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Outcrossing and Pollinator Limitation of Fruit Set: Breeding Systems of Neotropical Inga Trees (Fabaceae: Mimosoideae)

Suzanne Koptur
Evolution
Vol. 38, No. 5 (Sep., 1984), pp. 1130-1143
DOI: 10.2307/2408445
Stable URL: http://www.jstor.org/stable/2408445
Page Count: 14
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Outcrossing and Pollinator Limitation of Fruit Set: Breeding Systems of Neotropical Inga Trees (Fabaceae: Mimosoideae)
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Abstract

Species of Inga characteristically have large floral displays, but few of the flowers set fruit. Seven species were studied in lower montane wet forest at Monteverde, Costa Rica (Inga brenesii, I. densiflora, I. longispica, I. mortoniana, I. oerstediana, I. punctata, and I. quaternata) to elucidate breeding systems and determine what factors limit fruit set. Observations of visitor activity (hawkmoths, hummingbirds, and skippers) on Inga brenesii and I. punctata indicated that there was no shortage of pollination. Examination of stigmas of flowers collected from all species showed that far more flowers had received pollen than normally set fruit. Hand pollination of six spp. revealed them to be self-incompatible. Observations of pollen grain germination and pollen tube growth in I. brenesii indicated its incompatibility system is gametophytic. Pollen/ovule ratios were much lower than those usually associated with self-incompatible species, but this may be explained by the greater efficiency of fertilization provided by having pollen in polyads. None of the species that bloom simultaneously were cross-compatible. Intraspecific cross-pollinations in I. brenesii, I. punctata, and I. densiflora were more successful when the pollen sources were more than 1 km away from the stigmatic parent than when pollen sources were less than .5 km away. Long-distance pollinator movements are therefore likely to be of greatest consequence in fruit setting. The low fruit set of Inga is a result of a special case of pollinator limitation: much more geitonogamy and unsuitable xenogamy is effected by pollinators than optimal outcrossing to greater distances. This phenomenon may be widespread among tropical trees; long-distance pollinators may be even more important than formerly thought. The flowering strategy of Inga has evolved to attract many pollinators over extended periods of time, while the breeding systems ensure maintenance of genetic variability despite much selfing and crosses between near neighbors.

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