In order to look into species relationships, crosses were made in all possible combinations between 9 species belonging to the subgenus Eumelilotus; 504 crosses were made among 53 strains in total. The reproductive barriers found among the species were a crossing barrier (death of young F1 zygotes), F1 inviability and weakness due to chlorosis, F2 breakdown due to the occurrence of chlorotic segregants (classified into lethal and weak types), partial F1 sterility, and a segmental interchange. All the hybrids within species were normal. In crosses between M. alba and M. suaveolens, the hybrids of annual strains showed more pronounced F1 and F2 chlorosis than those of biennial strains. The hybrids of self-fertile species or forms tended to have a low reproductive potential. The F1 chlorosis between the above two species appeared to be controlled by a set of complementary dominant lethals, or by alleles at one locus which express a disharmonious interaction in heterozygote. The F2 chlorosis in the same species hybrid was controlled by two sets of duplicate recessive genes, chl1 and chl2 for the lethal type and chw1 and chw2 for the weak type. The chl2 and chw2 loci were linked with a recombination value of about 13%.

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SUMMARYEvidence from an experiment involving interspecific hybrids of Melilotus suggested that several genes causing isolating barriers, such as hybrid breakdown and non-crossability, are located on rearranged segments. In general, reproductive barriers were controlled by complementary or duplicate genes. The fixation of such genes within a population is possibly promoted when they are linked with the breakage points of rearrangements.