The additive and non-additive variance components were estimated from progenies derived from two samples of parents (representing a northern continental type climate) for five factors relating to plant survival and two composites of the factors. It was found that additive variance made up 90 and 100%, 91 and 100%, 91 and 100%, 100 and 100%, 82 and 59%, 91 and 100%, and 90 and 100% of the total genetic variance for leafing-out date, leafingout percent, tip injury, stem damage, root damage, a shoot composite, and a shoot-root composite for the two samples respectively. A third sample had 100% additive variance for plant height while, in contrast, a sample of rootstocks, differing from each other in their ability to dwarf grafted scions, had approximately 50-70% additive variance for plant height. It was shown that breeding progress for both winter survival and plant height could be achieved by exploiting the additive variance, the total genetic variance, or (where progenies were the selection unit rather than individuals) by progeny selection. By exploiting the additive variance, it should be possible to improve plant survival and change plant height in each of several successive generations. It is predicted that (with the exception of selection for vigor in a population having a range of dwarfing abilities) potential parents could be efficiently screened phenotypically and so obviate the need for genotypic evaluation. A total of 9180 progeny trees were involved in the analyses considered in this paper.

Heritability and genetic variance components for 20 fruit and plant characters were investigated in 64 progenies produced among 31 North American and 1 German strawberry clone under the crossing scheme similar to North Carolina Design II (Comstock and Robinson, 1952). The data were analyzed based on the assumption of random and/or mixed models of the parents. The estimation of the genetic variance components indicated that for more than half the 20 characters nonadditive variance (dominance + epistasis) constituted approximately 50% or more of the total genetic variance and that in most cases epistasis played an important role in the nonadditive variance. Heritability estimates were low (less than 18%) for total berry yield and some fruit quality characters such as firmness, easy capping, pH value, soluble solids, and external and internal appearances but were high (more than 37%) for a number of yield component characters such as average berry weight, berries/flower stalk, yield/flower stalk and flower stalk number, suggesting that substantial improvement in total berry yield could be achieved through selection on these yield components or a linear function of them.

The performance in fruit yield characteristics and fruit appearance of seven Premier S5 inbred selections was studied in their crosses with four tester varieties, Redcoat, Senga Sengana, Sparkle and Jerseybelle. Some of the progenies produced by crossing Premier S5, inbreds with testers were superior in a number of yield characters to those produced when Premier was crossed to the same testers. This suggests that inbreeding, although expensive and time consuming, may be a useful step in one type of approach to breeding for higher yield in the cultivated strawberry.General combining abilities of the inbred selections and of the testers were both significant for all but one character, while specific combining ability of inbreds × testers was significant for six of the eight characters. The estimates of general combining ability effects of the inbreds and the testers were found to be of limited value in predicting the most desirable single crosses. The significance of this is discussed together with a proposed procedure for exploiting inbreeding in strawberry breeding.

No abstract

Two methods of determining the relative importance of the additive, dominance and epistatic components of genetic variance indicated that the nonadditive variance (most of which was shown to be epistatic) constituted approximately 50% of the total genetic variance for 20 commercial characteristics of the cultivated strawberry. With nonadditive, particularly epistatic, variance being so important, genetic progress may best be achieved by a two-step breeding procedure involving small scale testing of all the progenies followed by large scale testing of the best progenies. Breeding procedures involving such methods as reciprocal recurrent selection, inbreeding and backcrossing would be more efficient as a means of generating special types of parents for such a program than they would be as isolated breeding procedures.