Linkage Relationship of Two Gametophytic Characters in Oenothera Organensis.

N THE gametophyte generation all genes are normally present in the I haploid state, and since the gene effects are not complicated by dominance, it is often possible to apply methods which cannot be used in the sporophyte, or diploid, geieration. In this connection, the method used in identifying crossovers between two genes effecting pollen tube growth in Oenothera organensis may be of interest. The growth responses of compatible and incompatible pollen tubes in this species have previously been described (EMERSON 1938, 1939, 1940). The points of importance in the present connection may be briefly recapitulated as follows: Since pollen carrying any one of the 45 known alleles‘ of the self-sterility gene fails to produce normally growing tubes in stigmas or styles of plants carrying the same allele, it is impossible for plants to receive the same allele from both parents. Each plant is thus heterozygous for two alleles, each of which is present in half of the pollen. By pollinating a stigma heterozygous for one of the alleles carried by the pollen, the two sorts of pollen may be readily distinguished. The pollen carrying the allele also present in the stigma is incompatible, producing extremely short pollen tubes (fig. IB), the pollen carrying the allele not present in the stigma is compatible, producing pollen tubes which continue to grow until they reach the embryo sacs in the ovules (fig. IA). The second gametophytic character in this species is a pollen lethal. Pollen carrying this gene, pZ, fails to produce normally developing tubes in any stigma, regardless of the genetic constitution of the stigma. The lethal pollen tubes are extremely short, much branched and appear to have a much less definite membrane than either compatible or incompatible tubes and so are easily recognized in prepared stigmas (fig. IC).