138
I. Washitani
5 Pollination Failure in a Common Heterostylous
Species, Persicaria japonic8, Due to Isolation
Seed set failure due to pollination limitation was also demonstrated for isolated
genets (Nishihiro and Washitani 1998b) and smaller populations (Nishihiro et al.
1998) of Persicaria japonica, which is a heterostylous perennial herb common to
various moist or wet habitats throughout Japan. The species has dish-shaped corolla, and is supposed to be a generalist concerning pollinator insects, since many
insects including syrphids, flies, bees, and butterflies can pollinate the flower.
Our study (Nishihiro and Washitani 1998a) demonstrated that the species has
several features advantageous for studying pollination success separately from postpollination processes: 1) compatibility of a given deposited pollen grain can be
relatively precisely judged from the pollen size, which markedly differs between
the floral morphs, 2) a flower can set a seed by the receipt of only a single pollen
grain from the opposite morph, and 3) there is little possibility of self-pollen interference with legitimate fertilization. Therefore, female reproductive success at the
pollination stage can be evaluated by presence or absence of pollen grains from the
opposite morph on the stigma.
The species is common in moist tall grasslands dominated by Phragmites australis W. Clayton in floodplains and lakeshores of lowland Japan. However, recently these habitats of the species have been SUbjected to extensive fragmentation
due to reclamation and other construction works. Moist tall grasslands on the shore
of Lake Kasumigaura, the second largest lake in Japan, are not exceptional, and at
present there remains a highly fragmented landscape of the grasslands with small
and/or low-density populations of P. japonica.
In the remaining patches of fragmented grasslands on the lakeshore, we compared pollination success and seed set between genets which markedly differed in
relative distance from a potential mating partner of the opposite morph (Nishihiro
and Washitani 1998b). Artificial bank construction left small isolated patches of P.
japonica containing only a single genet, while patches consisting of several different genets (congregating genets) remain near the shoreline. Pollinator insects are
abundant even in the fragmented grasslands and 28 insect species including hymenopterous, dipterous, and lepidopterous species frequently visited the pollinator-generalist flowers even in isolated patches. However, the pollination success
varied significantly according to the degree of isolation and was considerably higher
in the congregating genets than in the single genets. Seed set also differed significantly between them, and higher seed set was recorded for the congregating genets.
These results suggested high susceptibility of pollination success and thus female
fertility to isolation or solitude.
We compared seed set among 25 local populations of Persicaria japonica growing in remnant grassland patches of various sizes scattered along 25 km of shoreline of the lake (Nishihiro et al. 1998). The populations greatly differed in the area
of grassland patches inhabited (120-18000 m 2 ), genet number (1-53), and morph
I. Washitani
5 Pollination Failure in a Common Heterostylous
Species, Persicaria japonic8, Due to Isolation
Seed set failure due to pollination limitation was also demonstrated for isolated
genets (Nishihiro and Washitani 1998b) and smaller populations (Nishihiro et al.
1998) of Persicaria japonica, which is a heterostylous perennial herb common to
various moist or wet habitats throughout Japan. The species has dish-shaped corolla, and is supposed to be a generalist concerning pollinator insects, since many
insects including syrphids, flies, bees, and butterflies can pollinate the flower.
Our study (Nishihiro and Washitani 1998a) demonstrated that the species has
several features advantageous for studying pollination success separately from postpollination processes: 1) compatibility of a given deposited pollen grain can be
relatively precisely judged from the pollen size, which markedly differs between
the floral morphs, 2) a flower can set a seed by the receipt of only a single pollen
grain from the opposite morph, and 3) there is little possibility of self-pollen interference with legitimate fertilization. Therefore, female reproductive success at the
pollination stage can be evaluated by presence or absence of pollen grains from the
opposite morph on the stigma.
The species is common in moist tall grasslands dominated by Phragmites australis W. Clayton in floodplains and lakeshores of lowland Japan. However, recently these habitats of the species have been SUbjected to extensive fragmentation
due to reclamation and other construction works. Moist tall grasslands on the shore
of Lake Kasumigaura, the second largest lake in Japan, are not exceptional, and at
present there remains a highly fragmented landscape of the grasslands with small
and/or low-density populations of P. japonica.
In the remaining patches of fragmented grasslands on the lakeshore, we compared pollination success and seed set between genets which markedly differed in
relative distance from a potential mating partner of the opposite morph (Nishihiro
and Washitani 1998b). Artificial bank construction left small isolated patches of P.
japonica containing only a single genet, while patches consisting of several different genets (congregating genets) remain near the shoreline. Pollinator insects are
abundant even in the fragmented grasslands and 28 insect species including hymenopterous, dipterous, and lepidopterous species frequently visited the pollinator-generalist flowers even in isolated patches. However, the pollination success
varied significantly according to the degree of isolation and was considerably higher
in the congregating genets than in the single genets. Seed set also differed significantly between them, and higher seed set was recorded for the congregating genets.
These results suggested high susceptibility of pollination success and thus female
fertility to isolation or solitude.
We compared seed set among 25 local populations of Persicaria japonica growing in remnant grassland patches of various sizes scattered along 25 km of shoreline of the lake (Nishihiro et al. 1998). The populations greatly differed in the area
of grassland patches inhabited (120-18000 m 2 ), genet number (1-53), and morph