The Inheritance of Unilateral Incompatibility in Lycopersicon Hirsutum.

A M O N G the mechanisms restricting the flow of genes between discrete populations of plants, unilateral incompatibility (UI) is unique in that one of the reciprocal crossing combinations succeeds whereas the other fails. The term unilateral incompatibility is generally restricted to a phenomenon thought to be closely related to self-incompatibility (SI), for it occurs in matings of self-fertile to self-incompatible species and is controlled by the same physiological mechanism, inhibition of pollen-tube growth (LEWIS and CROWE 1958). The term does not include unilateral crossing failures associated with different ploidy levels, nuclear-cytoplasmic interactions, conditions of sexuality, or mechanical impediments. Although U1 most frequently OCCUTS between species, it is also found within species (MARTIN 1961a) and the term unilateral interspecific incompatibility describes the same behavior. Unilateral incompatibility was first described in the cross of self-fertile Nicotiana langsdorfii to self-incompatible N . data (ANDERSON and DE WINTON 1931). These species usually hybridized reciprocally but in one case the cross succeeded only when N . langsdorfii was used as female. Failure of the reciprocal cross was associated with the presence of a particular S (incompatibility) allele. MATHER (1943) found U1 between self-fertile Petunia axillaris X self-incompatible P. violacea. The selfand cross-compatibilities of the F, and backcross hybrids depended chiefly on the particular S alleles present. Any S allele from P. uiolacea in the style inhibited growth of any pollen-tube containing Sa, the self-fertility allele from P. axillaris. However, segregating polygenes affected the strength of the incompatible reactions. STOUT (1952) redefined MATHER’S species, and found U1 between his self-incompatible Petunia integrifolia and self-incompatible P. axillaris. Both species were also unilaterally incompatible with males of self-fertile P . parodii. Hybrids of the first two species were self-incompatible. About half of the 97 plants were bilaterally compatible with both parents, but 46 could only be classified as of mixed or ill-defined compatibility. Some of these were unilaterally or bilaterally incompatible with the parents. PUSHKARNATH (1953) found U1 between seyeral species of series Commersiana (Solanum) and S. aracc-papa. In the cross of self-incompatible S. subtillis X self-incompatible S. aracc-papa, all F, hybrids were Self-incompatible, reciprocally incompatible among themselves and with the male parent, and unilaterally incompatible with the female parent. He hypothesized a factor R from the male parent, inhibiting tube growth of pollen containing any S allele. GARDB (1959) found similar U1 in species crosses of Solanum and also postulated an S and R series of alleles. DIONNE (1961) found three mechanisms responsible for failure of Solanum species crosses: pollen-tube inhibition, failure of the ovule to develop after fertilization, and embryo abortion. SI and U1 were found in the F, hybrids of self-fertile Lycopersicon esculentum x selfincompatible L. peruuianum by MCGUIRE and RICK (1954). Of the F, hybrids, all were selfincompatible, 28 were unilaterally incompatible with females of L. peruvianum, and three were