Proposal for Shift to Reciprocal Recurrent Selections in Clone and Strain Synthesis Timothy Breeding using Molecular Marker Diversity

Integration of reciprocal recurrent selection (RRS) with the formation and use of heterotic groups is a key strategy for yield improvement in timothy (Phleum pratense L.) breeding. This study aims at utilizing putative heterotic groups based on molecular marker diversity in clone and strain synthesis (CSS) development. Strains are synthesized by combining two top-cross progenies, that is, comprising two seed parental clones (SPs) and a pollen parental strain (PP). A significant positive correlation (r = 0.64, p < 0.05) was found between forage yields of 15 (= C-6(2)) CSS strains and genetic distance estimates among all possible pairs of their six SPs using 28 simple sequence repeat markers. This result suggests that the adoption of elite clones as SPs of CSS strains, selected from putative heterotic groups formed on the basis of molecular markers, results in the efficient development of high-yielding strains with exploiting specific combining ability (SCA) effects among SPs. The use of CSS has some advantages compared with conventional cultivar synthesis. One is that RRS with two or three heterotic groups can be established in CSS. Another is that current leading cultivars can be utilized as the PP. These advantages and the results presented in this study has led us to a procedure that consists of (i) a division of germplasm pools based on molecular marker diversity and (ii) a confirmation of the separated groups based on test cross progeny tests. The procedure provides an effective introduction of RSS into CSS development.