Crossability of Self-compatible Mutant^|^lsquo;Osa-Nijisseiki^|^rsquo; (Pyrus serotina Rehd.)

Abstract

Crossability of self-compatible mutant‘Osa-Nijisseiki’was investigated in the field and in vitro by crossing with the other cultivars possessing various S-alleles (S1S4, S2S4, S4S5, S5S7).‘Osa-Nijisseiki’crossed successfully with all cultivars tested when used as female, whereas it did not cross well with cultivars having S2S4 genotype when used as male. The mechanism of cross-incompatibility of‘Osa-Nijisseiki’is elicited when S2S4-alleles are present in female tissue. However, the degree of incompatibility of‘Nijisseiki’(S2S4) was different from that of‘Kikusui’(S2S4) when crossed with ‘Osa-Nijisseiki’: the former combination was completely incompatible but the latter resulted in some fruit setting which persisted on a tree until harvest time. This shows that the S-gene in pollen of‘Osa-Nijisseiki’acts differently on the 2 cultivars, even though they have the same S2S4 genotype.By using the excised-stylar system, crossing tests were also carried out. Pollen tubes with various S-alleles penetrated‘Osa-Nijisseiki’styles well, but pollen tubes of‘Osa-Nijisseiki’were completely arrested in excised styles of‘Nijisseiki’and partially arrested in styles of‘Kikusui’. Pollen tubes of selfed‘Osa-Nijisseiki’ penetrated relatively well to the cut ends of its excised styles. Fluorescent microscopic observations showed that the growth ratio of pollen tubes in selfed style of ‘Osa-Nijisseiki’was inferior to that in a crossed style.These facts suggest that the style of‘Osa-Nijiseeiki’loses its ability to arrest sefl pollen tubes but that the crossing barrier of‘Nijiseeiki’and‘Kikusui’with ‘Osa-Nijisseiki’is present in their styles. However, it seems that ‘Osa-Nijisseiki’ still possesses the properties of weak self-incompatibility.