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In hybrid rice breeding programs, it is the most important to select donors for improving parental lines and to predict the hybrid performance. In this study, AFLP marker technology and a half-diallel method were used to address these related issues in hybrid rice. Based on the representative heterosis characters representing in the F 1 generation, an innovative method was designed to identify the genetic diversity related to heterosis for yielding with the detected AFLP. The results showed that the genetic diversity among the parental lines was certainly related to F 1 heterosis, and a few loci contributed to hybrid vigor. We discovered 11 favorable alleles with different genetic effects significantly contributing to the heterosis in grain yield among 85 AFLP markers, and 8 unfavorable alleles that significantly reduced heterosis. Based on the favorable and unfavorable alleles found, 42 accessions were evaluated for the exploitation potentials of heterosis, as reference for hybrid rice breeding. Moreover, some aspects related to heterosis formation are discussed in this paper.

Heterosis is a heterozygous trait, depending on the genetic diversity of both parents. An approach to tag the heterosis trait with a molecular marker should be different from any other approaches for homozygous traits. To achieve this objective, it is necessary to identify the genetic diversity related to heterosis between both parents. This part of genetic diversity should be closely related to the parental combining ability, which is the most important factor for the performance of the hybrids. Based on the principle of parental combining ability, the simple sequence repeat (SSR) markers favorable and unfavorable to heterosis were identified in the previous study. In the present study, using these markers, two restorer lines with a very high combining ability, MGR44 and MGR45, were developed from the elite restorer line, Minhui63. In these two lines the favorable alleles were maintained while the unfavorable alleles were removed, resulting in the obvious increase of both the general combining ability (GCA) and special combining ability (SCA). Also, the spike density and 1000-grain weight traits were enhanced as compared with Minhui63. These results indicate that the previously selected molecular markers are very useful for hybrid rice breeding programs. Also, it was confirmed that the method described in our previous report is suitable and effective for screening molecular markers for hybrid rice breeding.

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Random amplified polymorphic DNA (RAPD) markers were used to develop genetic fingerprints and analyse genetic relationships among 29 Ipomoea accessions from different geographical locations around the world, including unique wild species, and reproducible profiles were obtained for all accessions using random decamer primers. The primers generated 46 polymorphic markers, one primer alone having 10 products, enabling the discrimination of all 29 accessions. A high level of genetic variability in sweet potato collections was suggested by the degree of polymorphism. Half of the Japanese land races were closely related while accessions from Papua New Guinea and The Philippines were distinct and exhibited the greatest genetic diversity. The wild species Ipomoea gracilis and Ipomoea tiliacea formed a group distinct from the cultivated sweet potato. The wild tetraploid accession K233 and the species Ipomoea trifida were progressively more related genetically to the cultivated sweet potato and are the probable progenitors of Ipomoea batatas, and may be suitable as germplasm for genetic enhancement. RAPDs proved to be useful for sweet potato systematics and should be valuable for germplasm management, gene tagging and efficient choice of parents in breeding programmes.