The partially fertile F₁'s were obtained from a cross between the 1D(1A) disomic-substitution of Langdon durum as female parent and the (lo) durum line having scs^tscs^t and ViVi genes pairs. In the F₂ generation, scs^t and Vi genes were expected to segregate independently of each other, because scs^t is located on chromosome 1A andVi on chromosome 1B (Table 1). A cytologically selected disomic F₂ plant was male fertile. This plant was crossed to the (lo) scs^t male-sterile line and produced all plump seeds and male-sterile progeny, indicating the paternal F₂ had a scs^tscs^t gene pair but did not have Vi (Table 1). Thus, the(lo) male-sterile A-line and the euplasmic maintainer B-line both sharing the same scs^tscs^t pair were produced.

A similar procedure crossing the euplasmic durum line with the scs^t scs^t gene pair with (un) durum having scs^tscs^t and ViVi produced a male-sterile line having the Ae. uniaristata cytoplasm [(un) cytoplasm] (Maan unpub). These results also showed that the euplasmic durum line with the scs^tscs^t gene pair was male fertile but the (lo) or (un) durum lines with the scs^tscs^t gene pair were male sterile.

To maintain cytoplasmic male-sterile A-line, it is essential that a scs^tscs^t pair is present in the male-sterile A-line as well as in the fertile maintainer B-line. Crosses were repeated from time to time to make sure that the male-sterile A-line as well as the maintainer B-line carry a scs^tscs^t pair (Fig. 2): (a) A cross between (lo) male- sterile durum carrying a scs^tscs^t gene pair and the normal durum produced plump seeds and male-sterile progeny, because all female gametes carried a scs^t gene. In contrast, a cross between (lo) male-sterile line carrying one copy of the scs^t gene and normal durum produced plump and viable seeds and shriveled and inviable seeds. These results indicate that the euplasmic maintainer line carried a scs^tscs^t gene pair. (b) A cross between a (lo) male- sterile line having one copy of scs^t and an euplasmic maintainer line having an scs^t scs^t gene pair produced all male gametes carrying a scs^t gene.

The scs^t and Vi genes can be distinguished from the Rf genes by the differential effects they produce in the durum lines having (lo),(un), or other alien cytoplasms. For example, Vi and Rf produce fertility in the durum lines carrying cytoplasm from several species, including T. timopheevi, T. araraticum, or Ae. speltoides (Maan unpub). In contrast, the male fertility restorer lines (R-lines) having Rf genes from the above species when crossed to the (lo) male-sterile line having one copy of scs^tproduced plump seeds and male fertile progeny having scs^t and Rf, while seeds with Rf alone (without Vi) were shriveled and inviable (Maan unpub), like those from the cross with control durum. These results show that Vi produces plump seeeds that result in fertile progeny, while Rf alone (without scs or Vi) produces shriveled and inviable seeds in durum with (lo) or (un) cytoplasm. The Rf genes are dominant to rf as such a single copy restores fertility by epistatic interaction with scs^t in the alloplasmic wheat lines.