While breeding methods to exploit only general combining ability (GCA) have been widely adopted in perennial and self‐incompatible forage crops, maize breeders have had great success in improving maize yield by developing hybrid breeding, whereby not only GCA but also specific combining ability (SCA) can be effectively exploited. A number of trials to apply hybrid breeding to such crops have not been successful because of severe inbreeding depression and/or ineffectiveness in commercial seed production. The objective of this study was to evaluate the potential of ‘clone and strain synthesis’ (CSS), a method in which both GCA and SCA can be exploited effectively to breed perennial and self‐incompatible crops. CSS consists of two procedures: (i) harvest of syn‐1 seeds from two seed parental clones after mating of each clone with a pollen parental strain under separate, isolated conditions; and (ii) even mixing of the syn‐1 seeds to produce subsequent generations, as is done to develop conventional synthetic strains from a number of parental clones. Therefore, the yield of a strain developed by CSS depends partly on combining ability, including SCA, among the three parental materials. Fifteen timothy (Phleum pratense L.) strains were developed by CSS for verification. The productivity test results indicated that four strains in the syn‐2 had significantly higher yield levels than the commercial variety adopted as their pollen parent, suggesting the potential of CSS to breed high‐yield varieties for forage, biofuel, or other uses. Yield of the strains was applied to a diallel analysis to evaluate the relative importance of SCA toward GCA; the results suggested that SCA was important in determining yield level.
Assessment of genetic diversity based on DNA profiling contributes to the selection of superior parents for heterosis. The objective of this study was to investigate how closely the yields of timothy (Phleum pratense L.) topcross progenies are related to the genetic diversity between their seed parents and the pollen tester parent ‘Aurora’, and to predict their yield potentials from the index based on simple sequence repeat (SSR) marker polymorphisms. Genetic diversity among 67 genotypes, which consisted of 16 plants of timothy cultivar Aurora, the pollen parent for topcross testers, and 51 clones derived from breeding populations was evaluated based on 28 SSR markers carrying 408 alleles. Genetic distance (GD) was estimated for all possible pairs of genotypes and then the mean of the GD estimates between each clone and Aurora (GDA) was also assessed to find the relationship between GDA estimates and yields of topcross progenies derived from all crosses between each clone and Aurora. The GDA estimates of the clones ranged from 0.653 to 0.756, and a correlation coefficient between the GDA estimates and yields of their topcross progenies was 0.675 (p < 0.001). These results strongly suggest that GD would provide reliable information on the yields of the assumed topcross progeny lines and that it will help breeders to accelerate yield improvements for a practical breeding program of timothy.
Hybrid varieties have not become commercially successful in perennial and self-incompatible forage crops because of their severe inbreeding depression and/or ineffectiveness in commercial seed production. Here, a modified synthetic variety (MSV), a breeding method for forage crops is proposed, where specific combining ability (SCA) is exploited in a way different from that in hybrids. As Syn-l seeds from only two of its parental clones are used to produce its Syn-2, its performance in Syn-2 partially depends on the SCA between the two seed parents. The inbreeding coefficient of MSVs can be as low as that of conventional synthetic varieties sold today. To evaluate its advantages, 15 modified synthetic strains (MSSs) of timothy were developed from a set of Syn-l seeds of a conventional synthetic strain (CSS) having six parental clones. An MSS (Syn-2) showed a yield level equivalent to and a disease score significantly lower than the CSS (Syn-1) in a field test. The diallel crossing analysis implied that the SCA effects are influential for yield, and that MSV or other SCA-exploiting breeding methods have the potential to improve the yield level of timothy effectively.
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