聚果榕两种非传粉小蜂产卵与果实脱落的关系

在聚果榕与其传粉榕小蜂Ceratosolen fusciceps组成的互利共生系统中,与传粉榕小蜂共存的还有榕树的寄生性小蜂Platyneura testacea和Platyneura mayri.这些寄生性小蜂由于不能给榕树带来任何收益而只是利用榕树的种子或与传粉小蜂竞争植物的雌花资源,因而可能导致榕树与榕小蜂之间合作系统的崩溃.植物果实的脱落机制普遍被认为是维持系统稳定的关键因素之一.然而,定量实验和野外观测发现P. mayri产卵并不会引起果实脱落,只有P. testacea产卵会使果实大量脱落.通过对3株样树进行比较发现:当产卵时的P. testacea数量越多时,它产生的瘿花数就越多,榕果发生脱落的比例越高.P. testacea的过度产卵是导致榕果选择性脱落的主要原因.结果表明:脱落机制并不能完全阻止非传粉小蜂的寄生,榕树只能选择性地脱落掉先于传粉小蜂产卵的榕果.这也同时表明维持榕树与榕小蜂互利共存的机制不仅仅只有榕果的脱落机制,可能还存在其它未被发现的机制.

Relation between two non-pollinating wasps oviposition and the fruit abscission on Ficus racemosa

In the fig-fig wasp obligate mutualism, parasitic wasps Platyneura testacea and Platyneura mayri coexist with pollinating wasps. Theses non-pollinating wasps only use the seeds of figs or compete with pollinating wasp for floral resources, bring no any benefits to figs. The parasitism of these non-pollinating wasps may result in the disruption of the cooperation between figs and fig wasps. The fruit abscission mechanism of plant is generally considered as one of the key factors maintaining system stability. However, our quantitative experiment and field observations of fig-fig wasp system showed that the figs did not abscise fruits oviposited by Platyneura mayri, but abscised fruits over-ovipisited by P.testacea. The observation showed that tree oviposited by more P. testacea will have higher proportion of abscission. The over-ovipositing of P.testacea is the main reason for selective abscission of syconium. Our results indicate that the plant can only abscise fruits oviposited by parasitic wasps before pollinating wasps in fig-fig wasp mutualism, but do not abscise the fruits parasitized by wasps ovipositing simultaneous or later than pollinating wasps, implying that abscission can not sufficiently maintain the fig-fig wasp mutualism.