COEVOLUTION AND CONSTRAINTS IN A NEOTROPICAL FIG - POLLINATOR WASP MUTUALISM (COSTA RICA).

摘要

Partners in mutalisms often have conflicting evolutionary goals. I examined the nature and consequences of such conflicts within a Costa Rican fig pollination mutualism. The female fig wasp (Blastophaga silverstrii, Agaonidae) distributes pollen within the inflorescences of the fig (Ficus pertusa, Moraceae), then lays eggs in florets whose ovaries are accessible to her ovipositor. Her offspring eat developing seeds. Seeds successfully mature in ovaries inaccessible to her (those with long styles). Why hasn't the short-lived wasp evolved an ovipositor long enough to reach every fig ovary? I found that relative style and ovipositor lengths were in fact not important in regulating wasp fecundity. Limited egg loads and high larval mortality were more critical. Seed set did not fall as wasp production rose, implying that higher wasp fecundity is not costly for the fig. The absence of direct tradeoffs in success between partners helps explain the great evolutionary success of the fig pollination mutualism.; The lack of tradeoffs in success between partners also meant a lack of tradeoffs between male and female components of reproductive success for the fig, because wasp offspring are the only possible pollen vectors. Seed and wasp production were in fact positively correlated within inflorescences. Because developing wasps feed on some developing seeds, constraints on seed maturation (especially resource availability) may inevitably affect wasp maturation as well. Trees producing the highest total numbers of seeds and pollen-carriers were those experiencing intermediate pollination intensities. However, most trees were either very heavily pollinated or very poorly pollinated, despite evidence that pollination intensity is partially under the tree's control.; Species-specificity of pollination is maintained because F. pertusa evidently releases a species-specific chemical to attract pollinators. Specificity is continually reinforced because larvae cannot survive unless their mothers transfer compatible pollen among trees. Other organisms exploiting this mutualism are less specific to it, and probably less tightly coevolved with it. Three species of wasps (Torymidae) commensal to the mutualism are the most species-specific of these associates, probably because the timing of their development has to be closely synchronized with the pollinators' development. The least species-specific and most unpredictable associates were the avian seed dispersers. Compared to its obligate pollinator, F. pertusa's disperser assemblage is much less likely to be coevolved with it.