes,"It takes two to Tango". The wide spread development and use of hybrids in tomatoes in the last 50 years or so is based on a lot more than heterosis. In fact, tomatoes being a self pollinated crop do not show the degree of hybrid vigor seen in out crossing crops such as corn. Development of hybrids in tomato allows for combining resistance to multiple diseases since the genes used for disease resistance in tomato are dominant R genes that work well in heterozygous condition. It is easier to develop two lines with different sets of resistance genes than it is to put all the genes into a single op variety. Also, lots of F1 crosses are tested to select ones which have the best combinations of disease resistance and desirable plant and fruit traits. Total yield in tomatoes, particularly for fresh market, is secondary to marketable yield. Although a reasonable yield is important, I don't know of anyone who breeds only to increase total yield in fresh market tomato, and this is one of the primary uses of heterosis in developing hybrid field crops.

Having disease resistance genes in heterozygous condition can be an advantage for some resistance genes. For example , hybrids heterozygous for the Mi gene for root knot nematode resistance are generally more crack resistant than varieties that have Mi in the homozygous condition. The I-3 gene for resistance to race 3 of fusarium wilt is associated with smaller fruit size in the homozygous condition, but hybrids with much larger fruit can be developed when I-3 is hetrerozygous. Large fruit size in tomatoes with good fruit set under high temperatures is hard to achieve, but fruit size in F1 hybrids of this type can be achieved when a very large fruited heat sensitive parent is crosssed with a smaller fruited heat set parent. It may be that for other genes there is some deleterious characteristic associated with a particular gene when in the homozygous condition that is overcome to some extent when the gene is in the heterozygous condition.In some crosses, hybrids are earlier in maturity than either of the parents and it is easier to combine large fruit size with earliness in hybrids compared to op varieties. For some traits in tomato, expression of the trait in heterozygous condition is more desirable and shows better adaptability to a range of environmental conditions than does the homozygous condition present in op varieties. Although genes in tomato are classified primarily as dominant or recessive in their action, not many genes are completely dominant or recessive so there is some degree , although it may be only slight, of intermediate expression when two different conditions of genes are crossed.

The over riding factor in the development of hybrid tomatoes is related to economics and control of seed production for a particular hybrid. Hybrid seed cost much more to produce than op seed, but they also sell for a lot more money, which means greater profitability for seed companies. If the seed company owns both lines of a variety and can keep their identity and use in producing the hybrids from other companies, it gives them exclusive protection for the hybrid so no other company can produce and sell it. This also gives them greater control over marketing because they know how much seed will be in the market and don't have to compete with other companies for seed sales of a particular variety.

[tomato]

When gene expression is an intermediate form, it is called "penetrant" meaning that varying degrees of expression can occur. This is common in hybrids that are heterozygous for a given trait. It is what causes a red flower crossed with a white flower to produce a pink flower. This 'in between' condition is typical of a penetrant trait.


Here is a slightly different example from tomato. The P20 selection from OSU is a red tomato with 3 genes that produce anthocyanin. When mule crossed P20 with a PH selection, he brought in the gene for pink fruit. In the segregating population, I picked out a potato leaf plant with pink fruit and high levels of anthocyanin. Now that I am growing the seedlings, I can easily pick out the plants from the pink fruit. They have a royal blue/purple color on the underside of the leaves. The red fruited lines all have blue/black color under the leaves. It is very noticeable when they are side by side. This condition is not technically an example of penetrance, rather it is a modifier gene causing a similar effect.

So you could define penetrance as caused by a gene that directly affects a given trait and gives varying levels of expression in heterozygous condition. A modifier is a gene that does NOT directly affect a given trait but can cause the trait to have varying levels of expression.

[PVP]

A couple of questions:

"Large fruit size in tomatoes with good fruit set under high temperatures is hard to achieve, but fruit size in F1 hybrids of this type can be achieved when a very large fruited heat sensitive parent is crossed with a smaller fruited heat set parent." [GOBMater]

I've been told, and seen it in limited experience, that since small fruit is dominant over large fruit, such a cross will result in smaller fruit than the larger of the two parents. Question: When growing out the generations and selecting for the larger fruit, can one maintain the same degree of heat tolerance obtained in the F1 or is it lost or necessarily lessened with the loss of "heterogenous-ness?"



"In some crosses, hybrids are earlier in maturity than either of the parents and it is easier to combine large fruit size with earliness in hybrids compared to OP varieties." [GOBMater]

Same question: Is the earliness lost or necessarily lessened when growing out the subsequent generations?

[PVP]

Another question:

"In some crosses, hybrids are earlier in maturity than either of the parents and it is easier to combine large fruit size with earliness in hybrids compared to OP varieties." [GOBMater]

You say "in some crosses" you achieve earlier fruit maturity than in either parent. Why is this not the case "in all crosses?" How does one know ahead of time, or can one know ahead of time in which crosses the earliness will be increased?

[gobmaters]

papavic,
Lines with excellent heat set ability tend to set a much larger number of fruit under high temperature conditions than heat sensitive lines so this reduces fruit size. Also, under high temperature conditions fruit size tends to be smaller than under lower temperatures because the plants are not as efficient at photosynthesis at the higher temperatures and burn up more of the sugar manufactured during the day by higher night time respiration at the high temperatures. Because of this physiological response, it is very difficult to combine large fruit size with excellent set at high temperatures. The hybrid from a cross of a very large fruited heat sensitive line will not produce as large a fruit size as the large parent and will not produce as many fruit as the smaller fruited hot set line. The hybrid is intermediate in fruit size to the parents and also intermediate in fruit set. However, there is generally enough dominance for high temperature fruit set to make the hybrid productive enough to be grown in hot set conditions.

Earliness tends to be dominant in F1 crosses so crossing a very early flowering line with a later flowering line generally makes a hybrid that is more toward the earlier flowering parent in maturity. To get as large fruit size as possible in the f1, it is necessary to cross parents which both have large fruit size. It is difficult to select for really large fruit size combined with early maturity since there is a negative correlation between earliness and fruit size. Again, crossing an early maturing line with as large fruit size as possible with a later maturing line with very large fruit will improve the fruit size in the hybrid over that present in the early maturing parent but give earlier maturity than the later maturing parent because of the dominance for earliness. It is not always possible to predict what the earliness or fruit size will be in such crosses, so breeders search through numerous hybrids to identify those which combine earliness with as large fruit size as possible. Another important component of earliness particularly in determinate hybrids is the ability of the plant to produce a large crown set. This is generally achieved by developing hybrids that have the first two flower clusters separated by one leaf.

[gobmaters]

I grew a hybrid last summer from the cross of 'Chocolate Stripes' x a pink fruited late blight resistant line with 'Momotaro" in its background, and it had very good flavor. I will be growing a large F2 from the hybrid this summer and look forward to all the segregation this cross will provide- clear skin, stripes, fasciated type, flesh color,and flavor along with disease resistance. I am pretty excited about the prospects of developing some nice tomatoes with varied characteristics from this cross.
GOBmaters

[gobmaters]

mulio,
Yes, I have one I can operate with my fingers. Is that what you mean by digital?
GOBmaters

[jcm05]

Can you mail us the pictures? ;D