Unreduced 2n Pollen Production in 'Nishiuchi Konatsu' Hyuganatsu as Inferred by Pollen Characteristics and Progeny Ploidy Level

Hyuganatsu is a self-incompatible Citrus species with no parthenocarpic traits. Pollinizer requirements and seedy fruit production are therefore a limiting factor for Hyuganatsu growers. 'Nishiuchi Konatsu', a bud mutation of Hyuganatsu, can set fruit by self-pollination and aborts most seeds. To obtain a better understanding of the reproductive mechanisms of 'Nishiuchi Konatsu', its pollen was characterized by pollen observation and viability testing in vitro in addition to pollen tube observation of the pistil following various pollination combinations. Furthermore, the ploidy level of 'Nishiuchi Konatsu' and the progenies obtained after controlled pollination were analyzed using a flow cytometer. As a result, 'Nishiuchi Konatsu' pollen was shown to be functional in vitro, as indicated by both the pollen germination test and FDA staining. A curious feature of 'Nishiuchi Konatsu' pollen is that a giant pollen of about 42-44 mu m in diameter, which is about 1.3-fold the average diameter of original Hyuganatsu pollen (33.4 mu m), was partially produced. Observation of in vivo pollen germination and pollen tube elongation revealed that Hyuganatsu (or 'Nishiuchi Konatsu') x Konatsu' showed imperfect incompatibility; that is, some pollen tubes were able to enter the style while others could not. Flow cytometry analysis indicated that 'Nishiuchi Konatsu' is diploid while all of its self-pollinated progenies, which were rarely obtained as normal seeds, were tetraploid. Progenies from other combinations were diploid. These results strongly suggest that unreduced 2n pollen production occurred in 'Nishiuchi Konatsu'. In addition to self-compatibilty and seed abortion of 'Nishiuchi Konatsu' self-pollination, it has been reported that the proportion of normal to aborted seeds differs according to the cultivar of the seed parent crossed with 'Nishiuchi Konatsu' pollen (Honsho et al., 2009). Taking into account the unreduced pollen production as well as the effect of polyploidization on self-incompatibility and embryo development, we finally discussed why such a phenomenon was induced by pollination with 'Nishiuchi Konatsu' pollen and why 'Nishiuchi Konatsu' became self-compatible.