9. Creeping Fruitless Falls
135
Heterostyly, which has evolved repeatedly in approximately 25 angiosperm families is a genetic polymorphism in which plant populations are composed of two
(distyly) or three (tristyly) morphs that differ reciprocally in the heights of stigmas
and anthers in their hermaphrodite flowers (Ganders 1979; Barrett 1992). The
floral polymorphism is usually linked with a diallelic sporophytic self-incompatibility system. Most heterostylous plants are entomophilous (or pollinated by humming birds) and grow clonaHy, so that the breeding system is thought to be adaptive
to avoid geitonogamous self breeding. A sophisticated entomophilous breeding system like heterostyly is likely to be most vulnerable to the detrimental effects of
pollinator absence or isolation.
Our studies on pollination and seed set of heterostylous plants, Primula sieboldii
E. Morren, P. kisoana Miq. and Persicaria japonica (Meisn.) H. Gross, and a
highly endangered homomorphic species, Crepidiastrum ameristophyllum (Koidz.)
Nakai in their natural habitats suggested that fruitless fall has already been rather
common among wild plants which have suffered population decline or genet isolation due to habitat fragmentation or other threats to biodiversity.
2 Reproductive Failure of Primula sieboldii
Due to Po iii nator Loss
Primula sieboldii is a perennial clonal herb that occurs in a range of moist habitats
throughout Japan. Like many other Primula species, P. sieboldii is distylolls
(Richards 1986). Although the species was once very common, in recent years it
has declined and is now listed in the Japanese plant red list (Environmental Agency
1997).
Serious effects of pollinator loss have been recognized for a P. sieboldii population of a small nature reserve (4 ha) in the floodplain of the Arakawa River. The
nature reserve encloses a remnant moist tall grassland and is an 'insular habitat',
thoroughly surrounded by the urban area of Greater Tokyo. The nature-reserve
population of P. sieboldii contains approximately the same proportiolls of normal
heterostylous morphs, long- and short-styled morphs, with a very low frequency of
self-fertile short homostyle morphs, less than 1 %. Although such a morph composition can be expected for a population in which the heterostylous breeding system
is normally functioning (Charlesworth and Charlesworth 1979), it should not necessarily reflect the current state of reproduction of the population, since the genet
longevity of this perennial is likely to exceed several decades.
In the nature reserve population of P. siebold;i, seed set is generally low and
varies significantly among the JUorphs (Washitani et al. 1994a). Fertility of the rare
short-homostyle far surpasses those of normal heterostylous morphs, of which fertility is severely limited by pollinator availability (Washitani et al. 1991). Artificial
legitimate pollination greatly enhanced seed set in long-styled and short-styled
morphs, suggesting that limited supply of compatible pollen was the main reason
for the low seed set of the normal heterostylous morphs. We could not observe any
135
Heterostyly, which has evolved repeatedly in approximately 25 angiosperm families is a genetic polymorphism in which plant populations are composed of two
(distyly) or three (tristyly) morphs that differ reciprocally in the heights of stigmas
and anthers in their hermaphrodite flowers (Ganders 1979; Barrett 1992). The
floral polymorphism is usually linked with a diallelic sporophytic self-incompatibility system. Most heterostylous plants are entomophilous (or pollinated by humming birds) and grow clonaHy, so that the breeding system is thought to be adaptive
to avoid geitonogamous self breeding. A sophisticated entomophilous breeding system like heterostyly is likely to be most vulnerable to the detrimental effects of
pollinator absence or isolation.
Our studies on pollination and seed set of heterostylous plants, Primula sieboldii
E. Morren, P. kisoana Miq. and Persicaria japonica (Meisn.) H. Gross, and a
highly endangered homomorphic species, Crepidiastrum ameristophyllum (Koidz.)
Nakai in their natural habitats suggested that fruitless fall has already been rather
common among wild plants which have suffered population decline or genet isolation due to habitat fragmentation or other threats to biodiversity.
2 Reproductive Failure of Primula sieboldii
Due to Po iii nator Loss
Primula sieboldii is a perennial clonal herb that occurs in a range of moist habitats
throughout Japan. Like many other Primula species, P. sieboldii is distylolls
(Richards 1986). Although the species was once very common, in recent years it
has declined and is now listed in the Japanese plant red list (Environmental Agency
1997).
Serious effects of pollinator loss have been recognized for a P. sieboldii population of a small nature reserve (4 ha) in the floodplain of the Arakawa River. The
nature reserve encloses a remnant moist tall grassland and is an 'insular habitat',
thoroughly surrounded by the urban area of Greater Tokyo. The nature-reserve
population of P. sieboldii contains approximately the same proportiolls of normal
heterostylous morphs, long- and short-styled morphs, with a very low frequency of
self-fertile short homostyle morphs, less than 1 %. Although such a morph composition can be expected for a population in which the heterostylous breeding system
is normally functioning (Charlesworth and Charlesworth 1979), it should not necessarily reflect the current state of reproduction of the population, since the genet
longevity of this perennial is likely to exceed several decades.
In the nature reserve population of P. siebold;i, seed set is generally low and
varies significantly among the JUorphs (Washitani et al. 1994a). Fertility of the rare
short-homostyle far surpasses those of normal heterostylous morphs, of which fertility is severely limited by pollinator availability (Washitani et al. 1991). Artificial
legitimate pollination greatly enhanced seed set in long-styled and short-styled
morphs, suggesting that limited supply of compatible pollen was the main reason
for the low seed set of the normal heterostylous morphs. We could not observe any